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Dissertations, Theses, and Masters Projects Theses, Dissertations, & Master Projects
1982
The distribution and ecology of gammaridean amphipods in the plankton of the middle Atlantic Bight
Cathy J. Womack College of William and Mary - Virginia Institute of Marine Science
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Recommended Citation Womack, Cathy J., "The distribution and ecology of gammaridean amphipods in the plankton of the middle Atlantic Bight" (1982). Dissertations, Theses, and Masters Projects. Paper 1539617531. https://dx.doi.org/doi:10.25773/v5-3cwq-yk84
This Thesis is brought to you for free and open access by the Theses, Dissertations, & Master Projects at W&M ScholarWorks. It has been accepted for inclusion in Dissertations, Theses, and Masters Projects by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. THE DISTRIBUTION AND ECOLOGY OF GAMMARIDEAN AMPHIPODS IN THE
PLANKTON OF THE MIDDLE ATLANTIC BIGHT
A Thesis
Presented to
The Faculty of the School of Marine Science
The College of William and Mary in Virginia
In Partial Fulfillment
of the Requirements for the Degree of
Master of Arts
by
Cathy J. Womack
1982 APPROVAL SHEET
This thesis is submitted in partial fulfillment of
the requirement for the degree of
Master of Arts
LJ^qrYtouckj Author
Approved, August 1982
George C.
Robertr\ -v* +- Jordan T /I rtr> V
C^> ,
Christopher S. Welch
Richard Wetzel
Donald F. Boesch Louisiana Universities Marine Consortium This thesis is dedicated to my parents,
Andrew and Norma Womack
iii TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ...... v
LIST OF TABLES...... vii
LIST OF FIGURES...... ix
ABSTRACT...... xi
INTRODUCTION...... 2
MATERIALS AND METHODS...... 9
RESULTS...... 16
DISCUSSION...... 86
BIBLIOGRAPHY...... 108
VITA...... 115
iv ACKNOWLEDGMENTS
I would like to express my deepest appreciation to Dr. George C.
Grant for his patience and support as my major professor. I also
thank Dr. Don Boesch, Dr. Bob Jordan, Dr. Chris Welch, and Dr. Dick
Wetzel, for critically reading this manuscript and serving on my
committee.
A critical part of this thesis was data collected, analyzed, and
reported by the VIMS Department of Invertebrate Zoology. I appreciate
the work done by all members of that department and particularly thank
Dr. Don Boesch for allowing me access to the benthic data, Ms. Marcia
Bowen for her assistance in identification of amphipods, and Ms. Linda
Schaffner for allowing me to examine specimens in her care.
I cherish the memories of my stay at VIMS, not only because of all that I learned and did, but also because of the friends and acquaintances I made. I extend my warmest appreciation to all of
those people. I especially thank Alyce Thomson for always being a cheerful and considerate housemate during my time at Gloucester Point.
I feel that I was extremely fortunate to have been a member of the VIMS Department of Planktology. The following friends and co-workers each in his or her own way contributed to this thesis:
Steve Berkowitz, Shelia Berry, Burt Bryan, Willy Burton, Mike Canino,
Pat Crewe (our unofficial boss), Fred Jacobs, Cathy Meyer Lascara, John Olney, Jim Price, Jo Ellen Robins, Russ Short, Pete Smyth, Mike
Vecchione, and Roberta Wallace. I miss all of them.
In addition, I thank my present coworker, Ms. Anne Morrison, for her physical and moral support while I completed this thesis.
Ms. Norma Womack and Ms. Beth Knight did excellent jobs inter preting my handwriting and various scribbled notes in typing this thesis.
Finally, I most especially thank my husband David Ludwig. With out his support and encouragement this thesis most certainly would not have been completed. LIST OF TABLES
Table Page
1. Time periods in which sampling occurred...... 10
2. Stations sampled for zooplankton...... 13
3. Species list, frequency of occurrence and maximum abun dance in subsurface collections, arid occurrence in surface collections...... 17
4. Occurrence of gammaridean amphipods in neuston collections...... 27
5. Occurrence of Ampelisca agassizi in the Middle Atlantic Bight in the water column (this study) and in the benthos at stations also sampled for zooplankton (from Boesch, 1979)...... 30
6. Occurrence of Ampelisca vadorum in the Middle Atlantic Bight in the water column (this study) and in the benthos at stations also sampled for zooplankton (from Boesch, 1979)...... 34
7. Occurrence of Byblis serrata in the Middle Atlantic Bight in the water column (this study) and in the benthos at stations also sampled for zooplankton (from Boesch, 1979)....,...... 39
8. Occurrence of Argissa hamatipes in the water column in the Middle Atlantic Bight...... 50
9. Occurrence of Erichthonius rubricornis in the Middle Atlantic Bight in the water column (this study) and in the benthos at stations also sampled for zooplankton (from Boesch, 1979)...... 52
10. Occurrence of Unciola irrorata in the Middle Atlantic Bight in the water column (this study) and in the benthos at stations also sampled for zooplankton (from Boesch, 1979) ...... 56
11. Occurrence of mature females of the species Unciola irrorata in the plankton at B5...... 62
vii List of Tables (Continued)
Table Page
12. Occurrence of Microprotopus raneyi in the plankton of the Middle Atlantic Bight...... 66
13. Occurrence of Liljeborgia fissicornis in the plankton of the Middle Atlantic Bight...... 71
14. Occurrence of Monoculodes edwardsi in the Middle Atlantic Bight in the water column (this study) and in the benthos (from Boesch, 1979)...... 74
15. Degree of maturity of Monoculodes edwardsi by station 78
16. Occurrence of Synchelidium americanum in the water column. 82
viii LIST OF FIGURES
Figure Page
1. Location of stations sampled for zooplankton...... 12
2. Abundance and species composition of gammaridean amphipods across the shelf in subsurface collections during the fall...... 19
3. Abundance and species composition of gammaridean amphipods across the shelf in subsurface collections during the winter...... 21
4. Abundance and species composition of gammaridean amphipods across the shelf in subsurface collections during the spring...... 23
5. Abundance and species composition of gammaridean amphipods across the shelf in subsurface collections during the summer...... 25
6. Size and sex frequencies of Ampelisca agassizi at selected water column and benthic stations..... 32
7. Size and sex frequencies of Ampelisca vadorum at selected water column and benthic stations..... 36
8. Size and sex frequencies of Byblis serrata at station B5 in the water column and benthos...... 42
9. Diel cycle of Ampithoe longimana. in the neuston at station LI during the summer, 1977...... 45
10. Occurrence of Ampithoe longimana in the water column at station Ll and from the Chesapeake Bay...... 47
11. Size and sex frequencies of Erichthonius rubricornis in the water column and the benthos at B5 during the fall, winter, and spring...... 55
12. Size and sex frequencies of Unciola irrorata at B5 during the fall, winter, and spring...... 59
13. Size and sex frequencies of Unciola irrorata in the water column at L2 and N 3 ...... 61 List of Figures (.Continued)
Fi gure Page
14. Diel cycle of Microprotopus raneyi in the neuston at Cl during the summer...... 68
15. Size and sex frequencies of Monoculodes edwardsi in the water column at all stations during all four cruises ...... 77
16. Mean lengths and range of lengths of Monoculodes edwardsi by station for each cruise...... 80
17. a) Cumulative percent and percent of tows in which gammaridean amphipods were collected by distance-from-bottom intervals, b) Percent of tows which had no maximum depth within specified distance-from-bottom interval. 89
x ABSTRACT
Gammaridean amphipods collected during a plankton survey of the outer continental shelf of the Middle Atlantic Bight were identified, measured, sexed, and compared with amphipods collected during a con current benthic survey of the same area. Twenty-nine species com prising 18 families were identified from plankton collections. The occurrence of seven species in surface samples was indicative of important hydrographic phenomena which influence the composition of Middle Atlantic Bight zooplankton. Microprotopus raneyi and Ampithoe longimana were by far the most abundant gammaridean amphipods in surface collections. These were the only species to occur in more than two of the 75 neuston samples taken during any single cruise and comprised 80% of all neuston specimens.
Gammaridean amphipods were most consistently collected in subsurface tows at central and outer shelf stations and were absent from slope stations. Monoculodes edwardsi was the most frequently collected gammaridean amphipod. It is suggested that its superior dispersal capabilities facilitate exploitation of new habitats. The corophiids Erichthonius rubricornis and Unciola irrorata exhibited a constant flux of low numbers into the water column. Short-range dispersal via the water column by corophiids improved the ability of these species to recolonize small defaunated areas and to coexist with competitively superior species. Several species were apparently transported by southwesterly currents of the shelf indicating the occurrence of long-range dispersal via the water column.
Emergence activities of the ampeliscids Ampelisca agassizi, A. vadorum, A. verrilli, and Byblis serrata and the phoxocephalid Trichophoxus epistomus were related to reproductive behavior. Seasonally restricted mobility of ampeliscids may be one factor which increases the importance of density-dependent interactions in determining distribution.
The actual impact of gammaridean amphipods on zooplanktonic communities of the shelf was probably insignificant. Their occurrence in some surface samples was indicative of important water movements influencing the fauna1 composition of zooplankton communities. The occurrence of benthic amphipods in the water column may be important to the recovery of benthic habitats from disturbance, the composition of benthic communities, and the coupling of benthic and pelagic communities. DISTRIBUTION AND ECOLOGY OF GAMMARIDEAN AMPHIPODS IN THE
PLANKTON OF THE MIDDLE ATLANTIC BIGHT INTRODUCTION
Interaction of benthic and pelagic communities through periodic migration of adult benthic invertebrates into the water column has implications for the ecology, behavior, and evolution of the species themselves and for the ecology of planktonic and benthic communities as a whole. No matter how brief these migrations might be, the organism will be in contact with different species and environmental parameters than those it usually encounters in the substrate. Its behavior and morphology will reflect adaptations to minimize the risks and to maximize the advantages of swimming free in the water column.
Some possible impacts of emergence of benthic organisms from the sediment are summarized by Alldredge and King (1980) who speculate that:
"migration facilitates rapid recolonization of disturbed or defaunated sites, disrupts and mixes bottom sediments, and results in daily variation in the micro-distribution, patchiness, and species composition of the benthic fauna."
The negative relief in bottom topography created by the collapse of the surface as buried crustaceans move out of the sediment may be an important factor in determining community structure (Rhoads, 1974).
Fluxes of nutrients, pollutants, and matter/energy between the water column and benthos may be enhanced by the movement of organisms between the water column and the benthos and by the mixing of sediments as a result of the movement.
2 3
The emergence of individuals and their subsequent horizontal displacement may be important in maintaining a constant species composition in a specific area. In a habitat made up of patches of individuals in which one species is at a competitive advantage relative to other species, the movement of individuals into and out of the substrate may break up the patches frequently enough to prevent exclusion of species by the superior competitor. This was suggested by the hypothesis of contemporaneous equilibrium set forth by
Richerson, et _al. (1970) to partially explain the apparent paradox of high diversity in small phytoplankton samples, Dauer and Simon (1976) apply the hypothesis to the pattern of repopulation following defaunation in an intertidal habitat in which they found that through time the distribution of individuals among species changed greatly while species composition remained relatively constant.
On the opposite end of the scale, dispersion through the water column and settlement of populations may displace entire communities.
For example, a mudflat in Barnstable Harbor was regularly occupied by the mud snail Illyanassa (-Nassarius) obsoleta which seemed to keep the number of coexisting species low (Mills, 1967b). When the mud snail moved into deeper water during the winter, a breeding popula tion of Ampelisca abdita established itself on the mudflat, preventing the reoccurrence of I_. obsoleta the following spring.
Emergent benthic invertebrates are a significant portion of the diet of planktivorous fishes in eelgrass communities (Robertson and
Howard, 1978), inshore communities (Thomas, 1976), and near-shore habitats (Hobson and Chess, 1976). Studies detailing the contribution of migrating benthic organisms to planktivorous fishes in deeper waters are lacking. Such organisms are not considered important to pelagic communities of the Gulf of Maine and Bay of Fundy (Fish and
Johnson, 1937). However, Bigelow and Sears (1939) suggest that bottom- dwelling organisms may be important prey items for planktivorous
/ fish foraging near the bottom along the continental shelf.
The emergence of benthic organisms from the substrate may be studied by examining gammaridean amphipods from plankton samples taken on the continental shelf in the Middle Atlantic Bight and then relating their occurrence in the water column to their benthic distribution.
Gammaridean amphipods are generally adapted to a benthic lifestyle but are frequently taken in plankton collections.
Review of Studies of the Pelagic Occurrence of Gammaridean Amphipods
Studies describing planktonic gammaridean amphipods indicate that the purpose and manner of occurrence in the water column vary among the many different taxa. Nevertheless, certain patterns emerge from these studies.
Gammaridean amphipods in the plankton frequently are most abundant in the lower water column (Russell, 1925; Whiteley, 1948;
Williams and Bynum, 1972; Jones, ejt a l ., 1973; Hobson and Chess,
1976) . Apparently, many species, particularly burrowing species, re enter the substrate after only short periods of time in the plankton
(Alldredge and King, 1980). Migration is generally confined to hours of darkness (Russell, 1925; Fish, 1925; Mills, 1967b; Fincham, 1974;
Hobson and Chess, 1976), and the absence of light has been shown to be a major cue initiating migration (Jansson and Kallander, 1968;
Alldredge and King, 1980). A study of amphipods of the shelf region off southern California, however, found that some species of 5 gammaridean amphipods migrated upward during daylight hours (Brusca,
1967). The lunar cycle which often corresponds to periods of breeding and release of young is another factor involved in the timing of free swimming in the water column (Watkin, 1939; Fincham, 1974). This response appears to be greater in intertidal species than in subtidal species (Mills, 1967b; Fincham, 1970a).
Gammaridean amphipods in the water column may be classified into the following categories (Fish, 1925): 1) Species that breed in the water column, 2) individuals which are passively transported from the bottom by storm-generated currents, and 3) species which are attracted to the surface for reasons unrelated to reproduction. Two other possibilities should also be considered: species which use the water column as an active means of horizontal dispersal, and species that are adapted to pelagic life styles.
Many studies of gammaridean amphipods indicate that their occurrence in the plankton is attributable to breeding behavior (Fage,
1933; Watkin, 1939; Fincham, 1970a, b; Williams and Bynum, 1972). In some species, sexual pairings are brought about by apparently random collisions of individuals followed by amplexus (Holmes, 1903; Crozier and Snyder, 1923; Hynes, 1955). In other species, mating takes place freely in the water column without amplexus (Bousfield, 1973). There is evidence that females may emit pheromones attractive to males
(Dahl, €it ail., 1970).
