THE EFFECTS OF SEA STAR AND WALRUS ON BIVALVES IN NORTON SOUND,

Allan K. Fukuyama San Francisco State University 1985

groenlandicus, and hyperborea are abundant in sub- tidal, soft sediments in Norton Sound, Alaska. All exhibit distinct size distributions with many small, recent- ly settled , few of intermediate size, and a dis- tinct adult population. Predation by sea stars, especially

Asterias amurensis, is the most likely explanation for this size distribution. Gut content examinations of showed it to be an important predator on bivalves <10 mm.

There were several refuges from sea star predation. Burrowing and leaping are used by Yoldia and , respectively, while Macorna and Mya use a depth refuge, and a related size refuge, to avoid predation by sea stars. When bivalves reach approximately 40 mm, they become sub-

jected to seasonal predation by walruses. The preferred

prey is Serripes since it is a shallow burrower. Populations

of this species have been reduced; walruses now app.ear to be

feeding mostly on populations of Mya and Macoma. ACKNOV.'LEDG11ENTS

A study in remote areas could only be accomplished with the aid of many organizations and people. Foremost I thank the Benthic Bubs, M. Silberstein, P. Slattery, E.

O'Connor, and J. Oliver for three fun-filled field seasons.

Others who helped with field work included J. Oakden, J.

Beine, G. Van Dykhuizen, D. Canestro, B. Stewart, R. Kvitek,

A. Baldridge, R. Clevenger, and B. Matsen. The Alaska

Department of Fish and Game in Nome, Alaska kindly provided use of a boat and facilities and I especially thank R.

Nelson. The University of Alaska Institute of Marine

Science, Seward, Alaska, under the directorship of D.

Dieter provided laboratory space and accommodations. The crew of the R/V Alpha Helix, captained by S. Bailey, provided us with an atmosphere condusive for scientific exploration and I thank the crew for their patience and help. R. Kvitek, M. Silberstein, V. Hironaka, T. Herrlinger,

J. Oliver and J. Nybakken revised early drafts. I am

grateful for the editorial help and friendship of R. Gill

and c. Handel, who provided different perspectives. K.

Lohman gave advice with statistics and I give special

thanks to S. Baldridge for procuring many obscure and

sometimes hard to get references. I also thank the members

of my thesis committee, J. Nybakken, B. Wursig, R. Larson,

and a special thanks to "Oli" Oliver, who obtained funding

ill for this project, organized the expeditions, provided many hours of ear-bending discussions and showed me the science of banditry. Financial support was provided by the National

Science Foundation Grant #DPP 8121722 and the San Jose State University Foundation. Thanks also go to the many friends at Hoss Landing Marine Laboratories and to my parents and family.

v. TABLE OF CONTENTS

INTRODUCTION • ...... '"" .. 1 METHODS • • • . . . 3

Study Area 3 Field Methods • • . . • . • • . • • • • • • . . . 6 Laboratory Methods . . • 9 RESULTS . • • .12 Bivalve Densities and Population Structure • 12 Predators and Their Effects on Bivalves ••• 25 Ophi uroids...... 25 Asteroids ...... 25 Behavioral Observations • • • • • • • • 27 Walrus Predation •••.••••••••••.• 30 Behavioral Refuges •••.••••••••••.• 33 Depth Refuges ...... • 34 Size Refuges ...... 36

DISCUSSION • • • • • . . • • . . • • • • • • • • •• 38

Population Structure • • • • • • • • • • • • • • .38 Spawning and Settlement • • • • • • • 43 Spatial Refuges • • • • • • • • • • • • • • • • • 44 Behavioral Escape Responses • • • • • • • • • • • 48 Size Refuges • • • • • • • • • • • • • 49 Walrus Predation ...... • SO

CONCLUSION • • • • 56 LITERATURE CITED • ...... 58 APPENDIX • ...... • • .. .. 66

vi LIST OF TABLES

TABLE Page 1. Densities (mean nurnber/m2 + 1 SE) of the bivalves Mya truncata, Macorna calcarea, , and Yoldia hyperborea from three sites. • • • • 21 2. Mean abundances (+ 1 SE) of the asteroids and Lethasterias nanimensis from transects at three sites in Norton Sound, June-July 1982.. 26

3. Stomach contents of Asterias examined in the laboratory •.• • • . • 28 4. Prey items found on the oral surface of Asterias is examined in the field. 29

5. Numbers of bivalves eaten by Asterias amurensis in aquaria with and without sediment. Bivalves are separated into two size groups -- Macorna calcarea, Yoldia hyperborea, Lyonsia sp., Mya truncata, and Thyasira flexuosa range in size from 1 6 mrn and Macoma balthica range in size from 6-10 mm. • • • . 35

6. Time for Asterias arnurensis of three size classes to capture and eat bivalves of various species and sizes.. • • • •• 37

7. Maximum depths to which nine species of bivalves have been found burrowed •• • 47 8. Depths at which six predators have been found to dig to obtain bivalves. • • • • • • 51

vii LIST OF FIGURES

FIGURE Page

l. Location of study sites in Norton Sound, Alaska...... 4

2. Sizes of Yoldia hyoerborea collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. . • ••• • 13 3. Sizes of Macoma calcarea collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983 ...... • 15 4. Sizes of Mya truncata collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. . • • • . . • • • • • • • .17 5. Sizes of Serripes groenlandicus collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. . • • • • • • 19

6. Comparison of sizes of Mya truncata from the dead shell record at Cape Nome in 1981 and 1982 with live bivalves collected by digging from 1982 •••. 23 7. Combined zes of three bivalve species consumed by walruses from Cape Nome and Sledge Island, Alaska in 1981 and 1982 •••••••••• 31

vii.i INTRODUCTION

Studies of the of Arctic soft-bottom subtidal communities have been limited since these areas are remote and frequently inaccessible due to ice cover or inclement weather conditions. However, epifaunal and infaunal animals of the eastern are known from grab, dredge and trawl samples (McLaughlin 1963;. Fay et al. 1977;

Wolotira et al. 1977; Stoker 1978; Feder and Jewett 1978,

1980). These studies have described distribution and abundance patterns of invertebrates, including bivalves, but little is known about factors structuring bivalve communities, particularly the effects of predation on these communities.

The bivalves, Mya truncata Linne, Macoma calcarea

(Gmelin), Serripes groenlandicus (Bruguiere), and Yoldia hyperborea Loven form an important component of the infauna, especially as food for predators such as walruses (Vibe

1950; Fay 1982), bearded seals (Lowry et al. 1980), fishes

(Feder and Jewett 1978, 1980; Jewett and Feder 1980), large invertebrates such as king and snow crabs (Feder and

Jewett 1978, 1980, 1981) and sea stars (Kim 1969, Feder and

Jewett, 1980). Food analyses of these predators have shown

that bivalves of various sizes are preyed upon, but there

has been little quantitative information on the effects

that such predation may have on bivalve population

1, structure. 2 The Norton Sound region is utilized by walruses during their spring northward migration (Fay 1982). Thus, bivalve populations are subjected to intense seasonal predation by walruses, coupled with predation by sea stars, particularly

Asterias amurensis. There is little information on the effects that such predation have had on bivalve populations.