Emergence from the substrate during breeding periods provides tubicolous and burrowing species free access to potential mates
(Watkin, 1939; Williams and Bynum, 1972; Thomas, 1976) and aids in gene flow between populations normally restricted in movement (Jones, 6 e^t al. , 1973). Males of species which breed freely in the water column often have a terminal pelagic mating stage characterized by morphological adaptations to their pelagic existence such as enlarged eyes, elongated antennae, streamlined body, more powerful pleon, and foliaceous third uropod (Bousfield, 1973; 1978).
Passive displacement from the bottom and transport of individuals by storm generated currents may be the source of occasional collec tions of some species of gammaridean amphipods in the water column.
Several authors report that after the passage of storms with unusual winds, several species previously absent from the water column in their study area were found in plankton collections (Farrell, 1970;
Williams and Bynum, 1972; Thomas, 1976). Fish and Johnson (1937) suggest that accidental occurrences of bottom forms in the plankton could be detected by collections including a small number of individuals from many different taxa.
Other factors.associated with the collection of benthic amphipods in plankton tows are light, temperature, avoidance of demersal preda tors, and feeding. Fish (1925) suggests that some species of amphi pods captured in surface plankton hauls were attracted to the light of phosphorescent diatoms concentrated in the net. Fage (1933) collected
34 species of gammaridean amphipods at night within the light given off by a lantern and Barnard (1969) notes adult male phoxocephalids have been found swarming around lights. Whiteley (1948) and Fincham
(1970a) suggest that temperature may be a factor affecting swimming activity of gammaridean amphipods. Monoculodes edwardsi may actively migrate to areas of cooler temperatures (Whiteley, 1948). It is sometimes suggested that amphipods move into the water column to avoid 7 demersal predators (Jansson and Kallander, 1968; Williams and Bynum,
1972). Whereas the timing of migration may be determined by avoidance of predators, it is doubtful that benthic forms would leave the protection of the bottom as an avoidance mechanism. In fact, the risk of predation is increased during periods of emergence, even at night
(Hobson and Chess, 1976; Robertson and Howard, 1978). Feeding is an unlikely motivating factor since most gammaridean amphipods are adapted to feeding on the bottom.
Amphipods lack the pelagic larval dispersal stage that is charac teristic of many crustaceans (decapods, for example). Eggs are car ried in brood pouches by females until they are released as juveniles closely resembling adult amphipods. Rafting by adults on floating debris, logs, and plants has been considered a major means of disper sion (Barnard, 1970; McKinney, 1977). In a study of benthic fauna of the Cape Hatteras continental shelf, it was found that amphipods were more widely distributed across zoogeographic barriers than molluscs, many species of which have pelagic life stages (Weston, 1979). Weston suggests that the infaunal amphipod taxa are more likely transported limited distances in the plankton rather than by rafting. Studies of ampeliscids (Mills, 1967b), corophiids (Watkin, 1941) and the dexa- minid Tritaeta gibbosa (Jones, e^t aJL. , 1973) indicate that ovigerous females swim to new areas before releasing their brood. The ampeli scids move to previously unoccupied areas, perhaps reducing competition between juveniles and adults for food and space (Mills,
1967b) and the corophiids disperse to areas with favorable substrates
(Watkin, 1941). Alldredge and King (1980) suggest that dispersal over short distances may be effectively accomplished by these brief migrations into the water column. There are species of gannnaridean amphipods which have adopted pelagic lifestyles. Of the 4500 species described by 1964, 20% were regarded as demersal and pelagic (Barnard, 1969). Pelagic gammaridean amphipods comprise several families (Lysiannasidae, Pardaliscidae,
Hyperiopsidae, for example) and have typically been described from bathyal and abyssal zones of the world's oceans (Birstein and Vinogradov,
1958; Bowman and Manning, 1972; Thurston, 1976).
.Objectives of This Study
A multidisciplinary study of the continental shelf of the Middle
Atlantic Bight during which benthic and zooplankton samples were collected at corresponding stations within the same time frame was conducted by the Virginia Institute of Marine Science. This project offers an opportunity to examine the occurrence of benthic amphipods in the water column of the continental shelf. Specific questions which will be addressed by this thesis are: Which species of gammaridean amphipods are found in the water column of the Middle
Atlantic Bight? Is their presence in the water column related to breeding? Are they dispersed via their migrations in the water column?
How does their planktonic occurrence relate to the zooplanktonic and benthic communities of the continental shelf? MATERIALS AND METHODS
The gammaridean amphipods examined in this study were collected on four cruises conducted by the Virginia Institute of Marine Science
(VIMS) as part of Contract AA550-CT6-62 with the Bureau of Land
Management (BLM). The zooplankton and benthos of the Middle Atlantic
Bight area were surveyed to provide baseline data before the leasing of oil drilling rights off the New Jersey coast by BLM. The study began in fall, 1975, and the data for the present study were collected during the expanded second-year program beginning in fall, 1976 (Table
1).
Twelve stations along two transects crossing the continental shelf were sampled for zooplankton (Figure 1 and Table 2). Surface collections were made with a Woods Hole Oceanographic Institution neuston net: at nine of the twelve stations (LI, L2, LA, L 6 , Cl, E 3 ,
Jl, A2, and B5), 20-minute neuston tows were made every three hours during a 24-hour period, and at the other three stations (Dl, N3, and
F2), a single neuston tow was made per sampling period. At all stations, two tows of paired 60-cm opening-closing bongo nets (McGowan and Brown, 1966) were made at night, the first tow with 505 pm mesh nets (B505) and the second tow with 202 pm mesh nets (B202). The nets were opened just below the surface, lowered to within a few meters of the bottom, raised obliquely, and closed just below the surface. The maximum fishing depth was determined by triangulation during the tow and was verified using a time-depth recorder (Benthos). At three
9 10
Table 1. Time periods in which sampling occurred.
Season______Dates ______
Fall 5-28 November 1976
Winter 20 February - 6 March 1977
Spring 18 - 28 May 1977
Summer 19 - 29 August 1977 Figure 1. Location of stations sampled for zooplankton (open circles). 2000
• A2
NEW JERSEY
•Cl •01
• E3
DELAWARE
MARYLAND
VA.
• LI \# L2 \ L4«^i'VR \ ' , 13
Table 2. Stations sampled for zooplankton.
Station Latitude N Longitude W Distance from Depth Shore (km) (m)
A2 39°21.8 1 7 2 ° 31.8T 149 131
B5 39°28.3' 73°02.1T 94 63
Cl 39°22.2f 74°14.9f 10 17
D1 39°04.71 73°53.2f 56 37
E3 38°41.2' 73°32.5 * 112 60
F2 38°44.4f 73°09.21 132 108
J1 38°44.2' 73°00.7’ 141 355
LI 37°31.1f 7 5 ° 18.31 31 22
L2 37°20.1' 74°58.61 66 41
L4 37°08.1f 74°36.8' 105 95
L6 37°04.4’ 74°33.1* 113 322
N3 38° 51.4.’ 73°44.81 83 45
LIBRARY of the VIRGINIA INSTITUTE of ^ M A R IN E SCIENCE 14 stations (A2, B5, and E3) , three sets of replicate tows were made during which the 202 pm mesh net and 505 ym mesh net were towed simul taneously. Both neuston and bongo nets were equipped with General
Oceanics flowmeters to quantify the samples. Temperature and salinity data were collected concurrently with surface plankton samples.
The samples were preserved shipboard with 4% buffered formalin- seawater. After return to the laboratory, displacement and settled volumes were recorded for each total sample. The samples were then sorted to major groups, and each group enumerated and identified to species, whenever possible.
Gammaridean amphipods were identified to species level in all but two cases using Barnard (1969) and Bousfield (1973) as major taxonomic sources. The specimens were then sexed and examined for breeding condition using the same criteria, and terminology similar to that used by Nelson (1980). Total length to 0.25 mm was obtained by straightening the.amphipods with forceps and measuring from the tip of the rostrum to the posterior edge of the telson with an ocular micro meter. Males were determined by the presence of penes papillae or by secondary sex characteristics. Females were characterized by the presence of oostegites. Females possessing setose oostegites were classified as reproductively mature. The loss of eggs and newly- hatched juveniles from brood pouches during collection, sorting, and identification procedures prevented the separate classification of ovigerous females. All unsexed specimens were considered juveniles.
Selected amphipods collected from the benthos and identified by the Department of Invertebrate Ecology during concurrent BLM cruises were also sexed, measured, and examined for breeding condition. Linda 15
Schaffner provided the ampeliscids and corophiids from station B5. 2 Benthic samples were taken with a 0.1 m Smith-Mclntyre grab sampler at the stations indicated in Figure 1. Sampling procedures and laboratory methodologies are described in Boesch (1979). RESULTS
Table 3 lists the species of gammaridean amphipods found in zooplankton collections of the Middle Atlantic Bight. Of the 29 species comprising 18 families identified, nine were found exclusively in surface collections and 15 were found exclusively in subsurface collections. Nine species collected in plankton tows were not found in benthic samples taken in the Middle Atlantic Bight during the same time period (Boesch, 1979). Of these, seven were taken only in neuston collections.
Subsurface Collections
Gammaridean amphipods were primarily found in bongo samples.
Eighty-six percent of the specimens examined were from subsurface samples. Monoculodes edwardsi was the most frequently encountered and abundant gammaridean amphipod. It occurred in 20-40% of the bongo collections during every cruise. Unciola irrorata was the only species to occur more frequently than M. edwardsi during any cruise.
It was found during the spring in 40% of the bongo samples.
The trends of abundance and distribution across the shelf of gammaridean amphipods collected with bongo nets are shown in Figures
2-5. The zones of the continental shelf follow those used by Boesch
(1979) to describe the distribution of benthic organisms during the
BLM study. Amphipods were consistently collected only at central shelf stations N3 and L2 and outer shelf stations B5 and E3. They were scarce in collections made at the shelf break and completely
16 Table 3. Species IJst, frequency of occurrence and maximum abundance in subsurface collections, and presence In surface collections. 3C J3 U. S o* u o e x o w
< O <-«. cn O r>» O socn m H u"> H t
t-4 -F- ; < w C£ Z. 'Z 2 3 S c = j c 2 — 4ft f —4 3 • •Ft j r 3 o Cm i i rz c E nt " o evi sO t ^ t f 00 w 3 c CO 03 r c 3 a c 'J U 3 . •Ft t f c U S- U 1•H 01 03 a z 53 <5 E 2 3 3 'i-4 —t f-4 #-4 —4 w U i W i U w i * • > 3 CO u o 03 7) ZJ 3 3 . a 3 t r - 0) 3 O 3 3 £ *t ’* 03 a U c O t 3 E irt oi ^ —1 C~^4 O 1 i— N O ic— -
JD t i f o o X O a a 3 r* o CJ Cm — C J * r w 93 Q CQ 93 ?3 a X 03 3 5 3 u 3 a Ct •Ft / VQ C/3 _^ xj 3 CQ u £ c C 3 3 —4 -Ft •Ft X j a £ u S3 U c c 3 ra -X — c- « 33 o o C. w c
Total number of subsurface collections made during cruise 17 18
Figures 2. Abundance and species composition of gammaridean amphipods across the shelf in subsurface collections during the fall. At stations B 5 , E 3 , and A2 the abundance was the mean of all four tows made at these stations* The maximum abundance at these stations is shown by the dotted line. Bongo 505 is solid bar and Bongo 202 is cross-hatched bar. Species listed below the station were the species found at that station. NUMBER OF INDIVIDUALS / I OOm - 0 2 0 2 10 10 15 15- 15- 10 0 5 0 5 5- 0 - - - - - NE SHELFINNER M. o d w a rd s i s rd a w d o M. B202 5 0 5 8 8505 L I Cl n 22 55 B202 B505 8202 0 B0 B0 B202 B505 B202 505 B ■ ert Lfsiornis L.fissico serrata B■ a atipes ham A A. .r i ni S. americanum is rn o ric b ru £. t dsi hmatipes .ham A i s rd a w d At.e A. vadorum A. .agassi i iz s s a g a A. U . . U . mericanum am S. topakis k a p tro T fissieornis L. t d ardsi At.edw v N 3 D I ror ta ra irro ado ado CENTRAL r u m U. U. m u r SHELF
L2
ror ta ra irro STATIONS
A. vadorum A. . nguis u g in p L. . ongata a g n io e £. . dwar i s rd a w ed M. ta ra (J. irro A.agassizi . ror ta ra irro U. . obti i i hotbot P. si rd a w ed At. A. v a d o r r urn v o a d A. r ta rro e s B . ubrcornis n r o ric b ru £. 5 8202 05 5 8 8202 5 0 5 8 n 55 202 B B505 OUTER i i iL i i SHELF B5 L 4 E3
. dsi s rd a w d e M. . ta f la n i R. . zi iz s s a g o A. BREAK SHELF 50 8202 05 85 0 8 02 82 505 B A 2 F 2
55 8202 8505 SLOPE JI L6 20
Figure 3. Abundance and species composition of gammaridean amphipods during the winter. Symbols as in Figure 2. NUMBER OF INDIVIDUALS / 100 m 20 10 10 10 15 15 15 0 0 5 0 5 5 O . morc m u n rica o am S. t odwar i s rd a w d o At. t odwar i s rd a w d o At. l 55 B202 B505 5S B2Q2 B50S SHELF
INNER nguis u g in p Cl L I
55 22 55 B202 B505 B202 B505 zi iz s s a g a A . r is k a p T. tro 55 B202 8505 JZL . dsi s rd a w d o M. . radorum A. l N 3 Dl L2 CENTRAL SHELF . rrorata t a r o r ir U. At. od ards od w At.
STATIO STATIO NS . n
. ador m ru o d ra A. t odwar i s rd a w d o At. B. so r ra t t a so r ra B. orum vad A. . rrorata a r o r ir U. . i comis m o ic s fis L. . ubreomis m o rie b ru F. . rrorata t a r o r ir U. t dsi s rd a w d o At i s o tip a m a b A F. ru b ri corn is corn ri b F. ru OUTER 55 B202 8505 JZL B505B SHELF 55 B202 B505 i ! r*i L 4 3 E B5
r 202
At. od w ard i s ard od w At. . zi iz s s a g a A. . oftota t o t f io R. phi ppa p ip id h ip o M SHELF 8REAK 55 B202 8505 5S B202 B50S F A2 2
sp.
SLOPE 5S B202 B50S 55 B202 B505 L 6 J 1 22
Figure 4. Abundance and species composition of gammaridean amphipods during the spring. Symbols as in Figure 2. NUMBER OF INDIVIDUALS / 100 m 20 20 20 30 25 25 io- I 5 15 15 10 10 0 0 0 5 5 5
38.4 . a t a r o r r i U. . gi i y ie ig w P. . i s e tip a m a h A. . i corois o r o ic s fis L. .serata rra e s B. t dsi s rd a w d At.e . mericanum am S. t edwardsi s d r a w d At. e SHELF INNER 55 22 0 B202 8 505 8202 B505 5 8202 05 5 8 C I Q . . o s . L I L
|V
40.9
35 22 8202 5 0 5 8 B202 8305 n. t dsi s rd a w d At.e . rrorata a r o r ir U. .agassi i iz s s a g a A. i s rd a w d At.e .r um ru o d ra A. L 2 .serata rra e s B. J ir ata ra irro (J. I N3 DI CENTRAL .0 8 4 SHELF ft . ubr ns rn o ri c b ru £.
t dwardsi a w ed tt. . a atipes ham A. a t a r o r r i U.
r ta rra e s STATIONS
P. . ador m ru o d ra A. A. kamatipes . irrorata U. t dsi s rd a a t w a d At. r o o r ir U. B. B. rbc s i m r rubnco £ £ r u b r i c o r s r m o i c r b u r £ 55 B202 8505 202 B 5 0 5 8 HELF E SH 50 *02 0 * 03 85 OUTER EL o r c a m serrata i i i i I i i I I
L4 “i B5 E3 I i « I
m o c T"! 1 i 1 i I I I I ii
a
. dsi s rd a w d e M. SHELF BREAK 55 8202 8505 8202 5 0 5 8 F 2 A 2 SLOPE B202 5 0 5 8 8505 B 20 2 20 B 8505 J I L 6 24
Figure 5. Abundance and species composition of gammaridean amphipods during the summer. Symbols as in Figure 2. NUMBER OF INDIVIDUALS / 100 m 20 20 20 10 10 15 15 10 15 0 5 0 0 5 5 . i s e tip a m a H A. t r ayi y a n ra St. . i m u n a ric o m a S. 55 22 55 8202 B505 8202 8505 SHELF 55 B202 8505 INNER C! I L2 IL 40.3
55 22 55 8202 8505 B202 8505 t edwar i s rd a w d e St. . ror ta ra irro U. B. s e r r o t a t o r r e s B. . um ru o d a v A. 1 N3 01 CENTRAL HELF SH sorata rra o .s B . nguis u g in p O. . rrorata t a r o r ir U. dsi s rd a w d o M
STATIONS
. nguis u g in p 0. sorr us tu ra r o s H £ . a t a r o r r i U. . um ru o d a v A. . zi iz s s a g a A. serrata a r r e s . B . is r m o i c r b u r F. t dsi s rd a w d St.o dsi s rd a w d o U is n r a c t r & u r 55 2 0 2 8 8505 55 8202 8505 5 8 202 5 B50 OUTER SHELF n i [r** B5 L4 E 3
h
. 0 B202 505 8 8505 8 8 202 8505 BREAK HELF SH A2 F 2 55 8202 8505 B202 5 0 5 8 SLOPE L6 Jl 26
absent from samples taken at slope stations. Amphipods were
frequently, but inconsistently collected at inner shelf stations and
the central shelf station Dl.
Seasonally, there was a noticeable increase in abundance during
the spring cruise at the inner and central shelf stations. The
highest abundance of gammaridean amphipods in any sample occurred at
Cl where a high density of juvenile Monoculodes edwardsi was collected
by the Bongo 202.
Latitudinal differences in abundance were apparent only at L4.
Amphipods were consistently collected at the outer shelf stations to
the north but they were virtually absent from L4. In this respect, L4
more closely resembled the deeper stations where amphipods were rarely
collected.
Species of gammaridean amphipods found in subsurface tows were
generally found in more than one shelf zone. All but one species
collected at central shelf stations were also collected at either
inner or outer shelf stations. Only two species, Monoculodes edwardsi
and Argissa hamatipes, were found in all three zones. Tiron tropakis
was the only species unique to the central shelf region and it was
found only in the southern transect. Two of the four species taken at
the shelf break stations were also found at the outer shelf stations,
including the ubiquitous M. edwardsi. Rhachotropis inflata and
Melphidippa sp. A were limited to the shelf break.
Neuston Collections
The 15 species found in surface collections fell into three major
divisions (Table 4). Group A were ubiquitous and abundant components 27 Table 4. Occurrence of gammaridean amphipods in neuston collections.
Species Group Station Cruise
Also found in subsurface collections
A. Found in benthic collections
Liljeborgia fissicornis L2 Fall Ampelisca agassizi L2 Winter Monoculodes edwardsi Cl Winter LI Spring Unciola irrorata L2 Spring Ampelisca vadorum L2 Spring
B. Not found in the benthos
Microprotopus raneyi LI, Cl Summer
II, Only found in neuston collections from water column but also collected in the benthos
Corophium ascherusicum L4 Fall Cl Summer Trichophoxus epistomus LI Summer
III, Only found in neuston collections
A. Species associated with warm water
Sunamphitoe pelagica (Bousfield, 1973) L4 Spring Synopia ultramarina (Barnard, 1972b) L4, J1 Summer J1 Spring Hyale sp. L4 Summer
B. Species with arctic-boreal affinities
Calliopius laeviusculus (Steele and B 5 , A2 Spring Steele, 1973) Gammarellus angulosus (Bousfield, A2 Spring 1973)
C. Species with temperate affinities
Ampithoe longimana (Bousfield, 1973) LI Summer Stenothoe minuta (Bousfield, 1973) L4 Fall
SL Reference for distribution in parenthesis following species.
Distribution unknown; placed with southern group because of associ ated species and hydrographic events which will be discussed later. The genus Hyale is listed as cosmopolitan, especially tropical by Barnard, 1969. 28
of the benthos except Monoculodes edwardsi. Monoculodes edwardsi
occurred widely across the shelf but only in low numbers (Boesch,
1979). Only one individual of each of these species was found in the
neuston at inner and central shelf stations during the fall, winter,
and spring. Group II was mixed. Trichophoxus (=Paraphoxus) epistomus was ubiquitous and abundant in the benthos (Bowen, et al., 1979) but
Corophium ascherusicum was rarely encountered in the benthos. Groups
IIIA and B were at the limits of their ranges. These species were
found only in the neuston at outer shelf and deeper stations during
the spring and summer. Microprotopus raneyi and Ampithoe longimana
were the most frequently occurring species. They were the only
species which occurred in more than two of the 75 neuston samples
taken during any single cruise and comprised 80% of all neuston
specimens.
Individual Species
The following are results for each species identified from the
collections. Their distribution and results from previous water
column studies during which they were collected are included.
Family Ampeliscidae
Ampelisca agassizi (Judd) 1896
Ampelisca agassizi is found from shallow inshore waters to deep waters from southern Nova Scotia to the Caribbean Sea and along the
Pacific coast of North America (Mills, 1967a). It is a tubicolous
infaunal species which probably leaves its tube to swim freely while mating. The young settle in areas with a reduced adult population,
usually within a few centimeters of their maternal tubes (Sheldon
Pratt, personal communication). It has a terminal pelagic adult male
mating stage. 29
A. agassizi was an abundant but substrate specific species of the benthos of the Middle Atlantic Bight (Bowen, et al;, 1979) with greatest densities along the outer shelf. However, it occurred in low numbers and
infrequently in the water column (Table 5) and displayed no apparent
relationship with its occurrence in the benthos. It was most abundant
in the spring plankton collections at L 2, an area in which winter and summer benthic samples suggested it was a common species. Despite being more abundant in the benthos at F2 and B5 than at A2, it was only collected at A2 in fall plankton samples. Only single individuals were found in
the remaining samples and it is difficult to relate those isolated occur rences to the benthic distribution of A. agassizi. The planktonic popu lation at a station may be influenced by benthic populations from a much broader area within which there were considerable variations in the benthic communities and their constituent populations (D. Boesch, personal commu nication) .
All but one of the specimens of A. agassizi taken in plankton tows were mature adults and those were predominantly males. The frequency distribution of sizes and sexes is given in Figure 6. A comparison of the size and sex frequencies of planktonic and benthic specimens at A2 in the fall shows the dominance of mature males in the plankton samples.
At F2 where A. agassizi was common in the benthos but not collected in the plankton, the specimens were mostly immature (only five females and one male were reproductively mature out of a total of 66 specimens) and smaller than those found in the water column at A2. During the winter sampling period, specimens found at L2 in the benthos were larger and more mature than those found farther north at F2 (Figure 6 d,e). No benthic samples were taken at L2 in the spring but the water column samples were again dominated by mature males and some females. A. agassizi was Table 5. Occurrence of Ampelisca agassizi in the Middle Atlantic Bight in the water column (this study) and in the benthos at stations also sampled for zooplankton (from Boesch, 1979). cj rH o 3 u - a O J-«w 3 I u < u o 3 3 O 3 o CQ cn cQ =«ts /-N S-' N O rH 14-1 t 3 . O T3 cn •H HH •H rH -O X H O £-• E cn CJ H • H ■H 4-» 4-t 4-i H 4-1 4-i 3 3 5-i o 3 c CQ co o cd CL CJ £ 3 5-1 OJ 3 5-t o o 5 CQ £ CO C O 3 3 a) 3 O 3 CO 3 3 c 3 h tH »H rH 5H •H IH pH x iw •H E x •H X c X JH H 4-1 £ 5-i 3 3 3 § 3 3 3 £ h 3 g 3 3 3 3 N PC O 3 3 3 3 3 3 3 3 3 X £ O O 3 3 £ N N r—I CN CN st cnm H O O O O O «H O O m CN m o o o © O O iH CMCN o o o o o o o o O < P3 3 0 a p n r < o o o o n c cn m n i
n cn on o o o O O N O CN O N C o CN n c n i o o CN ON nin pa oa -J n c -H 00 . a 5-i c • • • • n c
cn o o o n o n i n pa pa cn 3 =Sa O x Q H • cn i-l U 3 CQ 3 CO 3 3 CQ rH rH E 3 h T P O CN O o \P NT O T N cn ON NT C O C 0 C Ol CO l O CO 05 CM ON -CT no o o o cn o ON ON lO tH CN rH r—5 H - 1—4 --T rH H c i in co - n ’rHN'n M O O P v ' rH O - r on rH O O Mature males determined by presence of elongated second antennae. Mature females possessed setose oostegites. 31 Figure 6 . Size and sex frequencies of Ampelisca agassizi. n = number of individuals in sample. a) Water column - fall - A2; b) Benthos - fall - A2; c) Benthos - fall - F2; d) Benthos - winter L2; e) Benthos - winter - F2; f) Plankton - spring - L2 FREQUENCY (%) - 0 3 “ 0 4 - 0 3 - 0 5 - 0 3 - 0 2 30 - 0 3 - 0 2 6 - 0 3 “ 0 2 -i 0 4 - 0 2 - 0 2 0 1 20-1 10 10 10 10 10 O 0 0 0 0 0 -7 - - - - - L2 Fall A 2 .0 Spring Winter n= I 5 n 60 = Benthos Plankton ae Column Water - 11 - n . 3.0 2.0 EGH (mm) LENGTH . 50 6.0 5.0 4.0 I I I I I H MS****n bsa i 2 Winter F2 Fall F 2 7.0 2 Fall A2 MALE | ~ | n 134 = Benthos Benthos n= 66 Benthos = 12n= 8.0 RE PRODUCTIVELY AUE FEMALE MATURE IMMATURE FEMALE 9.0 33 virtually absent from plankton samples during the summer, the reported breeding season (Bousfield, 1973). Only one individual was collected at the surface. A previous report that A. agassizi was taken at the surface of Narragansett Bay, Rhode Island (Kunkel, 1918) has since been questioned (Mills, 1967a). Ampelisca vadorum Mills 1963 The distribution of Ampelisca vadorum extends from southwestern Gulf of St. Lawrence to the Gulf coast of Florida (Mills, 1967b, Bousfield, 1973). It is a tubicolous infaunal species which becomes free-swimming while breeding and has a terminal pelagic adult male mating stage. A. vadorum was found in the benthos throughout the study area (Boesch, 1979), and was particularly abundant at the outer shelf stations in the B and E areas. There was some agreement between the benthic and planktonic distribution of abundances of A. vadorum (Table 6). The stations where it was collected in the water column were the same benthic stations (among the ones which were also sampled for zooplankton) at which it was most common— B5, E3, and L2. A. vadorum was collected most frequently in the plankton during the fall cruise. Reproductively mature adults dominated the collections but immature specimens were occasionally found. This is in contrast to the virtual absence from plankton collection of immature A. vadorum. The high number of immature specimens from L2 in the fall (see Table 6) may be the result of contamination from the net touching the bottom during the tow. A comparison of benthic and planktonic size and sex frequencies is presented in Figure 7. During the fall at B5 the distribution of sizes of individuals found in plankton collections was similar to that found in the benthos. It was a loosely bimodal distribution in which 34 y CO —b 0> CB /-I ib E >1 '■ 3 y *o *4 a 3 re y *4 £ re 03 y o e o o o o N —4 c o \C O G *4 co re y ib £ CO *4 •3 y y re V4 m n- e • 09 u re y Figure 7. Size and sex frequencies of Ampelisca vadorum. Symbols as in Figure 6 . a) B5 -Water Column - fall; b) B5 - Benthos - fall; c) E3 - Benthos - fall, d) B5 - Water Column - winter; e) B5 - Benthos - winter; f) L2 - Benthos - winter FREQUENCY (%) - 0 4 - 0 3 0 2 - 0 3 - 0 3 - 0 4 0 2 0 2 0 2 0 1 0 1 0 1 10 0 0 0 ------] 1.0 n 15= 5 Fall B5 5 Winter B5 a e ColumnWater ae Column Water 2.0 . 405.0 4.0 3.0 n=20 l ... EGH (mm)LENGTH . 1 —— L— — ... . eo __ 7.0 3 B & 1 sSJ 8.0 Benthos 2 Winter L2 Winter B5 Benthos = 10 n = 5 Fall B5 = 53 =n Benthos n= 81n= n = 68 Benthos 1 9.0 9.0 -- □ MALE □ 3 Fall E3 10.0 IMMATURE REPRODUCTIVE LY AUE FEMALE MATURE FEMALE 1.0 37 immature specimens were concentrated in the 1.5 to 3.5 mm size classes trailing off to 4.5 mm in the benthos and ranged from 2.0 - 3.0 mm in the plankton. There was a second, more dispersed size group of adults which at lengths of greater than 7.0 mm were reproductively mature in the benthos. In the plankton collections, the second group consisted entirely of reproductively mature individuals larger than 7.0 mm. The size distribution at E3 during the fall appeared similar to that found at B5 in that immature specimens were grouped from 1.0 to 3.0 mm trailing off to 4.5 mm and adults were spread out from 5.0 to 11.0 mm with reproductively mature individuals having lengths greater than 6.5 mm. However, despite seemingly similar composition of benthic populations at B5 and E3, A. vadorum was not collected in the water column at E3 during the fall cruise. The occurrence of reproductive adults in the water column at L2, B5, and E3 during the winter (Figure 7d) is surprising in light of Mills (1967b) findings. He suggested a minimum temperature of 8°C for breeding to occur in A. vadorum. Bottom temperatures during February and March were below 8°C at these stations. Mills also found no evidence that adults of A. vadorum survive after breeding. There was also a lack of breeding adults in the benthos at B5 (Figure 7e); only one individual exceeded 6.0 mm. Reproductive adults were found further south at L2 (Figure 7f). A. vadorum has been reported in the water column by other inves tigators. Whitely (1948) listed A. spinipes from his collection of planktonic crustaceans from Georges Bank, but Mills (1963) reported that A. spinipes has not been found in North American waters and was often confused with A. vadorum. Williams and Bynum (1972) collected 38 A. vadorum throughout the year in the nocturnal plankton on the coast of North Carolina. Thomas and Jelley (1972) found the ampeliscid in emergence traps set at night off Prince Edward Island. Ampelisca verrilli Hills 1967 Ampelisca verrilli is found on the east coast of North America from the southside of Cape Cod to North Carolina (Mills, 1967a) and along the Mississippi coast (Farrell, 1970). It is an abundant amphi- pod in the nocturnal plankton of Bogue Sound, North Carolina (Fox and Bynum, 1975). Its abundance in the benthos of the Middle Atlantic Bight is low. It was never taken in numbers greater than 0.260 indi- 2 viduals/0.1 m (Boesch, 