This study was initiated on the biva.lves inhabiting soft-bottom areas of Norton Sound, Alaska in order to seek answers to the following questions: [1] what determines the size structure of bivalve populations? [2] what animals prey on these bivalves and what sizes are utilized by these predators? [3] what effects do these predators have on various life stages of the bivalves? and [4] what refuges from predation exist for these bivalves? METHODS

STUDY AREA The seafloor of Norton Sound consists of extensive flat ~reas of fine and muddy sand (Sharma 1974).

Sedimentary characteristics of the area have been descp_bed by Nelson and Creager (1977) and Larsen al.

(198U, who observed distinct layering of the sediment with a soft silty-clay layer extending 10-15 em deep and a consolidated hard-packed clay layer below that. The area is co7ered with sea ice from the fall until late spring, but rD ice gouging of bottom sediments occurs in this area

(Lars=n et al. 1979). Current patterns are influenced by runoff from the Yukon River as well as by tides and winds.

Tbe p:edominant current flow is .from east to west and is

detailed in Muench et al. (1981). Bottom currents of

approximat~ly 1-2 knots were occasionally encountered

duri~ this study. Water visibility was usually 0.5-3 m

and bottom temperatures were l-l0°C.

Four study sites were established within Norton

Sound: the primary site (Cape Nome) was about 24 km

southeast of Nome (64°15'N, 165aOO'W); another site was

near Sledge Island (64°30'N, 166°15'W)(Fig. 1). These two

sites were feeding grounds for migrating walrus (Oliver et

al. 1983). Secondary sites were sampled at Square Rock (64°

30'N, 163"40'W), and Golovin Bay (64°20'N, 163°00'W)(Fig. 1).

3 4

Figure 1. Location of study sites in Norton Sound, Alaska. 5

. - 0 • z :!'l < w "'0 !!;"' ~·- u < <.. <"' ...1"' < ;;; u "'0 u ;: u \ a: ![ "\, < • •• (} ·" •. < w • " <( "z • ... 'q 0 "'m .ll c: • .. :"' o iE ..::> :j- ~ '!> " ""--. ~ Q CJP 6 Depths at the sites were as follows: Cape Nome, 18.3 m; Sledge Island, 24.4 m; Square Rock, 12.2 m; and Golovin

Bay, 6.1 m. The first three sites were characterized by similar silty-sand sediments, but the Golovin Bay region

consisted of coarser sandy sediments.

FIELD METHODS At each study site quantitative information was

collected on size structure and densities of the major

bivalve species and their invertebrate predators. All

field work was done using SCUBA. To determine size

patterns for populations of the major bivalve species, one

of two methods was used. Smaller bivalves were sampled

with a coffee-can core which sampled an area of 0.0075 sq

m. Cores were obtained in June, 1981 and June-July, 1982.

The second method sampled larger areas. A metal cylinder

which delineated an area of 0.25 sq m was placed on the

bottom. Then a plastic scoop with a 1-mm mesh bag attached to the end was used to remove the upper 10-15 em

of the sediment. All animals in the samples were sieved

through a 0.5 mm screen and preserved in a 10% solution of

formalin. Bivalves were later sorted, counted, and

measured to the nearest mm, and ophiuroids were sorted and

counted. Size-frequency histograms combined bivalves from

both cores and scoops. Scoop samples were obtained in

June-July, 1982 and July, 1983. An attempt was made to

sample at Cape Nome in September, 1982, but inclement 7 weather prevented scoop samples from being obtained. Only

qualitative information and six quantitative cores were

obtained at that time.

An alternate method was necessary to determine the

densities and size-frequencies of large, deep-burrowing

Mva truncata. This species has a large siphon that

emerges from the sediment and creates a distinctive

keyhole-shaped hole when the siphon is retracted. Density

estimates were obtained by counting all siphons and holes

of this species within 1 sq m quadrats sampled haphazardly

throughout the study areas. Other studies have also used

this method to obtain density estimates for large bivalves

(McErlean and Howard 1971; Hines and Loughlin 1980). An

underwater sampling device similar to a clam gun was used

to verify identification of these holes from the surface.

The device was placed over a bivalve siphon hole and water

was jetted into the hole. This created an excavation from

which live bivalves could be removed and identified.

Holes of different shapes and sizes were checked until I

felt I could count Mya truncata holes and siphons

accurately. The size structure of these larger bivalves

was determined by measuring these individuals. In

addition, as each was unearthed, a ruler was used

to measure the depth at which the posterior, siphonal edge

of the bivalve was situated.

Densities of asteroids and numbers of bivalve shells 8 discarded by predators were counted along haphazardly placed 50 m belt transects. Asteroids and shells were enumerated along the line and for l m to either side of the line. At least 19 replicate transects were done at each site in June-July, 1982.

The diets of the asteroid Asterias amurensis from

Cape Nome (1982) and Golovin (1983) were examined. Live sea stars were turned over as they were encountered in a straight line swim and prey items were observed on the tube feet or in the oral cavity were recorded. The size of each sea star was classified into three groups based on the longest length from arm to opposite arm (D) : small

(D=l80 mm).

The gut contents of 116 Asterias amurensis collected from the Cape Nome area on lB-22 June 1982 were also examined in the laboratory. Each animal was measured (D) and an incision was made to expose the oral cavity and gut.

All large prey items were removed, counted and measured. Unidentified "mush" was examined under a dissecting microscope for the presence of identifiable parts such as setae, worm tubes, spicules and ossicles.

The gut contents of 15 ophiuroids collected from Cape

Nome on 24 June 1982 were examined by similar methods.

The sample examined animals of both sexes and with disc diameters ranging from 3-6 mm. Females were not carrying eggs at this time. 9 LABORATORY.METHODS

Laboratory experiments were conducted at the

University of Alaska Marine Science Center in Seward,

Alaska, from September 14-24, 1982 and from July 13-19,

1983. The asteroid Asterias amurensis, ophiuroid

Diamphiodia craterodmeta, and bivalves of several species were obtained off Cape Nome in September 1982 and transported to Seward. In addition, Macoma balthica were obtained intertidally from Resurrection Bay at Seward to augment the experiments. All animals were maintained in lcrge tanks with circulating seawater at 9-l0°C.

The ophiuroid Diamphiodia craterodmeta was considered a potential predator on small bivalves. Ten ophiuroids with disk diameters 2-10 mm were placed into a 1 L beaker along with a Nucula tenuis (1 mm), a Mya truncata (3 mm), a Yoldia hyperborea (3 mm), and a Macoma calcarea (2 mm).

Observations were made periodically for 3 days to see if any of the bivalves were eaten.

All other experiments examined the sea star Asterias amurensis as a predator on bivalves. Each sea star was used for only one experiment and was deprived of food at

least•3 days before each experiment.

An·initial experiment tested the responses of bivalves to Asterias amurensis. A single Yoldia hyperborea was placed into an aquarium containing a fine'

sand-silt layer 10 em deep, and touched with an arm of a sea star. Behavioral responses and time to complete burial were recorded. After complete burial, the Y. hyperborea was excavated, placed into a different aquarium with a similar sand-silt layer and touched with a probe. Burial time was again recorded. This experiment was repeated with three individuals. To assure that the burial response was not biased by the order of stimulation, each test reversed the order of stimulation. In addition

I observed the behavioral responses of small (<5 mm) Mya truncata, Y. hyperborea, Macoma calcarea, and Macoma balthica to the touch of an A. amurensis arm. Larger sizes (>SO mm) of M. truncata, Mya arenaria, and Serripes groenlandicus were also touched by A. amurensis and responses were noted.

Another experiment evaluated the hypothesis that bivalves found refuges from predation by burrowing into sediments. Ten bivalves were placed into an aquarium containing sediment 5 em deep, and were allowed to burrow, while another aquarium had ten bivalves without any sediment. Each trial utilized either all Macoma balthica or a mixture of the bivalves Macoma calcarea, Yoldia hyperborea, Lyonsia norvegica, Mya truncata, and Thyasira flexuosa. Single equal-sized Asterias amurensis were added to each aquarium. Controls had ten bivalves in sediment and ten bivalves out of sediment without the predators. This experiment was repeated seven times. A 11

Mann-Whitney U-test was used to test differences in number of bivalves surviving after five days in each aquarium.

A final experiment evaluated the idea that larger bivalves may have a size refuge from Asterias amurensis.

In each of a series of trials, an individual A. amurensis was placed into an aquarium or large wash bucket with an individual Mvtilus edulis or Mya arenaria. Lengths of bivalves ranged from 22-54 mm while diameters of sea stars

ranged from 113-271 mm. The time it took for each asteroid to attack and to eat each bivalve was recorded.

If a bivalve was attacked but not eaten within 3 days the

experiment was terminated and size was assumed to be an effective refuge for the bivalve against a sea star of -

that size. RESULTS

BIVALVE DENSITIES AND POPULATION STRUCTURE

Bivalve size structure was generally consistent among years and between those study sites with a silty-clay sediment. The populations of Yoldia hyperborea, Macoma calcarea, and Mya truncata were all dominated by individuals less than 10 rnrn in length (Figs. 2,3,4). In contrast, populations of Serripes groenlandicus showed annual variation in both abundances and size structure. In

1982, there were low numbers of small animals less than 10 mrn at Sledge Island, and a large population of juveniles

from 10-15 mm in length (Fig. 5). At the Cape Nome and

Square Rock sites no S. groenlandicus of any sizes were

found. Cores and scoops taken in July and cores from

September 1982 at Cape Nome were also devoid of S.

groenlandicus and no animals were found in an additional

qualitative scoop sample. However, cores from 1981 showed that s. groenlandicus from 1-8 rnrn were present in the Cape Nome and Sledge Island areas and scoops from 1983 revealed

that low numbers of this species were now found at Cape

Nome but not at Sledge Island (Table 1, Fig. 5).

Macoma showed a decrease in abundance from

1981 to 1983 (Table 1). No ~ive large M. calcarea were

found although large (>45 rnrnl shells recently discarded and

presumably eaten by walruses were seen throughout the area.

}2 13

Figure 2. Sizes of Yoldia hyperborea collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. 14

~ hyperborea

Cape Nome SJedge Island

50 1981 40

30 n=17

20

10

10 30

1982.

1983

n=15 10

10 20 30 Size (mm) Size {mm) 15

Figure 3. Sizes of Macorna calcarea collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. 16

Mac om a calcarea Cape Nome Sledge Island 30 30

• ..~ "C 20 > 1981 '6 .E 0=51 o•24 0 ~ 10 "E z~

10 20 30

o=174 ..• ~ "C > '6 .E 1982 0 i "E z~

30 • 20 20 ••~ "C > '6 n=62 0=27 .E 0 10 1983 10 i "E z~

10 20 30 10 20 30 Size (mm) Size (mm) 17

Figure 4. Sizes of Mya truncata collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. 18

My a truncata

Cape Nome Sledge hland

• 20 Ci.., ;;" n=12 n=6 .=:'i5 1981 0 10 1 ~

"'E" z" 10 20 10 20

;;;.. 20 ..," n=59 n=45 'i5"' .5 1982 10

"~ "'E z" 10 20

30 n=59 n=S ;;;.. ..," ;; 20 'i5 .=: 1983 0 ..,~ E 10 z"

20 10 20 Size {mm) Size (mm! 19

Figure 5. Sizes of Serripes groenlandicus collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. 20

Serripes groenlandicus

.!1 Cape Nome Sledge Island " 20 20 ";;" ~ .5 n=17 n=13 0 1981 ~ ...~ 1 10 E z"

10 20 30 10 20 30

iii" 20 :!1" n=50 :;;> 1982 .5 ao indlvlduelc found 0 10 ...:. E z" 10 20 -30

.,1i" 2 " n=S ~ .5 1983 no lndlvlduol• fovnd 0 10 :. D E :z" _rf::: 10 20 30 Size (mm) Size (mm) 21

Table 1. Densities (mean number/m2 + 1 SE) of the bivalves Mya truncata, Mac6ma calcarea, Serripes qroenlandicus, and Yoldia hyperborea from three sites. (-)-no sample taken.

1981 1982 1982 1983 cores cores scoops scoops

M. truncata -Cape Nome 114.3±33.7 19. 0±13. 9 11.2±1.5 26.2±4.9 Sledge Is. 114.0±53.7 54.4±16.7 18.0±3.3 6.0±2.7 Square Rk. 30.0±4.9

M. calcarea Cape Nome 503.5±59.4 66.5±34.7 30.9± 3.7 27.6±4.1 Sledge Is. 456.0±57.0 326.5±42.6 70.8±12.5 21. 6±3. 7 Square Rk. 12.7± 2.4 s. groenlandicus Cape Nome 161.5±39.9 0 0 4 .0±1.5 Sledge Is. 247.0±79.1 42.3±16.1 20.0±3.9 0 Square Rk. 0

Y. hyperborea Cape Nome 712.5±123.6 110.8±45.8 48. 0±4. 4 41. 8±10.9 Sledge Is. 285.0±84.3 102.8±19.4 24.0±5.4 13.6± 3.9 Square Rk. 15.3±3.0

Sample Size Cape Nome 14 12 21 9 Sledge Is. 7 22 10 5 Square Rk. 6 22

Few animals of intermediate sizes (10-50 mm) were found

(Fig. 3). Yoldia hyperborea showed a distinct size distribution

at Cape Nome in 1982 (Fig. 2). Size groups from 1-10 mm

and 15-25 mm were present. I believe that a similar

distribution may also have been present at the other sites,

but small sample sizes precluded detection of this pattern.

The abundance of Y. hyperborea also decreased from 1981 to

1983 (Table 1).

Mya truncata showed a large decrease in numbers from

1981 to 1983 (Table 1). The population structure shov;ed a

distinct group of animals less than 10 mm in size (Fig. 4).

No individuals were found between 15-60 mm, but individuals

were present in the 60-95 mm range (Fig. 6). Collections

of dead shells also showed that most of the larger sizes

were 6.0-9 5 rnm (Fig. 6 l . Counts of siphons at Cape Nome

revealed a population density of 1.14 per sq m (SD=0.996,

n=l55) while the Sledge Island site showed a density of

1.09 per sq m (SD=l.657, n=35). The vertical distribution

of six M. truncata measured at Cape Nome showed it to occur

from 22-27 em below the surface of the sediment with an

. average depth of 24.1 em.