1979). Two females (length 15.2, 16.7 mm; oostegites setose) were collected in a subsurface plankton tow at LI during the summer. Byblis serrata Smith 1873 Bybilis serrata is distributed from the southside of Cape Cod to Georgia (Weston, 1979). It is a tubicolous infaunal species with a terminal pelagic male mating stage. J5. serrata has been taken in plankton tows from the surface to 11 fathoms in Fishers Island Sound (Kunkel, 1918). _B. serrata occurred widely across the shelf on the bottom and was caught in highest numbers in the B area (Boesch, 1979). It was found at B5 during all four plankton cruises (Table 7). It occurred in a greater number of samples during the spring and was least frequently collected during the winter cruise. Maximum abundance was 3.7 3 individuals/100 m at B5 in B505 collections during the spring and summer. The specimens were mostly reproductive adults of both sexes in accordance with the findings for the other ampeliscids, except for the high number of immature specimens at B5 in the spring. Table 7. Occurrence of Byblis serrata In the Middle Atlantic Bight in the water column (this study) and in the benthos at stations also sampled for zooplankton (from Boesch, 1979). N “ © •“* - O r-4 m © O in m © -4 © © r-i rsi 3 ea S3 1-4 cni O O © © © O ® ® N N « N n H H O© © © O © iH © m H ' O O o n i N O S N ^ * 4 ^ W n n07 S' O in ' n O n O m m in n i-^ © © © a 4 ^ H s o *-4 ^ n i n c *•■4 c o o <3 5 v ea a o S Q s 5 - S u B 0) * • * • © ^■4 o 1*4 ^ O C m c o • JS w i 4 - ■4* s . o ■ a • £ o a ■ 4 ^ o s j j y u - 4 4 a u 93 J 4 4*1 0 s y * s a a 3 W « 07 03 CO CO cc u © w C 3 0 y s > a. y e u V V 40 44 3 07 07CD e O OB ) . ) ) * — ) ) ) y a D C • « ai M C * * O V © 2 2 3 0 © J 4 4 4 u 4 4 « C 07 o U 07 CO S 07 U 07 c CQ C e * *■4 4 ^ e b W H • T-4 4 4 £ Q O O & U S »o co c o o» 07 U 3 o © c w 0 / O 0 v O m c ^ o 0 5 ^■4 a a < 0 < M C m c ^ O '• • « • n c tn co © D s f-4 tn rsi 4-4 n < 0.122 0.122 0.0 0.122 1.0402.018 1.749 1.402 0.201 0.513 1.040 1.570 39 1.378 2.253 0.189 0.189 40 The size composition of planktonic collections at B5 (Figure 8) once again shows the dominance of mature adults in the water column. There were more females at B5 than was found for A. agassizi and A. vadorum. During the summer, females outnumbered males. At L2, the collections were mostly males. There was some agreement between the composition of benthic and planktonic samples throughout the year though differences in sample sizes prevent any direct comparisons. Reproductive individuals were found year-round despite reports that breeding occurs between June and October (Mills, 1971). Family Ampithoidae Ampithoe longimana (Smith) 1873 Ampithoe longimana is a nestler, often found on algae and sea grass. It is a shallow water species, and has been reported from the Gulf of St. Lawrence to Florida (Bousfield, 1973), along the Texas coast (McKinney, 1977), and the southern California coast (Barnard, 1965). The habits of this species were described by Holmes (1901) who noted it had a marked disinclination for continuous swimming. He suggested the occurrence of A. longimana in surface net tows in the Woods Hole, Massachusetts vicinity was a result of tidal currents carrying it away from shore. It has since been found in the water column by Kunkel (1918) near Connecticut and Woods Hole, and by Williams and Bynum (1972) who collected it year round in nocturnal plankton tows in Bogue Sound, North Carolina. They found A. longimana in significantly greater numbers on nights with a full moon than nights with a new moon. A. longimana was collected in all eight neuston tows at LI during the summer cruise. It was absent from benthic samples, but no grabs 41 Figure 8. Size and sex frequencies of Byblis serrata at B5 during each of the cruises. Water column collections are the upper histogram and benthic collections are the lower histogram. Symbols as in Figure 6. FREQUENCY (%) ae ColumnWater Fall B 5 n - 36 - n B 5 Summer ae ColumnWater ae Column Water Winter 5 n= 16 B5 - n B-5=8 . 40 . 80 10.0 8.0 6.0 4.0 2.0 EGH (mm) LENGTH ae ColumnWater Spring 5 n 19= B5 A- F S S & W I2J0 B5 n= 112 n= B5 Benthos Benthos Benthos Fall Winter 5 n =B5 64 5 n=83 B5 Benthos Spring 5 n= B566 Summer 14.0 □ IMMATURE AUE FEMALE MATURE IVELY REPRODUCT FEMALE MALE 43 were taken at LI. Figure 9 shows its abundance throughout the 24-hour cycle of neuston collections at LI. Greater numbers occurred in samples taken during daylight hours. It was absent from subsurface tows which were made at 2200 DST at LI. The length-frequencies and composition of A. longimana are pre sented in Figure 10. There was no significant difference between 2 numbers of males and females collected in the plankton (x - 1«1» df = 1, a - .05). Juveniles were also collected. Eighty-one percent of the females collected at LI had setose oostegites or were ovigerous. For comparative purposes data from plankton collections of A. longimana in the lower Chesapeake Bay are also presented. These collections were made a year later in August 1978 using smaller mesh \ sizes (333 p m on neuston and bongo frames as well as 202 pm on bongos). Though the differences in sampling methodology precludes statistical comparisons, the apparent shift towards a smaller size of the Bay population, appears to be a real phenomenon, not a sampling artifact. The size at which sexes can be distinguished is smaller in the Chesapeake Bay population. A. longimana was collected in both neuston and bongo samples. Fifty percent of the females from neuston collections and 39% of females from all Bay collections were repro- ductively mature. Sunamphltoe pelagica (Milne - Edwards) 1830 Sunamphitoe pelagica is an epibiotic-pelagic gammaridean amphipod known from w a r m and temperate waters of the North Atlantic, from the Gulf Stream north to Cape Cod and Sable Island (Bousfield, 1973). It is also found on the. European coast north to western Britain (Bous field, 1973). Shoemaker (1945a) collected j3. pelagica near the 44 Figure 9. Diel cycle of Ampithoe longimana in the neuston at station LI during summer, 1977. 70.0 O (£w 00! O /=#) 30NvoNnav Q O O r Q CM O O _ o _ O CM CD o r CM CM •o to UJ to UJ to TIME OF DAY ( DST) Figure 10. Occurrence of Ampithoe longimana in the water column station LI (upper histogram) and' from the Chesapeake Bay (lower histogram). Symbols as in Figure 6. FREQUENCY (%) i - 0 2 0 2 10 15 10- - 5 1 - 5 0 0 — 0 0 — 5 - 5 - - - BL M BL r 8 9 n= I Summer LI HSPAE BAY CHESAPEAKE Summer FEMALE MALE MMATURE 48 surface in waters off Bermuda and Bousfield (1973) reports its occurrence on Sargassum drifting in from the Gulf Stream, at or near the surface. Two specimens, an immature male (length = 5.Q mm) and female (length = 3.75 mm), were collected in a neuston tow at L4 during the spring. It was not reported from the benthos of the area (Boesch, 1979) . Family Aoridae Leptocheirus pinquis (Stimpson) 1853 Leptocheirus pinquis is a tubicolous infaunal species distri buted from Labrador south to Virginia (Bousfield, 1973), and North Carolina (Fox and Bynum, 1975). It has been reported in plankton samples by Kunkel (1918) and Fox and Bynum (1975). _L. pinquis was a common widespread amphipod in the benthos (Boesch, 1979). Only one specimen, a female (length = 8.0 mm), was captured in a bongo tow at B5 during the fall. Family Argissidae Argissa hamatipes (Norman) 1869 Argissa hamatipes is an amphi-Atlantic species distributed on the western North Atlantic from Labrador south to Cape Cod Bay (Bousfield, 1973), North Carolina (Fox and Bynum, 1975) and along the Texas coast (McKinney, 1977). Bousfield (1973) described it as an epibenthic and pelagic species with ovigerous females appearing in late winter and early spring. The males have a terminal dimorphic stage. A. hamatipes has been identified from plankton tows made on Georges Bank (Whiteley, 1948) and in Bogue Sound, North Carolina (Williams and Bynum, 1972). 49 A. hamatipes was f o u n d in low numbers throughout the Middle Atlantic Bight in the benthos (Boesch, 1979). Among the stations at which zooplankton was also sampled, it was found only at E3 in the fall and B 5 , Dl, and E3 in the summer. Its abundance was never greater than 0.260 individuals/0.1 m at those stations. The occur rence of A. hamatipes in the water column was seemingly unrelated to its benthic occurrence (Table 8 ) . It was collected during all four cruises and while it was not consistently present at any one station, it was generally collected at inner and central shelf stations. The relationship of the appearance of A. hamatipes in the water column to breeding activity was unclear. All but one of the males collected were terminal pelagic males but some of the females, including some which were collected during the spring when A. hamatipes occurred most frequently and in highest number, possessed immature oostegites. At least one female (Ll, summer cruise) was ovigerous. No juveniles were collected. Family Calliopiidae Calliopius laeviuscuius (Kroyer) 1838 Calliopius laeviuscuius is an arctic-boreal species distributed from Hudson Bay to Long Island Sound on the Atlantic coast to North America and the Pacific coast from Alaska south to Washington State (Bousfield, 1973; Steele and Steele, 1973). It is common on rocky shores where it attaches itself to algae at the water’s edge. In late summer and early autumn it swarms at the surface and may be found in the plankton far out at sea (Steele and Steele, 1973). C. laeviuscu- lus has frequently been reported from plankton samples (Kunkel, 1918; Fish, 1925; Frost, 1936; Whiteley, 1948; Dunbar, 1954). Bousfield (1951) found it almost invariably in surface tows. Table 8. Occurrence of Argissa hamatipes in the Middle Atlantic Bight in the water column. CO OS 3 •H 3 N 00 3 03 •H o HrH C_> •H CO M ■u H 3 O 3 3 o a CU 3 ro a) 3 3 03 CO 3 3 H =tfe r—1 CM •H H co .2^1 4-) 4-J V-t 3 O 3 03 3 3 o. cu a 3 CO CO co CL) O o o rH P— rH rH tH o p 1—1 P s pH s .6 cu a 03 cu cti CU H 3 cu g e 03 CU 03 g m ' o o o u 0 U rH rH rH CM CO o m co uo m rH O rH O O O O rH o H tH rH OCO CO o P X o Q PQPQ 03 m 1 1 1 . . . . *. * • • . . . 3 5 CO o X o m m O m CO CN p o •H o PQ QP PQ PQ PQ H 3 U 3 . «. m . o o o o o rH m t O in © o CO •H CO CM O O m H U 0 6 Q. 3 3 . • a 1 in PQ • 3 0 1 m c in m 0 O o in CO O tH rH tH o Occurred in 1 of 4 tows. Mean abundance = 0.1 individuals/100 a a a 3 0 Occurred in 2 of 4 tows. Mean abundance =0.1 individuals/100 Occurred in 2 of 4 tows. Mean abundance =0.1 individuals/100 51 Spring neuston collections included C_. laeviusculus at the northermost stations of A2 and B5. It was not found in any subsurface plankton or benthic samples collected in the area. Family Corophiidae Corophium ascherusicum Costa 1857 The cosmopolitan species Corophium ascherusicum is found in the western North Atlantic in warm temperate coastal waters from central Maine (Bousfield, 1973) to Texas (McKinney, 1977). It builds tubes on hard substrates and is more commonly found in estuaries than the open sea (Barnard, 1961). Williams and Bynum (1972) collected C^ ascherusicum in nocturnal surface plankton samples. Two specimens were found in the surface plankton of the study area. A male (length - 2.2 mm) was taken at L4 during the fall and a female (length = 2.2 mm) with setose oostegites was taken in the summer at Cl. C_. ascherusicum was also identified from benthic samples collected at E2 during the spring (Boesch, 1979). Erichthonius rubricornis (Stimpson) 1853 Erichthonius rubricornis is an amphi-Atlantic species which has been reported along the American east coast from Labrador and Gulf of St. Lawrence south to Long Island Sound (Bousfield, 1973) and recently as far south as Cape Hatteras (Dickinson, et_ aA., 1980). This corophiid is an epifaunal tubicolous species usually found in colonies attached to rocks or tubes of other species (Bousfield, 1973). It lacks a terminal pelagic male mating stage. The collection of JE. rubricornis in the water column reflected its occurrence in the benthos at the northern stations (Table 9). It was most frequently found in the water column at B5 and E3, areas in Table 9. Occurrence of Erichthonius rubrlcornla In the Middle Atlantic Bight in the water column (this study) and in the benthos (from Boesch, 1979). i * e m 4k 1-4 4k w H j us < T3 l"4 s o o .■fa U-4 X H VI s U3 •rt c H cc u O 3 at 3 V C3 a U 3 e O CQ s S V 0 CL V 3 = V a 30 s u CU CQ IS 19 O JS W| •W H • <4-1 £ £ LL r £ i*H f-i fa. . s ( 3 OS W 3 u u O W to 5 m CQ o X S CU a 3 s CQ V a> V g a 0) CQ sa CQ c « ; Si in r» Si n -. r- m in m in in in in o o o a a sc ea ca I I I• n n c O n m m c o o d cn O © - S' -9 m n o i o cm c m in r*i m til a S3 ca 33 o un os m cm cm m M O M CM s CM r r-s O m O CM n m p. o ->3 in m o C O p. O m n.en p»in o 9 ■ -9 •9 ■? < m c r~>~r -4 m c m o c o o o o o c n i o 33 S3 30 3 3 S 3 3 3 S s a m I t t t i I i mm i p m c m c m c i in in o nrt r n n m c n m c cm m o is m m n p m c m c in O d 03 N fl o o o o o o o o m in m n i S3 33 3 3 S 3 S cm m c Oniso CM M3 cn O nM O M3 in o ^ M3 -3 CM m P-s M C M C o o o d au t j ^ to cau < 2 n n i n h o h n o n a i < 56.620 10.591 1.449 0.0 52 53 which it was most abundant in the benthos. However, it was never collected in plankton tows at L4 despite its presence in the benthos at that station. There was an increase in emergence activity during the spring and a much lower amount of activity during the summer. The size classes of benthic and plankton samples taken at B5 during the fall, winter, and spring are shown in Figure 11. Plankton samples lacked individuals of the smallest size classes and immature specimens. There appeared to be no selective migration into the water column among any segment of the mature benthic population. Unciola irrorata Say 1818 Unciola irrorata is reported from the Gulf of St. Lawrence to South Carolina (Shoemaker, 1945b) and along the Texas coast (McKinney, 1977). U_. irrorata often occupies tubes of other species and may be a facultative tube dweller (Schaffner, 1980). It has no male pelagic terminal mating stage. Williams and Bynum (1972) found IJ. irrorata in nocturnal surface plankton tows from North Carolina with a peak in abundance in December and January. It has also been reported in the water column of the Chesapeake Bay (Grant and Olney, 1979). IJ. irrorata was widely distributed throughout the sampling area in the benthos with greatest densities occurring along the outer shelf. It also appeared regularly in the plankton along the outer and central shelf areas and occasionally in the shallow inner shelf region (Table 10). It was most common in the water column during the spring, occurring at six of twelve stations sampled. The composition of the population of U. irrorata collected in the water column at B5 appeared similar to the benthic population during the fall and winter (Figure 12). Spring plankton samples contained 54 Figure 11. Size and sex frequencies of Erichthonius rubricornis at B5 in the benthos and plankton during the fall, winter, and spring. Symbols as in Figure 6. FREQUENCY (%) Fall 8 5 5 Spring B5 n 179 = 5 Winter B5 25 n= ae ColumnWater ae ColumnWater Benthos Winter B5 ns 43 ns ae Column Water n 179 = I “ . 30 . 50 6.0 5.0 4.0 3.0 2.0 ------ ssss 1 ------, ----- LENGTH (mm) 8253 n ------" r 5 Fall B5 5 Spring B5 Benthos n 161= 80 n = Benthos . . 