Although large live specimens of Macoma calcarea and

Serripes groenlandicus were not found, the population

structure of these bivalves may be approximated from

collections of dead, recently discarded shells in the area 23

Figure 6. Comparison of sizes of Mya truncata from the dead shell record at Cape Nome in 1981 and 1982 with live bivalves collected by digging from 1982. 10 1981 DEAD SHELL RECORD ne84

.. ;;; 10 20 30 40 ~ .:'1 :;;~ .5 ... 10 1982 DEAD SHELL RECORD ...0 n•104 0 "'e ll:•

10 20 30 40 00 80 ro BO DO 100

10 1982. LIVE nc43

10 30 •o 60 80 IOO Size (mm) 25 which showed that M. calcarea shells were larger than s. groenlandicus shells. A problem with this assessment is the questionable assumption that predators randomly feed on various sizes of these two species.

PREDATORS AND THEIR EFFECTS ON BIVALVES

Ophiuroids The ophiuroid Diamphiodia craterodmeta was very abundant at Sledge Island (2932/sq m, SD=lBOO.O, n=9), s'omewhat less abundant at Square Rock (1235/sq m, SD=455.6, n=6), and relatively sparse at Cape Nome (175/sq m,

SD=l22.6, n=13).

I found no food items in the guts of 15 Diamphiodia craterodmeta.

In laboratory exReriments, Diamphiodia craterodmeta did not feed upon live bivalves 1-3 mm in size. However, when a dead bivalve was placed into the container there was immediate movement of several ophiuroids toward the dead bivalve. After 15 minutes, five of the ophiuroids had converged on the bivalve and the other five ophiuroids were moving towards it. After several hours all of the soft tissue of the bivalve was consumed.

Asteroids Asterias was an abundant epifaunal animal in the area. Mean densities at Cape Nome were twice that at

Sledge Island and Square Rock (Table 2). Lethasterias 26

Table 2. Mean abundances (± 1 SD) of the asteroids Asterias amurensis and Lethasterias nanirnensis per 100 rn2 at three sites in Norton Sound, Alaska (June-July 1982.)

LOCATION n A. amurensis L. nanirnensis

Sledge Is. 27 17.8 + 35.8 1.1 ± 4.7 Cape Nome 43 34.7 ± 112.7 0.9 ± 8.5 Square Rock 19 14.4 ± 33.1 0 0 27 nanimensis was found in low numbers with densities around 1 per 100 sq mat both Cape Nome and Sledge Island (Table 2).

The digestive tracts of 116 Asterias amurensis were opened in the laboratory. A majority were empty (n=78).

Those with food ( 8) contained tunicates, , crustaceans, ophiuroids, bivalves and unidentifiable organic material (Table 3). Yoldia hyperborea was found in nine guts and ranged in size from 4-10 mm. A single 3 mm

Mya truncata and one 5 mm Macoma calcarea were also found in the stomachs.

A total of 16 0 Asterias arnurensis from Cape Nome "'ere examined in the field. Food items found in 46 animals included polychaete tubes, tunicates, bivalves, and unidentified organic matter. Four animals were found scavenging tissue from bivalve shells and one animal was cannibalizing another Asterias amurensis (Table 4). A total of 17 bivalves were found on the oral surface of sea stars. All of these bivalves were Yoldia hyperborea less than 10 mm in length.

Fifty-five Asterias amurensis were examined from

Golovin Bay. Most were feeding on bivalves, primarily

Tellina lutea (Table 4).

Behavioral Observations

Asterias arnurensis was observed in laboratory experiments using its tube feet to dig for buried bivalves. In one instance a sea star (D=215 mm) extended its tube 28·

Table 3. Stomach contents of Asterias amurensis examined in the laboratory. Seastars were divided into three size classes (see text). n=ll6 total; P=present in trace amounts; F.O.=frequency of occurrence.

Food Small (n=SB) Medium (n=28l Large (n=30) n F.O. n F.O. n F.O. ------~------EMPTY 49 13 16

POLYCHAETA Glycinde sp. l 1 2 2 Myriochele sp. 18 4 18 4 setae p 1 p 3 p 1 tube worm 1 1

CRUSTACEA Ostracod 1 l l 1 Cumacea l l Dulichia sp. l 1 shrimp exoskeleton 1 1

HOLLUSCA Yoldia hyperborea 4 4 4 4 1 1 Hya truncata l 1 Macoma calcarea 1 1 Gastropod l 1

ECHINODERMATA Strongylocentrotus sp. l l Ophiuroid l 1 Ophiuroid spicule p 1 A. amurensis ossicle p 1 p 2

TUNICATA Pelonaia sp. 2 2 5 3 2 2

UNIDENTIFIED Unid. organic matter P l p 6 p 6

------~ 29

Tabl= 4. Prey items found on the oral surface of Asterias amurensis examined in the field at Cape Nome (1982) and Golovin Bay (1983). Total number of prey found in all sea stars (Number Prey) and the number of sea stars containing each prey item (Number Sea Stars).

Food Cape Nome Golovin n=l60 n=55

l~umber Number Number Number Prey Sea Stars Prey Sea Stars

EMP'IT ll4 3

POLYrnAETA. tu3le worm 6 6 Pe;::tinaria sp. 4 4

MOLiiiTSCA ! .fuyperborea 17 13 Tell ina lutea 54 39 SEi.sula sp. 1 1 2 2

ECHJNODERMATA A.amurensis 1 1 l 1 sal!ld dollar 1 1

TUNI

SCAVENGING 4

Unid. org. matter p 5

P=present in trace amounts 30 feet up to 1 em into the sediment in order to capture buried Macoma balthica. A Protothaca staminea which was b~ried 5 em in the sediment was also excavated by an A. amurensis. However, a Mya arenaria which was buried 20 em could not be removed by the sea star within a three-day period, though a pit was dug to a depth of 5 em. In field observations, several sea stars were observed in pits estimated to be up to 1 em deep. The feeding behavior of Asterias amurensis was observed several times. When feeding upon bivalves between 1-10 mm, the sea star would touch the prey with a ray and then use its tube feet to move the bivalve into the oral cavity. Up to five bivalves were placed into the oral cavity at one time. When feeding upon bivalves over 30 mm, the sea star would move its oral surface on top of the bivalve and assume a humped position. It would then extrude its stomach and insert it into the bivalve. The

soft tissue of the bivalve was consumed and the empty shell

discarded.

Walrus Predation

The effects of~walrus predation on size and structure of bivalve assemblages can be reconstructed from analyses

of the dead shell record. Generally, greater numbers and larger sizes of Mya truncata were eaten than of either Macoma calcarea or Serripes groenlandicus. This trend was evident for both 1981 and 1982 (Fig. 7). Walruses have a ~1

Figure 7. Combined sizes of three bivalve species consumed by walruses from Cape Nome and Sledge Island, Alaska in 1981 and 1982. Maooma calcarea n .. 40 1981 "I 9 n [L, nyDfl?[L.;

1flD D .. " !! 101 ,.., "1 ..!1 ;: ! ~n n: 66 ;; Mya truncola .!l at 1981 0 nn n p6(:'1-,;t,np n -~ 9 .a~ E ""' 1 63 z" 10 1082

earrlpu groanlandloua n =15 7 10 1981

1962 n • 31

Cl[],_M I q "! I I 1 ~ 1o •• ao •• GO ••• 10 no •• 100 Slzo (mml

w N 3_3 selectivity towards larger sizes. When live animals of M. truncata and the dead shell record were compared, it was evident that bivalves greater than 60 mm were predominately fed upon and no shells less than 35 mm were found as prey (Fig. 6).

Behavioral Small bivalves were tested for an escape response to Asterias amurensis. When small Yoldia hyperborea (5-10 mm) were touched with an arm of A. no escape response could be elicited (n=5). Similarly, a 10-mm Mya truncata and small (2-9 mm) Macoma and Macoma balthica did not respond when they were touched by an A. amurensis (n=24).

When a large hyperborea (19-21 mm) was placed into a tank with a layer of sediment and touched with a probe it responded within four minutes (x=l55 seconds,

SD=99.9, n=3) by extending its large foot into the sediment and rapidly burrowing. It took about 12 strokes of the foot to completely disappear into the sediment. -When an

Asterias amurensis was introduced into the tank and an arm touched the Y. hyperborea the response of the bivalve was quicker (x=lO seconds, SD=5.0, n=3). The foot expanded within several seconds and the bivalve was completely buried within 15 seconds. The depth to which these Y. hyperborea were burrowed was measured and all were found 34 to be at the bottom of the 4.5 em sediment layer. It is conceivable that they may be able to burrow and sustain themselves at even greater depths.

Small sizes (<5 mm) of Mya truncata, Yoldia hyperborea, Macoma calcarea, and Macorna balthica as well as larger sizes of Mya truncata and Mya arenaria (>50 mm) did not respond to the presence of Asterias amurensis. In contrast, s=rripes groenlandicus exhibited a leaping response. First, the large foot was extruded and bent into a V-shape. Then the' animal straightened its foot and a rapid leapic,g movement was generated. Up to 14 leaps in rapid succession were recorded for one animal.

Depth Refuges

Over periods of 3-5 days Asterias amurensis of three different sizes were not able to excavate a 19-mm Yoldia hyperborea buried at a depth of up to 15 em. When a 22-mm

Y. hyperborea was placed into an aquarium without sediment an ~· amurensis (R~88 mm) was able to capture and eat the bivalve within three hours.

Depth of burial in the sediment provided a refuge from sea star predation. When Asterias arnurensis was present, there was greater mortality of bivalves in aquaria without sediment compared to aquaria with a 4.5 em layer of sediment (Table 5). This difference was statistically significant for the larger (6-10 rnm) bivalves (p

Whitney U-test, n~4). It was not significant for smaller 35

Table 5. Number of bivalves eaten (summation of seven experiments) by Asterias amurensis after 2-6 days in aquaria with and without sediment. Bivalves were separated into two groups by size. Individuals of Macoma calcarea, Yoldia hyperborea, Lyonsia sp., Mya truncata, and Thyasira flexuosa were 1-6 mm while Macoma balthica were 6-10 mm.

Number Number Offered Eaten

MIXED BIVALVES (1-6 mm)

Sediment present 30 3 (10%)

Sediment absent 30 10 ( 3 3%)

Macoma balthica (6-10 mm)

Sediment present 40 12 (30%)

Sediment absent 40 32 (80%) ~6 (1-6 mml bivalves (O.lO

Size Refuges

The results of tests for a size refuge for bivalves produced few general trends. Bivalves up to 43 rnrn could be eaten by any size sea star. Small sea stars could capture but not eat bivalves that were 53-54 mm in length. In general, the smaller sea stars attacked bivalves more quickly but took longer to consume them than the larger sea stars. Within each size class of sea star, predators were able to digest smaller bivalves faste~ than larger bivalves

(Table 6). 3l

Table 6. Time for Asterias amurensis of three size classes to capture and eat bivalves of various species and sizes. NE=not eaten.

Sea star Bivalve Prey !:I ours to Diameter Species Size Capture Eat (mm) (mm)

SMALL 123 Mya arenaria 40 0.1 31 113 Mytilus edulis 43 0.1 42 115 My a arenaria 53 0.2 NE 123 My a arenaria 53 0.5 NE 130 My a arenaria 54 5.5 NE MEDIUM 131 Yoldia hyperborea 22 4.0 4+ 134 Mytilus edulis 30 0.9 15 169 Mya arenaria 54 2.0 30 LARGE 246 Myti1us edu1is 42 16 25+ 271 Myti1us edulis 43 11 20 211 Myti1us edulis 44 2.0 12 DISCUSSION

The ilforton Sound area of Alaska supports a rich bivalve fuuna. The bivalves studied generally show

consistent yearly recruitment patterns. The primary predators on these bivalves are sea stars and walruses.

Many sea 3tars are known predators on bivalves and their

diets hav~ been reviewed by Sloan (1980). Bivalves are

considereK the primary prey for walruses. This has been

known formany years by the natives, and has been

docurnenteif from stomach analyses by Vi be ( 1950), Fay et al.

(1977) an[ Fay (1982).

Other predators are known to adversely affect settling

bivalves. Some tube-dwelling polychaetes feed on settling

bivalves ((Breese and Phibbs 1972; Daro and Polk 1973). In

the Balt~, arnphipods are important consumers of settling

bivalves ((Segerstrale 1962; 1965; 1973) and ophiuroids are

also knoWJL to feed on settling. bivalves (Thorson 1955;

Feder 198]). In addition, filter-feeding bivalves also

feed upon small, settling bivalves (Hancock 1970) and

adults mas~ preclude spat. settlement by substrate

conditioning (Stoker 1978). Norton Sound does not have

large nunfuers of. most ofcthe aforementioned predators that

are known to affect settling bivalve spat (Oliver et al.

1983). Aa exception are ophiuroids which form dense beds

in some areas. Unfortunately, the guts of all ophiuroids

3.8 39 examined were empty. Small-sized bivalves may be eaten by other large invertebrates and fishes. Fishes in the Norton Sound area are generally sparse due to cold water temperatures and low (Stoker 1978). Of the few species able to tolerate low water temperatures, none is considered a predator of large-sized bivalves. However, most are able to feed upon smaller-sized bivalves. The starry flounder

Platichthys stellatus occupies Norton Sound and the

Chukchi Sea in relatively high numbers, especially in warm­ water years (Jewett and Feder 1980). Analysis of gut contents by Jewett and Feder (1980) revealed that Yoldia hyperborea was an important component of the diet of starry flounders, occurring in 36% of the fishes examined in

Norton Sound. Serripes groenlandicus was also important, occurring in 22% of the starry flounder stomachs.

The king , Paralithoides camtschatica, and large sea stars such as Lethasterias nanirnensis are able to prey upon some large bivalves (Feder and Jewett 1978). L: nanimensis is able to feed upon Serripes groenlandicus, but other deeper burrowing bivalves have not been found in their diet. Although these fishes and large invertebrates are known to prey upon bivalves, they were sparse in Norton

Sound in 1982 and 1983 (personal observations), and

probably did not have a great impact on bivalve 40 populations. In other years, however, these mobile predators may have an impact.

Additionally, bearded seals are seasonal inhabitants

of Norton Sound (Lowry et al. 1980). Their diet consists predominantly of fishes, but invertebrates including

shallow-burrowing bivalves such as Serripes groenlandicus

are also eaten (Lowry et al. 1980). There is no evidence

that deeper burrowing animals are fed upon. The feeding traces of bearded seals may be distinguished from walruses

(Oliver al. 1983) and no evidence was found that bearded

seals affected bivalve populations within my study areas.