9.0 8.0 7.0 IMMATURE MALE REPRODUCT IVELY AUE FEMALE MATURE 56 Table 10. Occurrence of Unciola lrroraca in Che Middle Atlantic Bight in the water column (this study) and in the benthos at stations also sampled for plankton (from Boesch, 1979). Occurrence in che Water Column Cruise S Cation Gear Total 9 # Tow8 with Abundance (#/100 a3) Tocal # of Soeclaens 2 Maturea Size of Tows 0. irrorata Mean or Maximum Male Female Immature Female Range all Tows (mn) Fall L2 8505 1 3.1 3.1 7 7 .1 0 2.5-4.7 N3 B505 .1 1 1.5 1.5 0 0 1 4.5 33 B505 4 3 0.5 1.4 2 4 0 50 3.0-6.7 3202 4 1 0.2 0.9 1 3 0 33 3.0-7.2 E3 3505 4 1 0.1 0.3 0 0 1 3.7 Winter 33 3505 4 0.5 1.4 10 3 0 3.7-6.5 3202 4 3 1.6 4.9 5 3 1 0 5.5 E3 3505 4 0.3 0.7 2 4 0 0 4.2-6.5 3202 4 1 0.1 0.3 1 0 0 6.0 M3 3505 1 1 1.1 1.1 4 2 0 0 4.7-6.0 B202 1 1. 0.3 0.3 1 1 0 0 6.7-8.0 Spring LI B505 1 0.7 0.7 0 2 0 0 3.5-3.7 B202 1 1 3.8 3.8 2 1 0 0 4.5-5.7 L2 B505 1 1 2.5 2.5 7 5 0 0 3.5-6.0 B202 1 1 1.6 1.6 3 3 1 0 3.2-5.0 N505 3 1 0.1 0.5 0 1 0 0 6.0 E3 B505 4 1 0.3 1.1 1 0 0 4.5 3202 4 2 3.4 7.5 5 6 0 33 4.0-8.0 N3 3505 1 1 40.7 40.7 34 64 3 55 3.2-11.5 3202 1 1 14.4 14.4 24 3:8 0 50 4.7-12.2 D1 3202 1 .1 0.3 0.3 1 0 0 9.5 35 B505 4 4 1.3 2.3 2 13 0 77 5.0-10.7 B202 4 0.9 3.1 1 14 0 93 5.6-12.2 Sumner 11 3505 1 1 0.3 0.3 1 0 0 6.0 12 B505 1 1 1.3 1.8 3 4 0 25 4.2-7.7 S3 3505 1 1 .3.1 3.1 0 5 0 0 4.2-5.2 E3 B505 4 2.1 7.5 4 4 0 0 4.5-6.2 3202 4 1 0.5 2.1 0 2 0 0 4.7-5.0 3 Mature females possessed setose oostegites. Occurrence in che 3encho3 Station Geometric Mean Density 0)/0.1 n^) Fail Winter Spring Summer A2 0.260 0.260 0.0 0.122 35 21.236 148.320 34.338 56.071 01 0.0 0.122 1.828 2.053 S3 1.884 13.336 15.363 23.467 F2 4.386 2.141 2.344 11.005 12 1.632 10.940 14 6.341 13.358 16 0.0 0.316 57 larger IJ. irrorata than specimens found in the benthos, mostly breeding females (85% of the plankton specimens). The size and sex composition of collections of IJ. irrorata in the water column varied among stations (Figure 13). The highest abundance of IJ. irrorata in the water column was found at N3 during the spring. The largest (lengths greater than 8.0 mm) were almost all reproduc- tively mature females, as at B5 (Figure 12). However, the collections at N3 contained a higher percentage of males (36%) than at B5 (10%) and were not dominated numerically by the larger specimens. Specimens collected at L2 (Figure 13) were generally smaller and less mature than those found at the northern transects during the spring. The collections of _U. irrorata at E3 and L2 coincided with increases in abundance in the benthos at these stations. It was absent from collections at L4 and F2 despite being present in the benthos but there was a general lack of gammaridean amphipods in subsurface collections during all four cruises at these two sites. Movement of IJ. irrorata into the water column from the sediment appears to include all but the smallest segment of the benthic population and does not seem to be simply a result of breeding activity. The high percentage of reproductively mature females collected in the water column during the spring was not repeated in the fall and summer collections despite an equal or higher percentage of mature females in the benthos (Table 11). There was no increase in number of benthic organisms migrating into the water column from the winter when breeding activity was the lowest to the spring when it increased again. In general the occurrence of IJ. irrorata in the plankton may be the result of a constant flux of a low number of 58 Figure 12. Size and sex frequencies of Unciola irrorata at station B5 during the fall, winter, and spring in the water column and benthos. Symbols as in Figure 6. REPROOUCTIVELY MATURE FEMALE B 5 Fall FEMALE 3 0 W ater Column m a l e 20 n= 10 10 IMMATURE 0 10 8 5 Fall Benthos 20 n = 171 40 B 5 W inter Water Columm 30 n = 22 20 10 0 turoF 10 8 5 Winter Benthos 20 n = 128 20 B5 Spring Water Column 10 n = 3 0 0 10 B 5 Spring 20 Water Column n = 3 0 “ I "I------'—T------1------1------r~ 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 LENGTH (mm) 60 Figure 13. Size and sex frequencies of Unciola irrorata in the water column at L2 and N3 during the spring. Symbols as in Figure 6. RE PRODUCTIVELY MATURE FEMALE < UJ u. ui 2 -i O O O O O M O CM CO -J -J . Q UJ — 1 O * o M — CM ro (%) AO N 3 0 0 3MJ 0 N0 3 AO (%) CO Q.O o o o O CM rO CM CD LENGTH (mm) 62 Table 11. Occurrence of mature females of the species Unciola irrorata in the plankton at B5. Cruise % Mature Females % Mature Females % Benthic Pop. in Benthos in Plankton in Plankton Fall 47 43 0.01 Winter 11 0 0.007 Spring 39 83 0.007 Summer 39 _ 63 organisms emerging from the sediment. In areas where its density is relatively low, it has less chance of being included in a plankton collection than in areas where its density is greater. Family Eusiridae Rhachotropis inflata (G. 0. Sars) 1882 Rhachotropis inflata is an epibenthic species known from the North Atlantic and northeast Pacific Oceans (Barnard, 1971). It occurs in the western Atlantic from the Arctic to the Middle Atlantic Bight (Bowen, et al., 1979) but it is rare south of Cape Cod (Dickinson, e_t al. , 1980). Eusirids are known to be one of the few predaceous groups of gammaridean amphipods (Barnard, 1969). R. inflata has previously been reported from plankton collections made by Frost (1936) off Newfoundland. Dunbar (1954) also found it in plankton samples taken in Ungava Bay but thought it was taken in the benthos when the net touched bottom. R. inflata occurred infrequently and in low numbers in benthic samples taken in the Middle Atlantic Bight (Boesch, 1979). It was found at B5 in the fall and G6 and 13 in the winter. _R. inflata was collected in subsurface plankton tows at F2 in the fall and A2 in the winter. Females and males, lengths ranging from 2.5 to 4.7 mm (n = 22), were found. Both females examined in the fall had setose oostegites. The females from the winter collections were in various stages of maturity including ovigerous females. Family Gammaridae Eriopisa elongata (Bruzelius) 1859 Eriopisa elongata is reported from the North Atlantic along the European coast and Iceland, and from the Pacific off Oregon (Barnard, 64 1971). In the western North Atlantic, Dickinson, e t _ al. (1980) reported it in low numbers from Cape Cod to Cape May along the outer shelf. Boesch (1979) found it in low densities in both the northern and southern transects. A single male (length = 5.25 mm) was collected in a bongo tow during the fall at B5. Gammarellus angulosus (Rathke) 1843 Gammarellus angulosus is an amphi-Atlantic boreal species reported from the North American coast from Newfoundland to Connecti cut (Bousfield, 1973). It is usually collected on rocky, highly exposed shores clinging to algae (Steele and Steele, 1972) or swimming in the swash zone at low tide level (Bousfield, 1973) . It was not collected from the benthos by Boesch (1979), but a single specimen (length =4.0 mm) was collected in a surface plankton tow at A2 during the spring. Spartina was also found in the sample suggesting the possibility that the amphipod was rafted in southward flowing currents. Family Haustoriidae Protohaustorius wigleyi Bousfield 1965 Protohaustorius wigleyi is a burrowing species distributed from Maine to North Carolina (Bousfield, 1973). Pelagic mating has not been observed for haustoriid amphipods but species from high-energy sand habitats both passively and actively leave the substrate (Grant, 1980). .L* wigleyi was found in benthic collections at central and outer shelf stations of the northern transects with its greatest density at D1 (Boesch, 1979). A single male (length = 3.7 mm) was collected in a spring bongo tow at LI. 65 Family Hyalidae Hyale sp. The genus Hyale generally occurs in marine intertidal and coastal freshwater habitats. No specimens of the genus were found in the benthic collections of the outer continental shelf, but Hyale sp. was collected in the neuston at J1 in the spring and L4 in the summer. Barnard (1970) reported dense populations of Hyale species rafting on Sargassum far from shore which may account for the occurrence of Hyale at the offshore stations in this study. Family Isaeidae Microprotopus raneyi Wigley 1966 Microprotopus raneyi has been reported from Cape Cod Bay south ward to northern Florida and the Gulf of Mexico (Lowry, 1972). It is a small tubicolous amphipod and does not have a terminal pelagic male mating stage. Watling and Mauer (1972) found M. raneyi extensively in epibenthic dredge samples but not in bottom grab samples in Rehoboth Bay, Delaware. It was recorded from occasional plankton samples taken in Drum Inlet, North Carolina (Fox and Bynum, 1975) and the Chesapeake Bay (Grant and Olney, 1979). The benthic grab samples from the Middle Atlantic Bight did not include M. raneyi (Boesch, 1979) but it was collected by Dickinson, e_t al. (1980) at a single station off Virginia in 8 meters of water. It was collected in both neuston and bongo collections at the inner shelf stations LI and Cl in August, 1977 (Table 12). Surface occurrence was not confined to a specific time of day (Figure 14). A relationship between reproductive activity and the planktonic occurrence of M. raneyi was suggested by these collections. A large Table 12. Occurrence of Microprotopus raneyi in the plankton. ~pA *te S ^ 8 rH r— 3 3 . ) 3 O < S3 CO fa SJ •H o t CJ •H 4-1 4-> J 4 n i V Figure 14. Diel cycle of Hicroprotopus raneyi in the neuston at Cl during the summer. INDIVIDUALS / 100 E 0.5- 2 0- .0 3 3.5-1 2.5 1 . . 5- .5 0-1 0 - 2100 2400 0300 IE F A () DST DAY OF TIME 0600 0900 1200 ! 500 1800 69 percentage (85%) of females found in the water column were either ovigerous or possessed setose oostegites. This concurs with results of surface and subsurface collections made in the Chesapeake Bay in August 1978 in which 100% of females collected were reproductively mature (personal observation). Photis macrocoxa Shoemaker 1945 Photis macrocoxa is distributed along the North American coast from the Gulf of St. Lawrence to the Middle Atlantic states (Bous field, 1973), Boesch (1979) collected it in the benthos on the central and outer shelf where it was most abundant at B1 and B5. A female (length = 2.5 mm) was taken in a bongo tow at B5 during May, 1977. Family Liljeborgiidae Liljeborgia fissicornis (Sars) 1858 Liljeborgia fissicornis is an amphi-Atlantic species, its range having been extended from the eastern North Atlantic to the western side after it was found in the Middle Atlantic Bight at depths of less than 75 m (Watling, 1979). Little is known about the ecology of _L. fissicornis. Members of the family Liljeborgiidae are known to live commensally with polychaetous annelids and a species of the genus Liljeborgia has been found living in association with the hermit crab, Pagurus hemphilli (Taylor, 1979). In Bousfield1s (1978) classifica tion of gammaridean amphipods, the superfamily Liljeborgioidea, though lacking a pelagic terminal male stage, is considered morphologically similar to the synopiids and pardaliscoids which include many epiben- thic and pelagic species. 70 _L. fissicornis occurred in low abundances at only two benthic stations, FI and D4, during the winter cruise (Boesch, 1979). It was collected in the water column during the fall, winter, and spring (Table 13). Though its abundance was low in the water column, it was consistently taken in more than one tow at all sites except N3. It never occurred at any station for more than one sampling period. An examination of its benthic occurrence during both survey years showed a similar pattern. It was present in benthic collections for more than one sampling period at only one of ten stations. .L. fissicornis was collected in highest abundances during the fall. All of the females collected at that time had setose oostegites or were ovigerous. Reproductively mature females were absent during the other cruises. Family Lysianassidae Hippomedon serratus Holmes 1905 Hippomedon serratus is distributed form the Gulf of St. Lawrence to North Carolina (Bousfield, 1973). It is a benthic scavenger with a free-swimming terminal male mating stage. Whiteley (1948) collected II. serratus in the plankton over Georges Bank. _H. serratus was a common member of the benthos in the Middle Atlantic Bight. It was found in low densities along the central and outer shelf of the northern transects (Boesch, 1979). Dickinson, al. (1980) also collected it in uniformly low densities throughout the region. Only one specimen was collected in the water column during the present study. It was a non-breeding female (length = 4.5 mm) found at E3 during August, 1977 in a bongo tow. Mature males (n = 6, length = 11.0 - 13.0 mm) were collected during the previous year’s study in the water column at D1 and Cl (personal observation). 7 1 G 0 0 cn CN 4-1 CO ••• -co CO cn cn cn 0 0 pO •H ■i I l CQ G j§ N ' —' o CN p - un CN p> «H *••••• C/0 cn cn CN CN cn CN CN G H CO o o o o o 3 G o o O 4-1 »H rH rH iH T3 cO cO n3 E B G E Jh g X CO G rH co CO un tH iH cn o cn O O £ 4-> G Pt4 CO CO =Ste rH a . a> x t CO CN G CO rH o •H cO CN OO CN rH o rH E co CO co •H *H •H =Cfc C CO CO H S & O o O a ‘ 4~* 4-4 *H co G cn cn G S 'H CO cO J 2 u m m un un CN un CN hJ cO o o o o o o o G m in m un CN n CN co <4-4 O CQ z PQ PQ PQ PQ PQ o o 4-4 Orchomenella pinquis (Boeck) 1861 The amphi-Atlantic species Orchomenella pinquis is found on the North American east coast from the Arctic to North Carolina (Bous field, 1973; Watling, 1979). It is a scavenger and its males have a pelagic terminal mating stage. Males are often pelagic during the winter (Bousfield, 1973). Immature specimens were collected in plankton hauls by Dunbar (1954) at Ungava Bay. (). pinquis was found infrequently in both the benthos and water column of the Middle Atlantic Bight. It was found on the bottom at D1 2 during the spring (0.260 individuals/0.1 m ) and at G5 during the 2 summer (0.189 individuals/0.1 m ). A male (length = 7.2 mm) was collected in the water column during the winter at Cl and non-breeding females were found at N3 (n - 2, length = 3.2, 4.5 mm) and E3 (n = 1, length = 5.2 mm) during the summer sampling period. All plankton specimens were from subsurface samples. Family Melphidippidae Melphidippa sp. A Several specimens belonging to the genus Melphidippa were collected in the water column during the winter at A2. Comparison with benthic collections showed that it was also found in grab samples and was designated Melphidippa sp. A (M. Bowen, personal communica tion). Melphidippa sp. A was encountered in the sediment only in low densities at G6 during the winter and A4 in the spring (Boesch, 1979). Sixteen females, all with setose oostegites (range of lengths =2.7 - 3.5 mm), four males (2.5 - 2.7 mm) and one immature specimen (2.0 mm) were collected in subsurface plankton hauls. Its density in the water 3 column was 1.2 and 1.8 individuals/100 m in two B505 tows and 5.6 3 individuals/100 m in a B202 tow. 73 The genus Melphidippa has been reported from plankton tows by Whiteley (1948) who found two species during his work over Georges Bank. Members of the genus Melphidippa are epibenthic and are known to sit upside down on the bottom in a cradle formed by their elongated legs (Enequist, 1949). The superfamily Melphidippoidea was described as being epibenthic and pelagic and found mainly in tropical and temperate continental regions (Bousfield, 1978). Family Oedicerotidae Monoculodes edwardsi Holmes 1905 Monoculodes edwardsi has been identified from oligohaline to euhaline habitats and geographically from the Gulf of St. Lawrence to the Gulf of Mexico (Bousfield, 1973). It is a burrower, feeding upon buried detritus (Biernbaum, 1979). M. edwardsi has frequently been reported from plankton collections (Fish, 1925; Dunbar, 1954; Mills, 1967b; Feeley and Wass, 1971; Williams and Bynum, 1972; Grant and Olney, 1979). M. edwardsi was the most abundant "larger" crustacean (meaning amphipods, decapods, euphausiids, and mysids) in oblique plankton tows made over Georges Bank (Whiteley, 1948). It was more abundant in the lower water column than at shallower depths, even after it migrated upward at night. There was no marked seasonal maximum in abundance over Georges Bank but in North Carolina estuaries there were peaks in abundance from February to April (Williams and Bynum, 1972). Benthic collections of M. edwardsi in the Middle Atlantic Bight were sparse (Table 14). It occurred widely across the shelf, but in low numbers (Boesch, 1979). In contrast, M. edwardsi was the most abundant and ubiquitous gammarid taken in plankton tows. It was Table 14. Occurrence of Monoculodes edwardsi in the Middle Atlantic Bight in the water column (this study) and in the benthos (from Boesch, 1979). .9 o O 3 J_l !