One issue concerned a possible overlap in food

resources utilized by sea stars and walruses. However,

these two predators were found to feed on different sizes

of bivalves, with Asterias amurensis feeding on bivalves

less than 20 mm in size, while walruses concentrated on

bivalves greater than 35 mm. The intensity of predation by

Asterias arnurensis on smaller sizes is likely to be an

important factor on the number of bivalves able to reach

adult sizes. Stoker (1978) found A. arnurensis to be a

dominant animal in Norton Sound in numbers and ,

which still appears to be true. The diet of A. amurensis

contained many bivalves less than 10 mm, and this indicates

either size selectivity or limited availability. In

laboratory experiments, when given a choice between

bivalves of several species that were 1-6 mm or Macoma 41 balthica that were 6-10 mm in size, A. amurensis ate many more of the larger bivalves. This may have been due to a

prey preference. Allen (1983) has demonstrated that at

least one species of Asterias showed selectivity towards

prey. Alternately, the choice may have been due to

energetic considerations. Small prey are usually

unattractive to large predators since the energy gain is

low compared to effort expended. Animals may become more

desirable as prey as they grow larger, until, in some cases,

a size refuge is reached. Predation intensity, in

conjunction with bivalve reproductive success and physical

factors could account for size/age class failure and the

population structure seen in these bivalves.

Size/age class failure is a common event for bivalves,

and it is likely that the iteroparous species are adapted

to periodic failures. Evidence for the occurrence of

complete loss of bivalve size classes due to predation has

been documented since the turn of the century. Both • Petersen (1918) and Jensen (1919) found that successful

·recruitment for Mya truncata occurred every 8-9 years

because of predation by bottom fishes.

Though the time frame for looking at the bivalve

species in this study was relatively short, there is

evidence that size class domination occurs in the

population of Mya truncata studied. Large numbers of

juveniles were found, an adult population was sampled and 42 no intermediate-sized bivalves (15-60 mm) were seen.

Though live ~ults of Serripes groenlandicus and Macoma calcarea werE not located, the presence large-sized discarded shalls indicated that adult populations were present, most likely in patchy distributions. The populations o.E these two species also show size-class domination.

Size distributions as I found for Mya truncata have been seen in other invertebrate populations. Bivalves such as (Feed and Brown 1975, Paine 1976, Pollock 1979),

Soisula (Fra:~ 1977, Hughes and Bourne 1981), Yoldia

(Hutchings an:1 Haedrich 1984), Protothaca (Paul and Feder

1973), and~ and Macoma (Commito 1982), as well as other invertebrates such as sea urchins (Tegner and Levin 1983) and gastropo~ (Paine 1965, Vince et al. 1976) have exhibited th5..s pattern.

The presence of the larger-sized bivalves indicates either that ~ize classes are periodically able to pass through initial predation or small numbers of bivalves survive yearRy and eventually accumulate as adults. Alternately, recruitment may be variable, though i found no evidence for this over a three year period for any of the

.. species other than Serripes groenlandicus. Several factors are likely ~ be involved. One major factor may be occasional rEduction of sea star populations. Evidence for a large-scale fluctuation on a seven-year cycle was 43 presented by Burkenroad (1946) for .

However, this evidence was based on qualitative observations rather than on quantitative data and Loosanoff

(1964) dismissed this idea. He quantitatively sampled the same area over 25 years and did not find the patterns of abundance as predicted by Burkenroad. However, there were some years when numbers of sea stars were reduced which could allow bivalves to recruit and attain a size refuge.

Weather-related conditions or other physical or biotic factors may affect sea star populations. Examples of massive mortality occurring in sea star populations were seen by Menge (1979) for Asterias vulgaris and A. forbesi in New England. The cause of these mortalities was determined to be disease in one ~ase and storms in two other cases.

Another hypothesis which may permit successful recruitment by bivalves is that may not feed,

at least not as voraciously 1 during their period of reproduction and/or during the winter (Thorson ~955, 1957,

1966). This lag period may allow settling larvae an opportunity to attain depth and/or size refuges. Evidence for such cessation of feeding was initially directed towards ophiuroids. Thorson ( 1957,· 1966) desc;ribed a npassive period" for Amphiura and Ophiura during their spawning period when feeding ceased. Here, guts of the ophiuroid Diamphiodia craterodmeta examined during two 44 seasons were all empty. This may have been due to seasonal variation or complete· digestion of food items. In contrast

Feder (1981) found that Ophiura texturata reduced its feeding during the winter, though feeding did not cease altogether.

The asteroid Asterias forbesi suspends feeding for one month during its reproductive period (Galtsoff and

Loosanoff 1939). Asterias amurensis spawns from January to

!-larch in , but there was no reduction in feeding

activity during this time (Hatanaka and Kosaka 1959).

However, these sea stars were subsisting mainly on

crustaceans rather than on bivalves.

In order for bivalves to successfully pass through

initial predatory pressures, several other strategies may

be utilized. An initial growth rate which enables an

animal to attain a size refuge would be of selective

advantage. Any behavioral escape response generated by the

presence of predators or an ability to obtain a depth

refuge would be another possible mechanism for survival­

through vulnerable life stages.

SPAWNING AND SETTLEMENT

Each individual species spawns and settles at

different times of the year. Mya truncata may be able to

spawn year-round (Thorson 1950, Muus 1973; Petersen 1978).

This adaptation would be advantageous in settlement during 45 periods of lower feeding, leading to speculation that there may be an advantage to winter spawning. Indeed,

Holland et al. (1980), found that cohorts of Macoma balthica released in the late spring or summer were heavily

cropped, whereas cohorts released in the late fall or winter had lower predation and were more likely to attain

large sizes and higher densities. Spring recruitment

pulses of Mya arenaria infrequently succeeded in

establishing themselves due to seasonal differences in

predation pressure (Virnstein 1979, Holland et al. 1980).

The winter diet of Asterias amurensis is unknown so no

conclusion can be made about the effects they may have on

bivalves during this period.

The spawning period for Serripes groenlandicus, Macoma

calcarea; and Yoldia hyperborea is early spring to summer

(see Appendix 1). This period is after settlement of

Asterias amurensis (Hatanaka and Kosaka 1959), and

increased numbers of feeding sea stars are likely to be

present in Norton Sound. Thus timing of settlement for

these bivalves to escape sea star predation does not appear

to be of primary adaptive significance.

SPATIAL REFUGES

Once settlement occurs, burrowing may result in the

establishment of a spatial refuge. Depth seems to. be an

effective refuge for some bivalves against predators such 4£ as sea stars, fishes, and crabs. Size is related to depth

of burial (Connell 1955) and depth of burial is related to

length of siphons (Stanley 1970). Serripes groenlandicus

is generally found at the surface, while Yoldia hyperborea,

Mya truncata, and Macoma calcarea may be found at depths

greater than 20 .em (Table 7). Hya arenaria has been found

to a depth of 25 em and Macoma balthica to 35 em, which is

sufficient to escape predation by blue crabs (Blundon and

Kennedy 1982b). A depth of 12 rnm was sufficient as a

refuge for H. arenaria against the mumrnichog, Fundulus

heteroclitus (Kelso 1979). I found in laboratory

observations that H. arenaria and Yoldia hyperborea which

were buried to a depth of 20 em or greater could not be

excavated by Asterias amurensis, but bivalves buried at

depths of 5 em were easily excavated. In field

observations, A. amurensis were found in pits up to 10 em

deep, some with bivalves or other prey items on their tube

feet~· Arima et al. (1972) observed in laboratory

experiments that A. amurensis could excavate bivalves that

were buried up to 7.5 em.

Hacoma calcarea may be able to take advantage of an

ability to move vertically in sediment as a refuge.

WMembers of this have long siphons (Reid and Reid

1969) allowing them to burrow to depths of up to 20 em

(Muus 1973). Reading and McGrorty (1978) found that Macorna

balthica exhibited a seasonal movement vertically in 47

Table 9. Maximum depths to which 9 species of bivalves have been found burrowed.

Species Depth (em) Source ------~------Serripes groenlandicus surface Lowry et al. 1980 Lyonsia hyalina surface Virnstein 1979 Lyonsia norvegica shallow Ansell 1967 Protothaca staminea 4 Paul and Feder 1973 Macorna balthica 7.5 Reading and McGrorty 1978 Macorna bal thica 35 Blundon and Kennedy 19B2b Macoma calcarea 20 Muus 1973 Mya arenaria 25 Blundon and Kennedy 19B2b 1-lya tnmcata 25 this study Yoldia ~oerborea up to 40 this study 48 response to predatory pressure.

The ability to burrow quickly and deeply would be an effective escape mechanism for a bivalve. Evidence for such a response was seen for the protobranch Yoldia hyoerborea. Members of this order are characterized by having thin, elongated shells and a large foot {Drew 1900).

These attributes would allow faster burrowing (Trueman et al. 1966, Trueman 1968, Stanley 1970). Once an animal had settled into a fossorial habitat, its ability to move deeper in response to a predator would aid in its survival.

Such responses to reduce sea star predation have been found

in the bivalves Mercenaria mercenaria {Pratt and Campbell

1956, Doering 1982a, 1982b) and Macoma {Reading

and McGrorty 1978).

BEHAVIORAL ESCAPE RESPONSES

The use of escape responses to avoid predation is

widespread among marine animals. l'lany potential prey

species exhibit escape responses when stimulated by the

presence of sea stars (see Margolin 1964 for example). In

molluscs this respons:e takes the form ·of autotomy,

discharge of .secretions 1 or locomotion such as leaping,

swimming or burrowing (Ansell 1969) .-

I believe that an escape response is of significant

value in allowing larger sizes of Yoldia hyperborea to

avoid predation by sea sta_:~::s. By using its large foot to 49 move, then digging into the substratum, it is able to escape from sea stars. The anatomical characteristics and laboratory observations strongly suggest that escape by movement generated by the foot would be of primary adaptive significance in this genus. In contrast, small individuals did not exhibit this escape behavior. I believe that this behavior may be size-dependent, since only smaller sizes of

Y. hyperborea were found in the stomachs of sea stars.

A behavioral escape response was also elicited in

several specimens of Serripes groenlandicus when touched by an Asterias amurensis. This reaction by S. gro=e~n~l~a~n~d~i~c~u~s~ was also reported to occur in response to Pycnopodia by

Margolin (1964). In contrast to large Yoldia hyperborea, which immediately burrows, s. groenlandicus uses its large foot to propel itself away from a predator.· The large L-

shaped foot is used for rapid leaping and burrowing and

many members of this family (Cardiidae) exhibit this

behavior (Ansell 1969, Stanley 1970, Sloan and Robinson

1983).

REFUGES

Size is an effective deterrent to some predators,

especially against smaller predator~ such as sea stars. A

size refuge may be of greater importance in rocky areas

where animals are more exposed. For example, Paine (1976)

found that large Mytilus are able to attain sizes that ~0 cannot consume. Other examples include the Cardium echinatum which reaches a size refuge from fishes after two years of growth (Ford 1925). Macoma balthica obtains a size refuge at 0.7 mm from the amphipod

Pontopreia affinis (Segerstrale 1962) and Mya arenaria has a size refuge from the gastropod Polinices (Edwards and

Huebner 1977).

Bivalves in the Norton Sound region may have a size refuge at around 53 mm, but only from smaller Asterias

(Table 6). This could be important only for Serripes groenlandicus, which is a surface-dwelling cockle (Lowry al. 1980). Since it lives at a depth to which Asterias can dig (Table 8) it could rely on size as a refuge, though I

believe that a behavioral escape response is the primary means of escape.

Macoma calcarea and Mya truncata rely on a depth

refuge to escape predation by seastars. Since depth of

burial is also related to size of bivalve (Stanley 1970),

the size of bivalves is also an important factor.

WALRUS PREDATION

Benthic-feeding marine mammals in Norton Sound subject

bivalves in Norton Sound to another level of predatory

pressure. There are no refuges for bivalves from walruses.

Analyses of their stomachs have shown that up to 97% of the

diet was comprised of bivalves (Vibe 1950, Fay 1982). I ';

Table 8. Depths at which six predators have been found to dig in order to 'obtain bivalves.

Species Depth (em) Source

Rhinoptera bonasus 45 Orth 1975 Callinectes sapidus 15 Blundon and Kennedy 1982a Pycnopodia helianthoides 23 Mauzey et al. 1968 Pisaster brevispinus 15 Van Veldhuizen-& Phillips 1978 Asterias amurensis 10 this study Odobenus rosmarus at least 30 Oliver ~ al. in prep. 52 The Pacific walrus population has been steadily increasing since its low point in the 1900's (Fay 1982;

Bockstoce and Botkin 1982). Recent estimates are a population size of around 200,000 animals (Fay 1982). The status of bivalve populations in Norton Sound ultimately depends on the numbers of walruses which move through the area. Walruses may adversely affect juvenile bivalves.

The disturbances associated with walrus .feeding are two­ fold. First, in digging for Mya truncata a pit is made by jetting water and blowing out liquified sediment with relatively small areas disturbed (Oliver et al. 1983).

When searching for and feeding upon Macoma calcarea and

Serripes groenlandicus, however, furrowing of the bottom occurs (Fay 1982; Oliver et al. 1983) and large areas are disturbed (Oliver et al. 1983). These activities smother, displace, and expose young bivalves to scavenging predators. While digging artificial pits and sampling for bivalves, scavengers such as seastars and gadid fishes were seen attracted to disturbances. They were presumably attracted by the displacement of food items from within the

sediment.

The_most obvious and"drarnatic effects that walruses have on bivalve populations are seen from their predatory

activities on the adults. Walrus predation on the three

large bivalve species found in Norton Sound is selective

upon larger size classes. Large numbers of Mya truncata 53 are consumed at Cape Nome, and at Sledge Island Macoma calcarea are primarily eaten. The dead shell record for

Serripes groenlandicus was sparse, with the few shells found being smaller in size than the other two species.

This is likely due to a preference for this species (Vibe

1950; Fay 1982) and accessibility due to shallow burial depths which has resulted in a depletion in numbers of this species. Heavy predation on S. groenlandicus may thus result in patchy distributions and a subsequent switching to other more abundant bivalves. Bockstoce and Botkin (1982) have speculated that some areas have been left undisturbed from intensive feeding during the decline of the walrus population. With the increase in numbers of walruses, these areas are now subject to heavy predation. In the principal wintering and summering grounds, removal of benthic stocks has approached or exceeded annual net productivity, which suggests that the walrus population may be at or near its carrying capacity (Fay 1977).