-4 u U 5-t cd o m c SO CM M CM 00 CM O' CM CM O rH O O rH rH © © rH rH © O O rH CM HC M 00 CMCM rH MC ©\ CMCM 74 75 collected during all four sampling periods with densities as high as 3 180.1 individuals/100 m at Cl during the spring. Amphipods of varying stages of maturity and of both sexes ranging in size from 1.2 - 9.2 mm were collected (Figure 15). Reproductively mature females were in the water column during the fall and spring but were completely absent during winter and rare during summer. Immature specimens were very numerous during spring at inshore stations and accounted for the highest density of any species of gammaridean amphipod in the plankton. Mature males (distinguished by elongated second antennae) were found during all four sampling periods. The apparently bimodal distribution of lengths (Figure 15) during the fall and spring disappeared during winter and summer. The length- frequency distribution is related to maturity (Table 15) and location of samples (Figure 16). During the fall, specimens larger than 6.0 mm were limited to stations B5 and E3. Specimens at these outer shelf stations displayed a more advanced maturity than specimens taken from other stations and included reproductively mature females. Breeding individuals were absent during winter and the bimodal distribution of length frequencies from the fall merged. Figure 16 shows however that mean and range of lengths differ by station. Individuals from inshore stations were longer and more mature than those from offshore stations. By spring the inner shelf collections were dominated by small immature specimens while specimens from central shelf .collec tions were longer and reproductively mature. There were few reproductively mature females and little disparity in lengths of individuals from August collections. 76 Figure 15. Size and sex frequency distribution of Monoculodes edwardsi in the water column at all stations during the four cruises. Symbols as in Figure 6. FREQUENCY (%) 20 - 0 4 50 -i 50 30-i - 0 3 0 0 - 0 - - .0 2 . 4.0 3.0 ■ LENGTH (mm) 5.0 60 WINTER - AH WINTER Stations- n = 233 n= 279 PIG l Stations All - SPRING Stations All - FALL UMR-Al Stations AllSUMMER- n 87 = n= 28 7.0 8.0 □ . 10.0 9.0 REPRODUCTIVELY AUE FEMALE MATURE IMMATURE MALE FEMALE Table 15. Degrees of maturity of Monoculodes edwardsi by station. When percentages do not equal 100, it is because condition of some specimens were poor and so maturity was undetermined. - cdB-S & *4H rH •H rH •H o *H pci C •J O •H cn 4-1 4-1 4-1 4-1 4-1 4-1 00 cn OO > CN CN m in H r o v s n C C 0 N 00 CO CN in N os o oo o m o H H N CN O C m O C CN rH rH O C o . o - o o o o o O O O CN O O CN CN O O CN O O O o o o m o O CN CO *H d - rH O C rH - cN O a O co O o c O ao O N c r-^ & O rJ 4-1 (0 H 3 o o N N -d" CN CN O C o o m oo so N O C CN O O 1-1 CN 3 2 Q o o o o o o o o N H O OS O rH OS S O -O'as so rH CN r~- as as O o o CO N O O O 0 0 0 0 0 - 0 0 s o m so as O O NJ- o o o o o •H n c 3 00 P. rH CN iH CO CO CN CN CO CO iH CN rH O V-J Q 23 W C N O N O O CN CO cn CN o c in o o o o © so o o o H rH rH cn cn o o o o o O O O o oo oo o O O N O O CN O 0 0 o c n u o c o o oo oo co CO o o H O iH CO rH cn H CO rH O C 3 Q fH PQ 23 N so CN co rH rH n m s O 78 79 Figure 16. Mean length (open circle) and range of lengths (horizontal line) of Monoculodes edwardsi by station for each cruise. Vertical marks are ± 2 standard deviations from the mean. STATION 1 I n =2 I o A 2 —1 2H n I n= o H L2 LI ~ LI 5— <■ B5 — CI -| 2 I — I n = I 4 o— I F2— - 3 E N 3 H Cl Cl - I ] - I n= I2 -o • —]DI E3 - 3 N LI - 3 E - 2 L - 5 B D I A2 L2 —iL2 N3~i L2 E3— B5H -J L 0 . 30 . 50 . 70 . 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 .0 I 20 . 40 . 60 . 80 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 0 ------ UMR 97 n © I = 1977 SUMMER ITR 1977 WINTER ______I I I I I I I I f I 1 — ————o——— n 161= I ------I ______...... ------n= 2 PIG 1977 SPRING n 92 = I o ------o I EGH (mm) LENGTH ______no = I n=1 FL 1976 FALL n =10 I n I =4 I ------o n =8 I —q —o i -o ------n I =o I ______------o i ------o ------I h ------I ------______------1 n= 8 no = 1 o— 1 n 148= n= 66 1 n= 12 o h- n= 19 n= h- o - l -o ------____ n =10 n i=5 » ______n=9 1— n 12 = 1 n = I 37 n=9 i 81 Synchelidium americanum Bousfield 1973 Synchelidium americanum is distributed from Maine to Georgia (Bousfield, 1973) and along the Texas coast (McKinney, 1977). The genus Synchelidium has pelagic males bearing large eyes and elongated sensory antennae (Barnard, 1972a). Williams and Bynum (1972) reported finding Synchelidium sp. in nocturnal surface plankton samples in North Carolina estuaries. Table 16 lists the occurrences of _S. americanum in the water column. It was found only at inner and central shelf stations reaching its greatest density at LI during the spring and summer. Boesch (1979) found SL americanum in low numbers in the benthos throughout the study area with greatest densities at L2 and K2 during 2 2 the winter (0.565 individuals/0.1 m and 2.464 individuals/0.1 m , respectively). Benthic sampling was conducted at LI only during the year previous to this study. S_. americanum was present in densities 2 of 0.698 individuals/0.1 m during the winter and 0.782 individuals/ 2 0.1 m during the summer. The occurrence of J5. americanum in the plankton was apparently unrelated to breeding behavior. Very few reproductively mature adults were among the individuals collected during the spring when densities were high. The spring collections were dominated by females but the sex ratio was closer to 1:1 during the summer. A greater percentage of reproductively mature adults was also present during the summer. Family Phoxocephalidae Phoxocephalus holbolli (Kroyer) 1842 Phoxocephalus holbolli is an arctic-boreal species whose range has recently been extended south to the Middle Atlantic Bight Table 16. Occurrence of Synchelidium americanum in the water column. -Q •H rO •H CO O < O 4-> 4-1 d cO cO on a) CO 0) d (Watling, 1979). It is a burrowing detritivore (Biernbaum, 1979) and has been collected in plankton tows (Whiteley, 1948). P. holbolli occurred frequently in the benthos of the Middle Atlantic Bight (Boesch, 1979) and was most abundant at B 5 . Only one specimen was collected by the plankton survey - an immature female (length = 3.5 mm) in a bongo tow at B5 during the fall. Trichophoxus epistomus (Shoemaker) 1938 Trichophoxus epistomus has been reported from southern Maine to North Carolina (Bousfield, 1973). It is a burrowing species and feeds upon buried detritus (Biernbaum, 1979). Phoxocephalid males are known to leave burrows at night and swarm at the surface (Barnard, 1969). Mating is thought to take place during relatively brief, essentially nocturnal swarming periods (Bousfield, 1970). Williams and Bynum (1972) collected Paraphoxus epistomus (=T. epistomus) in nocturnal surface plankton samples. T. epistomus was a common and ubiquitous member of the benthic community of the Middle Atlantic Bight (Boesch, 1979). It was found in the northern and southern transects with highest densities occur ring at L2. T_. epistomus was collected in only on plankton sample. Mature males, ranging in length from 3.2 - 4.5 mm, were collected in a neuston tow made at 2100 DST at LI during August. Abundance was 9.0 3 individuals/100 m . An examination of neuston collections from the previous year’s study showed that a swarm of males within the same C 3 ) Barnard (1979) recently reassigned Trichophoxus epistomus to the new genus Rhepoxynius. However, for consistency with the benthic report (Boesch, 1979) and because there is some controversy re garding the identity of the species collected during the VIMS-BLM study (Bowen, personal communication), the name Trichophoxus has been retained. 84 size range were taken at night at Cl in June 1976 with densities of up 3 to 51.0 individuals/100 m (personal observation). Family Stenothoidae Stenothoe minuta Holmes 1905 Stenothoe minuta is distributed from the southside of Cape Cod southward to Georgia (Bousfield, 1973) and is also found along the Texas coast (McKinney, 1977). It is found on jetties and among sea weed in shallow bays and estuaries. It is sometimes planktonic, though males lack a pelagic terminal mating stage. Williams and Bynum (1972) collected it in North Carolina plankton samples. JS. minuta was not found in benthic collections by Boesch (1979). It was collected in one neuston tow at L4 during the fall cruise. The sample included two females (length = 2.5, 3.2 mm) with large, but not setose oostegites, seven males (length = 1.7 - 2.2 mm), and two immature specimens (length < 1.7 mm). Family Synopiidae Synopia ultramarina Dana Synopia ultramarina is a tropical and subtropical species (Barnard, 1972b). Barnard (1972b) described the general behavior of the family Synopiidae as demersal and Synopia as a "neritic, if not a pelagic genus." Synopia has pelagont coxae which Barnard suggested may be an adaptation to a nonbenthic existence. S_. ultramarina was not captured in benthic grabs, but was taken in neuston tows at L4 and L6 during the spring and at J1 and L4 during the summer. The spring collections were all male, size range * 3.2 - 4.7 mm, and the summer collections were all female with setose oostegites, size range = 2.7 - 3.5 mm. 85 Tiron tropakis Barnard 1972 Tlron tropakis is found from Virginia to Venezuela and from California to Peru (Barnard, 1972b). Tiron is a shallow water genus and its morphology suggests nestling behavior (Barnard, 1972b). Tiron tropakis was not collected in the benthos by Boesch (1979) but Dickinson, elt al. (1980) found it in the benthos off the mouth of the Chesapeake Bay.in 20 m of water. It was taken in bongo collections during the fall (n = 2) and winter (n = 1) at L2. The only female collected possessed setose oostegites. The size range of the speci mens was 3.7 - 5.5 mm. DISCUSSION Recent studies have demonstrated the numerical importance of gammaridean amphipods in the benthos of the continental shelf of the Middle Atlantic Bight. In a survey of the macroinfauna of the area, 40% of the specimens collected were gammaridean amphipods (Dickinson, et al., 1980). In a separate study, it was found that in some area of the outer shelf amphipods accounted for greater than 70% of the total number of individuals sampled (Boesch, 1979). As would be expected of a group of organisms which generally exhibits a benthic lifestyle, gammaridean amphipods occurred infrequently and in low numbers in plankton collections. The relatively few specimens collected during this study do permit observations on the behavior, distribution, and interactions with other components of the shelf ecosystem. These must be made, however, with full knowledge of the limitations of the sampling program. Evaluation of the Sampling Methodology There are three basic questions which must be addressed before examining the results of this study. Were gammaridean amphipods, as an entity separate from the rest of the zooplankton population, sampled adequately? How do the problems inherent in plankton sampling relate to this study? How valid are comparisons between the plank- tonic occurrence of gammaridean amphipods and their benthic occurrence? 86 87 In surveying the zooplankton populations of the Middle Atlantic Bight, plankton tows were made to sample the entire water column. The bongo nets were towed as close to the bottom as possible, but there are problems in the determination of the maximum depth of tow by tri angulation, the method used while the tow was in progress. The tow may be made too close to the bottom with the nets actually dragging in the sediment or it may be made too far above the bottom. The magni tude of any error increases with increasing depth of stations. Since it is better to avoid towing the net on the bottom, most of the errors were made in towing the net too far above the bottom. Since previous studies suggest that most emergent amphipods stay close to the bottom (Russell, 1925; Whiteley, 1948; Williams and Bynum, 1972; Jones, e t _ al., 1973; Hobson and Chess, 1976), towing the sampler too far from the bottom at its maximum depth of tow may result in undersampling gammaridean amphipods. A comparison of the occurrence of gammaridean amphipods in plankton samples with the distance from the bottom of the maximum depth of tow was made to determine if the distance from the bottom that the net was towed was related to the occurrence of gamma ridean amphipods in the samples. Figure 17 shows that gammaridean amphipods were mostly found in tows which were no more than 30 meters from the bottom at their maximum depth. More than 70% of the tows within the distance-from-bottom interval of 0-5 m contained gammaridean amphipods and more than 90% of all amphipods collected were found in tows which were no more than 25 m from the bottom. Even though there was an increased chance of collecting gammari dean amphipods in tows made close to the bottom (within 5 meters), they were collected throughout the water column at all areas of the 88 Figure 17. a) Cumulative percent and percent of tows in which gammaridean amphipods were collected by distance- from-bottom intervals. Distance is number of meters from bottom of maximum depth of tow. Number of tows in that distance interval is in parenthesis. b) Percent of tows which had a maximum depth within specified distance-from-bottom interval. Distance intervals on side of bars. Shaded areas are percent of tows within that distance which collected gammari dean amphipods. PERCENT OF TOWS WITHIN DISTANCE INTERVAL PERCENT OF TOWS WITH AMPHIPODS 100 lOO—i 0 5 - 0 4 0 4 90 30 80 30 60 20 0 7 50 70 90- 10 0 (26) -5 0 m / SHELF INNER / (26) 6-10 / / / / / if) m CENTRAL SHELF (24) 11-15 (53) ITNE NEVL ( ) (m INTERVALS DISTANCE 16-20 (13) CM < 0*0 i m if) OUTER SHELF (73) 21-25 ) 5 0 OJ 30 -3 6 2 BREAK SHELF (40) ( 12 ) 40 -4 1 3 ( 10 in ) SLOPE 1-50 (7) >50 (7) 90 shelf except the slope. The absence of gammaridean amphipods from subsurface plankton samples from the slope is more likely attributable to their low abundance in the benthos rather than sampling error. It seems that gammaridean amphipods were collected in the water