Populations of Serripes groenlandicus have been primarily affected by this increase in preda_tion. In the mid-1970's this bivalve was a dominant animal sampled by

-grabs in the_Norton Sound area _(Fay et al. 1977; Stoker

1978). It was also the primary. food item in the stomachs

of walruses from the Nome area -(Fay et 1977) and the

dominant invertebrate in the guts of bearded seals from 54 Nmton Sound (Lowry et al. 1980). Populations of~- grnPnlandicus now appear to be declining in Norton Sound.

~ trend is suggested by inconsistent recruitment of s~ll-sized animals in 1982 and 1983 and the relatively low nrrnbers in the dead shell record. The prediction made by

Lony et al. (1980) that stocks of S. groenlandicus would be overgrazed in the Nome area due to intensive predation by walruses and bearded seals appears to be confirmed.

In contrast, fair numbers of Mya truncata are still pr=seht in Norton Sound. These animals are not adequately sanpled by grabs so it is hard to compare present numbers with previous densities. Walruses have switched their diet I from primarily feeding on Serripes groenlandicus to feeding on Mya truncata. In previous stomach content analyses Mya din not appear in any of the samples (Fay et al. 1977).

The dead shell record and observations of feeding pits now

shows Mya to be the most frequently eaten bivalve in the

None area (Fig. 7) with an estimated 5% of theM. truncata

population in Norton Sound being removed annually by

wauuses (Oliver et al. 1983).

Thus it appears that walruses first concentrate their

feeding efforts on shallow-dwelling, easily accessible

bivalves such as Serripes. Once these stocks become

depleted, the deeper burrowers such as Mya are utilized

along with smaller sizes of Serripes. A steady increase in

walrus numbers undoubtedly will result in declines in the 55 bivalve p~lations along their migration corridors as has been alreaey seen in the major wintering and summering grounds. CONCLUSIONS

(1) The bivalves Yoldia hyperborea, Mya truncata, Serripes groenlandicus, and Macoma calcarea form an important component of the subtidal, benthic, soft-bottom community of northwestern Norton Sound, Alaska. These four species all exhibit size-class domination.

(2) The most important predators on these bivalves are the asteroid, amurensis and the Pacific walrus, Odobenus rosmarus divergens. Asterias is present year-round, feeds heavily on bivalves <10 mm and is probably responsible for the size structure seen in these bivalves. Walruses are seasonal predators on bivalves >40 mm and seem to exhibit a preference for S. groenlandicus.

With a decline in the population of~- groenlandicus, walruses are currently concentrating their feeding efforts on Mya truncata in the Nome area.

(3) Refuges may be important for bivalves to alleviate predation by asteroids. Yoldia hyperborea and S. groenlandicus are surface~dwelling bivalves which may be excavated by asteroids, but they use size-dependent, behavioral escape responses to avoid capture. Asterias amurensis can dig up to 10 em into the sediment to excavate 57 bivalves and M. truncata and M. calcarea utilize a depth refuge (>15 em) and, in some cases, a related size escape to avoid predation by seastars. LITERATURE CITED

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LIFE HISTORY INFORMATION ON THE BIVALVES STUDIED

Yoldia hyperborea Loven in Torell, 1859 (Nuculanidae)

Yoldia hyperborea is a thin, fragile, compressed, yellowish-green bivalve with a gape at both ends. The most characteristic part of its anatomy is the presence of a large foot that protrudes from the posterior gape. It reaches lengths of 45 mm and is a selective deposit feeder

(Ockelmann 1954, 1958). The spawning season in southeastern

Greenland begins in July and it releases pelagic larvae which are planktonic for a short time (Ockelmann 1954,

1958). This bivalve is circumpolar in its distribution, ranging from Greenland to Norway and across northern Canada and Alaska (Abbott 1974). Water temperatures of 2-3°C are preferred (Stoker 1978). It is found at depths of 4-677 m

(Ockelmann 1958). An Atlantic species~, Yoldia limatula lives for five years and grows about 5-13 mm/yr (Lewis et al. 1982). Populations of Yoldia thraciaeformis have eXhibited up to 14 external bands and maximum sizes to 49 mm (Hutchings and Haedrich 1984).

Serripes groenlandicus (Brugiere, l789)(Cardiidae)

This cockle has weak radial ribs with a slight posterior gape. The foot is very large (Abbott 1974). This

6~6 67 animal is found from the Arctic seas to Cape Cod and

Greenland and Norway, and from Alaska to Hakodate, Japan and to Puget Sound, Washington (Abbott 1974; Ockelmann

1958). It is found to 70 m depth in sand, gravel, clay or mud, with a preference for sandy or sandy-mud substrata

(Ockelmann 1958). The maximum recorded size is 110 rnm and aging of animals by growth rings has determined that lengths of 58, 53 and 47 mm are equivalent to ages 14, 12 and 11 years respectively (Petersen 1978). The estimated mean growth rate is about 4.34 mm/yr with a range of 2.56 to 6.35 mm/yr (Stoker 1978). Serripes groenlandicus is a protandric hermaphrodite with planktotrophic, pelagic larvae (Thorson 1936). Spawning occurs in March-April and is probably induced by the presence of

(Petersen 1978). It is a filter feeder (Stoker 1978).

Macoma calcarea (Gmelin, 1791) (Tellinidae)

This bivalve is found from Greenland to Long Island,

New York and to the Baltic Sea in the Atlantic, and from the Bering Sea to Washington and northern Japan in the

Pacific (Ockelmann 1958; Abbott 1974). Shells to 60 rom in length have been found (co·an 1971). Shells of 30 and 40 mm have been estimated to be 9 and 18~years old (Stoker .1978).

Mean growth rate was estimated at ~.5 mm/yr (Petersen 1978) to 3.0 rnm/yr (Stoker 1978). It i~most common deeper than

50 min silty-sand to sandy substrata (Coan 1971). Adults &8 are found living at depths up to 20 em in the sediment

(Muus 1973). A preference for water temperatures of -1 to l°C has been found (Stoker 1978). It is a selective deposit feeder and/or filter feeder eating small diatoms and flagellates (Reid and Reid 1969). Pelagic larvae are produced (Stoker 1978). Spawning occurs in June to

September (Muus 1973, Petersen 1978).

Mya truncata Linne, 1758 (Myidae)

This bivalve has a thin shell with a wide gape at its abruptly truncate, posterior end. It is found from Arctic seas to Massachusetts and the Bay of Biscay, France in the

Atlantic and to Washington and Japan in the Pacific (Tebble

1966~ Abbott 1974). It is found in mud, sand Dr gravel bottoms to depths of 625 m (Ockelmann 1958). Larger individuals of Mya arenaria are found living at depths to

14 em with larger individuals living deeper in the sediment r than smaller individuals (Connell 1955). Spawning occurs mainly from May to July, but there is evidence that spawning may occur year-round (Muus 1973; Petersen 1978).

Planktotrophic larvae with long pelagic stages are produced

(Thorson 1936). It is a filter feeder (Stoker 1978).