column no matter how far from the bottom the net was towed. It is possible the net hit the bottom at one station - L2 - during the fall. Eight species were collected in the B505 tow but none were collected in the B202 tow and the actual depth of the tow (calculated from the Time-Depth Recorder tracing) was equal to the depth of the station. All of the species collected were found in the water column at other times and most did not differ in size and sex. Many immature specimens of Ampelisca vadorum were collected, which was unusual for that species. The net apparently did not hit the bottom during any other time. The bongo nets were lowered to the maximum depth of the tow, then raised through the water column either continuously or in a step-wise fashion, depending upon the depth of the station. The actual volume of water filtered at any one depth was actually only a percent of the total volume filtered. If gammaridean amphipods are more dense at a specific depth (in particular, the lower water column), then their density may be greater than indicated and some of the rarer species may be missed entirely. The duration of the tow affects the number of species collected. Longer tows would be expected to collect more species, including the less common ones. Since replicated tows were made at A2, B5, and E3, the length of time the net was in the water was greatest at these stations. Ten different species were collected over four sampling 91 periods at B5, the most found at any station, and eight different species were found at E3. Also identified from collections at B5 were single specimens of Phoxocephalus holboHi, Eriopisa elongata, Leptocheirus pinquis, and Photis macrocoxa which suggests that rarer species (in the water column) were also collected as a result of increased duration of sampling. Only four different species were collected at A2, but this probably was indicative of a general lack of gammaridean amphipods in the water column at that offshore station. Each station was visited only once every three months. For those species which emerge from the benthos on a seasonal basis, such infre quent samples may completely miss their occurrence in the water column. Fish (1925) found 24 gammaridean species in surface collec tions at Woods Hole but none were present continuously throughout the year. Only eight species were present for more than three successive months. Williams and Bynum (1972) found that although most of the gammaridean amphipods they collected in North Carolina estuaries were present in the plankton year-round, almost all of the species had a seasonality of occurrence. The absence of collections of Hippomedon serratus suggests that species may not have been collected in the Middle Atlantic Bight plankton as a result of infrequent samples being taken. H. serratus has free swimming terminal mating males but only one specimen (and not a mature male) was collected during the study. Collections made in the same area during the previous year included many male H. serratus. H. serratus was present in the benthos during both years. There are several problems inherent in sampling zooplankton popu lations which could bias the results. These include net avoidance and 92 size selectivity of the mesh. The choice of sampling gear and mesh size is naturally a compromise between the net best able to sample the population and the practical ability to fish it. McGowan and Fraun- dorf (1966) found that active avoidance was a significant source of variation among samples taken with different net sizes and that even small, poorly swimming animals were caught more efficiently in a larger net. Since only one size net (60 cm bongos) was used to make collections, no conclusions can be drawn about variation in collec tions due to net size. However, among gammaridean amphipods one would suspect that there would be a bias against catching large terminal pelagic mating males since they metamorphose into better swimmers in their final molt, thus are more capable of active avoidance of the net than females or juveniles. Females are more likely to be caught since variation in body shape associated with carrying eggs or young may decrease mobility. Molting and recently molted crustacea are also not likely to be capable of effective avoidance (Clutter and Anraker, 1974) . The choice of mesh size will influence the size selectivity of the sampling gear. Colton, et al. (1980) found that a significantly greater number of hyperiid amphipods (especially Hyperia sp.) were collected in a net with 253 ym mesh than with 333 ym mesh. There was a proportionately greater loss of amphipods than copepods of similar length through the 333 ym mesh net possibly because amphipods have longer appendages, greater compressibility, and better capabilities of active escapement (Colton, et^ al., 1980). In the present study, a comparison of B505 collections with B202 collections showed that in most cases B505 was more effective in collecting gammaridean 93 amphipods. The exceptions are the very small amphipod species Microprotopus raneyi (maximum length = 2.5 mm) and the spring collec tion of immature specimens of Monoculodes edwardsi at Cl. During all four cruises, amphipods in B505 collections were more abundant than in B202 collections at a greater number of stations and included all size ranges found in the water column. When surveying organisms with such low densities, it is not sur prising that the results were inconsistent. For example, species present in one replicate tow at B5, E3, or A2 were often absent from preceding or succeeding tows. Grant (1979) found much variability in abundances of dominant species in the plankton though order of dominance was fairly stable. Such variability in abundances of organisms in plankton collections suggests that the occurrence of those species with low mean abundances will be inconsistent in replicate tows. In general, it appears that some underestimation of the abundance of gammaridean amphipods in the water column occurred and some rare species and species limited to seasonal excursions into the water column were not collected. However, the collections were probably a fair representation of the species of gammarids and their population structure (sizes and sexes) present in the plankton. The size selectivity of the methods used to collect benthic organisms and plankton organisms affects the validity of size compari sons between benthic and planktonic populations. Benthic organisms were elutriated onto a 500 pm mesh screen and as discussed previously, plankton was collected with 505 and 202 pm mesh nets. Both larger and smaller organisms may be collected by the benthic methodology. Larger 94 organisms are best able to avoid the plankton net and the gentler elutriation may retain smaller benthic organisms while comparable sizes may be extruded from the towed 505 ym mesh plankton nets. In fact, benthic collections did generally include more juveniles than were found in the water column. However, since the B202 net did not selectively collect more juveniles than the B505, it appears that juveniles were truly less frequent migrants to the water column. The upper range of sizes in the benthos and plankton were comparable. In considering which species of gammaridean amphipods occurred in the water column, consideration must also be given to those species in the benthos which were not collected in the water column. Dickinson, et al. (1980) collected a total of 101 species during their survey and Boesch (1979) collected 97 species during his two-year survey of the benthos of the Middle Atlantic Bight which included a greater area and more stations than were sampled for plankton. Twenty-one species found by Boesch were also found in the water column. Of the first eleven benthic species occurring most frequently in the benthos (when ranked according to the number of stations at which they were collected), nine were also collected in the water column. Of the most frequently occurring species in the benthos that were not in the plankton, only Unciola inermis was abundant (its geometric mean 2 density was greater than 10 individuals/0.1 m during any cruise) and 13 other species were common (geometric mean density between 1-10 2 individuals/0.1 m )* These were Corophium crassicorne, Pseudunciola obliquua, Siphonoecetes smithianus, Melita dentata, Dyopedos monocantha, Stenopleustes inermis, Photis dentata, Photis pugnator, Listriella barnardi, and the haustoriids Acanthohaustorius spinosus, 95 Parahaustor ius attenuatus, Acanthohaustorius millsi, Pseudohaustorius borealis. The absence of species from the water column may have been due to inadequate sampling as previously discussed or they may not migrate into the water column. The reason for a species’ absence from the water column may be important. Migration, or lack of it, into the water column has implications for the distribution of the species, its availability as prey items, its interactions with other species, and its ability to withstand or recolonize after disturbances. Relationship of Free Swimming in the Water Column to the Behavior of an Individual Species: Monoculodes edwardsi Monoculodes edwardsi was the most abundant and frequently occur ring gammaridean amphipod in the water column. Its relatively high abundance in the plankton was out of proportion to its low abundance in the benthos (Boesch, 1979; Dickinson, e_t a3.. , 1980). The occurrence of Monoculodes edwardsi in the water column seems to be a significant factor in its life history. All sizes and stages of maturity are found in the water column. Though it is considered an infaunal burrower, its reproductive patterns differ from other infau- nal gammaridean amphipods (Van Dolah and Bird, 1980). Females brood large numbers of small eggs which is typical of epifaunal amphipods. Such an adaptation suggests to Van Dolah and Bird that it lives in a high risk environment. The small eggs develop quickly, increasing the chance of a successful release of its offspring if predation pressure or unstable environmental conditions increase adult mortality. They contend that it probably burrows through the mud-water interface with a portion of its body exposed as do other Monoculodes species. Swimming in the water column also would most certainly increase the 96 risk of predation and, in fact, M. edwardsi is found to be an important part of the diet of several species of demersal fish of the Middle Atlantic Bight during the summer (Sedberry, 1980). The abundance of M. edwardsi in plankton samples suggests it has good dispersal abilities and is able to exploit new habitats. It was found at Cl in the water column during the fall following the complete elimination of the peracarid fauna in the benthos as a result of hypoxic conditions on the inner and central shelf off New Jersey. It has a wide-spread distribution, occurring in oligohaline to fully marine habitats from all along the east and Gulf coasts of North America. Its dispersal capabilities which are apparently not seasonally limited, along with the facts that it broods its young and produces many small eggs with short brooding period (thus increasing the intrinsic rate of growth of M. edwardsi populations) suggest that it is an opportunistic species. It is able to discover new habitats quickly and reproduce rapidly. Grassle and Grassle (1974) suggest that an important component of adaptation to an opportunistic life style is genetic variation. This may explain some of the discrepan cies in descriptions of M. edwardsi. Bousfield (1973) suggests that specimens from very low salinities in some areas may be different species. His description of M. edwardsi also differs in some details from the original description by Holmes (1905). These variations may be normal variations reflecting its ability to exploit different habitats by having a diverse gene pool. Further studies of this species are needed to confirm or deny such speculation. 97 Relationship of Gammaridean Amphipods in the Water Column to Benthic Communities The relative mobility and dispersal capabilities of benthic species may be important in determining the overall composition of a benthic community. The corophiids Unciola irrorata and Erichthonius rubricornis occurred year-round in the water column. The distribution on the outer shelf and the population structure as depicted by histo grams of size and sex frequencies of benthic and plankton collections was similar with all but the smallest individuals found in the water column. The pattern of occurrence of corophiids in plankton samples suggests a constant flux of short excursions into the water column. While this would probably result in little overall change in species composition of the sampled area, the distribution of individuals would constantly be changing. Dauer and Simon (1976) suggested that local variations in time and space of species distributions may tend to minimize effects of important density-dependent interactions. In the case of the corophiids on the outer continental shelf, it may permit the co-occurrence of trophically similar species. Work done by Schaffner (1980) in which she studied resource partitioning among corophiids and ampeliscids in the B-area (see Figure 1) shows considerable spatial and temporal overlap of Unciola irrorata and Erichthonius rubricornis. She suggests that the mobility of _U. irrorata may be one factor allowing its coexistence in areas of very dense populations of Ampelisca agassizi. The ampeliscid amphipods on the other hand, were more restricted in their mobility. They emerged only during periods of mating activity. Plankton samples were less similar to benthic samples and 98 were composed of mostly reproductively mature adults. Density- dependent interactions may be important in determining the distribu tion of the ampeliscids. Schaffner found that the ampeliscids had a tendency to be spatially limited. A. agassizi, which occurred least frequently in the water column, was apparently competitively superior to A. vadorum and Byblis serrata in the finely grained sediment of swales of the outer shelf. There was little spatial overlap between A. agassizi and the other two ampeliscid amphipod species. A. vadorum and J3. serrata were found more frequently in the water column and Schaffner found that they exhibited greater spatial overlap. Other factors besides mobility into the water column may facilitate their coexistence such as the maintenance of different sizes as a result of different periods of reproduction (Schaffner, 1980). Another consequence of emergence activity by gammaridean amphipods is an increased ability to recolonize defaunated habitats. The effectiveness of a species in recolonizing different size scales of defaunated areas depends on its mobility. Swimming species may effectively recolonize areas of large scale disturbances. Crawling species may effectively recolonize small scale areas (Boesch, 1979; Santos and Simon, 1980a). The area of defaunation of the continental shelf as a result of hypoxia was large. Boesch (1979) suspected that since amphipods brood their young, thus lacking a broad planktonic dispersal mechanism, they would have to rely on several generations of short-range dispersal to slowly repopulate the area. Monoculodes edwardsi was possibly able to quickly establish itself in the area following defaunation because of its ability to swim. The dominant amphipods on the inner shelf prior 99 to hypoxia, Protohaustorius wigleyi and Pseudunciola obliquua, never returned during the course of the study. Except for a single specimen of Protohaustorius vigleyi collected at LI during the spring, these species were absent from plankton samples and probably have limited swimming abilities. Though some haustoriid amphipods may inten tionally enter the water column if food availability is decreased (J. Grant, 1980), they are generally confined to the sediment and do not breed in the water column (Bousfield, 1970). If P^. obliquua is similar to other corophiids, it may move into the water column for only short periods of time and is only capable of short range dispersal. Another aspect of the benthic study of the continental shelf was the placement of azoic sediment boxes on the bottom to follow recolo nization. In this case, swimming would not be the most effective method of recolonization. Some species of amphipods were found to be important recolonizers of the boxes which suggested to Boesch (1979) that the timing of the local disturbance was less limiting to them than to species dependent upon seasonal recruitment via planktonic larvae. Among amphipods it was found that corophiids colonized the boxes more quickly than ampeliscids. Boesch felt that reflected the relative mobility of the two families and that the ability of ampeliscids to recolonize depends on the timing since they are more capable of dispersal when mating. These contentions are supported by the results from the plankton collections. The corophiids appeared to have a constant flux of individuals moving into the water column but only for a short period so they were displaced only a small distance. Ampeliscids were more seasonal in their emergence patterns and were 100 possibly transported greater distances through the water column. For example, mature males of Ampelisca vadorum were found at B5 even dur ing the cold winter sampling period at a time when they were most likely not to be breeding and were absent from the benthic samples of the area. Other studies have shown that repopulation of defaunated areas by ampeliscids may be dependent upon the timing of the disturbance. McCall (1977) used azoic boxes for a two-year study of recolonization in Long Island Sound. Ampelisca abdita, the sibling species of A. vadorum, was a dominant early colonizer during the first year of the study during which sampling took place in July and October, but was present in reduced densities during the second year during which sampling took place in June. Santos and Simon (1980b) found A. abdita to be among the dominant colonizers following an annual defaunation in a South Florida estuary in two out of the three years of the study, but in one year it was virtually absent. Resuspension of sediments by currents or waves could cause the passive transport of bottom-dwelling invertebrates. Studies of bottom sediments are frequently disturbed on the inner and central shelf, primarily by oscillatory bottom currents created by surface waves. On the outer shelf there are long periods of apparent quiescence with little resuspension occurring except during winter storms (Boesch, 1979). It is possible that surface sediments may be reworked during the tranquil summers by trawlers and there was some evidence of this during August 1977 (Butman and Noble, 1979). Changes in bottom micro topography may also be brought about either by internal wave activity or by the activity of fish - hake were seen in photographs of the bottom - during August (Butman and Noble, 1979). 101 If a physical disturbance of the bottom resulted in the passive displacement of benthic invertebrates, samples may be expected to contain a few organisms from many different species (Fish and Johnson, 1937). Except for the sample when the net probably hit the bottom (at station L2 during the fall), no single sample appeared to include a group of organisms which were transported from the bottom as a result of a physical disturbance. At B5 during the fall, however, successive bongo tows each contained one individual of a different species that were not found at any other time in water column collections. These were Leptocheirus pinquis, Phoxocephalus hololli, and Eriopisa elongata. It is possible that some disturbance on the bottom may have transported these individuals into the water column, but there is no definitive evidence of this. The occurrence of a single individual in the water column could also be attributed to other causes such as occurrence in low abundances on the shelf, which would reduce the chance of its collection in the plankton, or errant behavior by the individual resulting in its planktonic excursion. Leptocheirus pinquis is apparently a mobile species which often crawls along the bottom (Pratt, 1972). The life history of Phoxocephalus holbolli makes it unlikely that an immature female would be in the plankton. Eriopisa elongata is widely distributed across the shelf life but was not found in any other plankton sample. Their occurrence in succes sive tows at the same station suggests a common cause was driving them into the water column. There were other species found in the benthos which only occurred once and were represented by only one individual in subsurface samples. These were Hippomedon serratus, Protohaus torius wigleyi, and Photis macrocoxa but there was no evidence that they were passively transported by sedimentary disturbances. 102 Currents of the outer continental shelf flow predominantly south westerly parallel to the New Jersey coast. Rhachotropis inflata and Melphidippa sp. A were both found at plankton stations which were located southwest of the benthic stations from which they were reported, suggesting they were transported in the plankton by the predominant currents. Argissa hamatipes, Liljeborgia fissicornis, Orchomenella pinquis, Synchelidium americanum, and of course, Monoculodes edwardsi occurred in the water column despite having low mean abundances in the benthos. These species were distributed widely across the shelf in both the water column and benthos but there appeared to be no relationship between their benthic and planktonic distribution. Free swimming may be a frequent activity of these species resulting in a highly variable distribution. Relationship of Gammaridean Amphipods in the Neuston to Zooplanktonic Communities The gammaridean amphipods collected in neuston samples were closely related to water mass movements. Gammarellus angulosus and Calliopius laeviusculus are amphi-Atlantic arctic-boreal species col lected at A2 and B5 during the spring. They were associated with the northern boreal zooplankton community which drifted southward in the Middle Atlantic Bight shelf waters over the New Jersey transect as the aftermath of the cold winter of 1977 (Grant, 1979). Several species of gammaridean amphipods were associated with the passage of Gulf Stream eddies along the shelf edge. Sunamphitoe pelagica and Synopia ultramarina are subtropical species collected at Jl, L4, and L6. The faunal composition of the neuston at these 103 stations was similar and dominated by warm water communities. Physically defined Gulf Stream water was sampled at the surface during the spring and summer of 1977. Synopia ultramarina was also found at F2 and J1 during the autumn of 1975 (personal observation) and again the influence of the Gulf Stream was observed in the zooplankton community (Grant, 1979). The occurrence of Hyale sp. in the same collections indicates that it too is a subtropical species. Species of Hyale have been reported to raft in surface currents (Barnard, 1970). By far the most important gammaridean amphipod species, numeri cally, in the neuston were Ampithoe longimana and Microprotopus raneyi. They are a part of a benthic group of species found in the "Transhatteran" zoogeographical province defined by Watling (1979) as being composed of "shallow nearshore and estuarine species which range extensively north and south of Cape Hatteras and which are endemic to American Atlantic waters." In the water column they are found with the zooplanktonic community associated with the Coastal Boundary Layer - a band of low salinity water extending from the mouth of the Hudson River to the mouth of the Chesapeake Bay (Grant, 1979). Both of these species were also found in the plankton of the Chesapeake Bay. Though Vecchione (1979) found only limited signs of internation between the bay and coastal communities, he felt that such interactions may be important. Corophium ascherusiicum is a cosmopolitan species frequently found on ships1 hulls (Crawford, 1937). Its occurrence offshore, away from its normal habitat of enclosed bays, may have been the result of dropping off of the hulls of the many ships passing through the area. 104 The occurrence of Liljeborgia fissicomis, Ampelisca agassizi, Monoculodes edwardsi, Undola irrorata, and Ampelisa vadorum in the neuston was probably not indicative of the usual pattern of swimming behavior of these species. Only single individuals of each of these species, all of which were found to occur in subsurface tows, were collected in the neuston, and they may be considered outliers of the normal population. The Relationship of the Occurrence of Gammaridean Amphipods in the Plankton to Their Availability as Prey Items Amphipods were an important part of the diets of several demersal fish analyzed from the B area during the course of this study (Sedberry, 1980). The demersal fish appeared to feed selectively on corophiids, and to a lesser extent on ampeliscids, consuming them far out of proportion to their numbers in the sediment (Boesch, 1979). Boesch suggested that these species are more vulnerable to predation than more deeply burrowing or smaller infaunal forms. Corophiids and ampeliscids were also found in the water column and it is possible that their emergence activity may increase their availability as prey items. An examination of Sedberry’s results shows that these species were important prey items to fish no matter what their densities or if they were present or absent in the water column. Argissa hamatipes and Monoculodes edwardsi may be available as prey items because of their planktonic occurrence. They were consumed by the demersal fish examined by Sedberry despite being absent or with low mean abundance in the benthos. M. edwardsi was the most abundant gammaridean amphipod in the water column at B5 during the summer. It was identified from fish stomachs during the summer but only in those 105 fish in which Parathemisto gaudichaudii was also an important prey item. The co-occurrence in stomachs of M. edwardsi and 1^. gaudichaudii, a pelagic amphipod, suggests that M. edwardsi was consumed while it was in the water column. Argissa hamatlpes was first collected in the water column at B5 during the winter, when it was also identified as a prey item. It was not found in the benthos until the summer. Concluding Remarks Gammaridean amphipods were collected in the plankton at all stations with the overall pattern of distribution dependent upon benthic and zooplankton distributions. Amphipods which occurred in low abundances in the benthos but were frequently in plankton collections were found at all areas of the shelf except the slope stations and L4. Their occurrence (except for the ubiquitous Monoculodes edwardsi) was somewhat sporadic, not consistently at any station over the course of the sampling period. Amphipods were most frequently collected at L2, B5, N3, and E3, stations where corophiids and ampeliscids were dominant components of the benthos. The sub surface gammaridean amphipod fauna at L4, L6, and J1 was truly depauperate. Except for the collection of Ampelisca vadorum in the plankton during the fall, gammaridean amphipods were absent from bongo tows at L4. Their absence is somewhat curious since the benthic fauna included corophiids and ampeliscids. No amphipods were dominant members of the benthic communities at the slope stations and they were also absent from the subsurface water column. Grant (1979) found that three hydrographic phenomena were major forces influencing the composition of Middle Atlantic Bight 106 zooplankton. The influence of these factors could also be seen in the neustonic occurrence of gammaridean amphipods. These were the general southward drift of shelf waters (Calliopius laeviusculus, Gammarellus angulosus), the passage of Gulf Stream rings (Synopia ultramarina, Sunamphitoe pelagica, Hyale sp.) and the presence of a coastal boundary layer (Microprotopus raneyi, Ampithoe longimana). The occurrence of gammaridean amphipods in the water column has received increased attention in recent years in studies of "demersal zooplankton" of nearshore waters (Robertson and Howard, 1978; Alldredge and King, 1980; Hammer, 1981). These studies review the diverse purposes and benefits of emergent activities of benthic invertebrates which include facilitation of mating, dispersal to new habitats, and adaptation to short-term variation in microhabitat. Gammaridean amphipods of the continental shelf derive similar benefits from being transported in the water column. Emergence activities related to reproductive behavior was most definitely observed in the ampeliscids and Trichophoxus epistomus. Short-range dispersal via the water column by corophiids aided their recolonization of small defaunated areas and their coexistence with competitively superior species. Monoculodes edwardsi was capable of long-range dispersal enabling it to exploit disturbed habitats. Long-range dispersal was also achieved by Rhachotropis inflata and Melphidippa sp. A which were transported in the southwestemly currents. A hazard of the swimming activities of Monoculodes edwardsi and Argissa hamatipes was increased availability as prey. The actual impact of gammaridean amphipods on zooplanktonic communities of the shelf was probably insignificant. Their occurrence 107 in some surface samples was indicative of important water movements influencing the faunal composition of zooplankton communities. 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Zoogeographic affinities of northeastern North American Gammaridean Amphipoda. Bull. Biol. Soc. Wash. No. 3: 256-282. Watling, L. and D. Mauer. 1972. Marine shallow water amphipods of the Delaware Bay Area, U.S.A. Crustaceana, Supp. 3:251-266. Weston, D. 1979. Distribution of macrobenthic Mollusca and Amphipoda in relation to a sharp thermal front: Cape Hatteras region, North Carolina. M.A. Thesis, College of William and Mary, Williams burg, Virginia. 120 pp. Whiteley, G. C. 1948. The distribution of larger planktonic Crustacea on Georges Bank. Ecol. Monogr. 18:235-265. Williams, A. B. and K. H. Bynum. 1972. A ten-year study of mero- plankton in North Carolina estuaries: Amphipods. Ches. Sci. 13:175-192. VITA CATHY JO WOMACK Born in Baltimore, Maryland, 11 October 1952. Graduated from Lansdowne Senior High School in 1970. Earned B.A. from the University of Maryland Baltimore County, Catonsville, Maryland in June 1974. Employed as a Laboratory Scientist at Springfield Hospital Center, Sykesville, Maryland from 1974 to 1976. Entered the School of Marine Science of the College of William and Mary, Gloucester Point, Virginia in 1976. Presently employed at the University of Georgia, Athens, Georgia. 115M 3 04 ■ a O CO O ■ a £ £ « X h Ml 41 3 0) a y —4 o o CO 4) CD 07 § CQ u 07 <8 07 S 9 e S a 'fH H cc