THE BEHAVIORAL ECOLOGY OF THE SCAPHOPOD MOLLUSC

ABERRANS: BURROWING MECHANISM AND RHYTHMICITY

A thesis submitted to the faculty of San Francisco State University in partial fulfillment of the requirements for the degree

Master of Science in Marine science

by Jenifer Lynn Levitt san Francisco, California December, 1990 THE BEHAVIORAL ECOLOGY OF THE SCAPHOPOD MOLLUSC CADULUS ABERRANS: BURROWING MECHANISM AND RHYTHMICITY

Jenifer Lynn Levitt san Francisco State University 1990

ABSTRACT The burrowing mechanism of Cadulus aberrans

(Whiteaves) was studied to explain the mechanism by which this infaunal mollusc can burrow despite considerably less foot musculature than other infaunal molluscs. circadian burrowing rhythms and changes in burrowing depth measured at various times of the year are also described. Locomotion is described from observations of foot movement in seawater, and burrowing in submerged, sediment-filled, narrow aquaria • .Q. aberrans, unlike other burrowing molluscs, extends its foot by eversion. As the tubular foot is everted it is inserted into the sediment. Foot elongation involves hydraulic movement of hemocoelic fluid and not seawater. After complete elongation the distal pedal disk flares and establishes an anchorage. Contraction of four longitudinal pedal retractor muscles pulls the shell down over the foot. Complete burial, one body length, is accomplished in approximately 3 seconds. Cadulus aberrans burrows deeply, and extensively in the laboratory and in nature. This life style contradicts the generalizations from studies of Dentaliumspp. that scaphopods are only superficial burrowers. Time-lapse observations of burrowing were recorded on video tape and revealed a cycle of daily burial and nightly emergence. Burrowing depth was quantified by removal of scaphopods from six replicate, experimental aquaria in 4 em strata, over 1 1/2 years. Burrowing depth varied, sometimes were found throughout the depth strata to the greatest depth of 20 em. At other times most animals were within 4 em of the sediment surface. A possible explanation for the two burrowing trends is correlation with periods of gametogenic activity; shallow burrowing may occur during periods of high gametogenic activity. However not all

data support this hypothesis. ~- aberrans was found throughout a 46 em deep box core from Monterey Bay, California, also indicating deep burrowing.

I certify that the Abstract is a correct representation of the content of this thesis.

[~ h...... ~ n- "-1- '"to Thomas M. Niesen Date (Chair, Thesis Committee) ACKNOWLEDGEMENTS

Many people have contributed time, expertise and equipment to this endeavor. Drs. T. Niesen, M. Foster and w. Gilly were kind enough to sit on my thesis committee and guide me through the thesis process. Dr. R. Shimek provided insightful information about scaphopods. G. Steiner edited a draft. Several of my peers at Moss Landing Marine Laboratories helped with scaphopod sampling and sorting, particularly J. Wolgast and E. Sawyer. s. Baldridge and s. O'Neil were instrumental in locating reprints. Dr. J. Nybakken introduced me to the study of mollusks. M. Sylvan, of ucsc, greatly helped with data interpretation and statistical analyses. After the earthquake in october 1989, I found myself without a lab, functional experimental enclosures, or a desk, and with all my lab possessions packed in boxes in a warehouse. At this time it was the faculty and staff of the Hopkins Marine station who made it possible for me to continue working. In particular I wish to thank Judy Thompson for allowing me use of the water tables, and Bruce Hopkins for paving the way toward working in Gilly's lab and for continuously helping me with problems as they arose. Dr. w. Gilly provided desk and lab space, supplies, and guidance. M. and J. Lucero and F. Horrigan helped with inspiration, technical assistance and an occasional pizza. Dr. D. Mazia and c. Patton provided the time-lapse video equipment. Last and perhaps most importantly, Ilene Meyers, my mother, provided emotional support and helped me with literature searches. I whole heartily thank all these people and many others who contributed directly and indirectly to this thesis. This work has taught me not only about scaphopods but the joy and necessity of cooperation. Financial support was provided by the Myers Oceanographic and Marine Biology Trust and the Santa Barbara Shell Club.

vi TABLE OF CONTENTS

List of Figures ...... viii

List of Tables ...... ix

Chapter 1. The Mechanism of Locomotion

Introduction ...... 1

Methods ...... 2

Results ...... 4

Discussion ...... 10

Chapter 2. Burrowing Rhythmicity

Introduction ...... 15

Methods ...... 16

Results ...... 21

Discussion ...... •.•... 3 3

Literature Cited ..•...... 3 8 Vll LIST OF FIGURES

Figure Page

1. Pedal Everson ...... 6

2. Pedal Retraction ...... 7

3 . Burrowing Sequence ...... 9

4. Distributions in Ant Farms ...... 24

5. Time of Emergence ...... 30

6. Surface-stay Duration ...... 30

7. Distribution in Box Core ...... 3 2

viii LIST OF TABLES

Table Page

1 . Burrowing Depth summary ••.••.•... 2 6

ix 1 Chapter 1 THE MECHANISM OF LOCOMOTION

INTRODUCTION Molluscan burrowing through soft substrates usually combines muscular contraction with hydraulic hemocoel expansion (Trueman, 1969, 1975). Burrowing has been described for many bivalves, gastropods, and the scaphopod Dentalium.

The bivalves use a combination of four muscle groups, circular, longitudinal, oblique, and adductor, and movement of both seawater and hemocoelic fluid to penetrate the substrate (Trueman et al., 1966; Trueman and Brown, 1985; Trueman, 1968a). Most burrowing gastropods use a variety of muscle groups and hemocoelic spaces, ranging from pedal plows to lobed cephalic shields (Brown, 1964; DeFresse, 1989), to form thrustable insertion appendages and expandable anchors. Other gastropods combine hydraulic-muscular mechanisms with ciliary movement or muscular pedal waves (Trueman, 1968b). Scaphopods in the order have a protrusible, muscular foot similar to the bivalves. The foot is equipped with longitudinal, circular and transverse muscles, that hydraulically erect epipodial lobes to form an anchorage in the substrate (Trueman 1968c). These scaphopods are shallow burrowers (Morton, 1959). The foot of scaphopods in the order is considerably less muscular than those of the other burrowing molluscs. It contains two paired longitudinal muscles and lacks circular or other musculature, being predominantly a 2 longitudinally expandable hemocoel (Petitte-Fischer & Franc, 1968). Despite this muscular disadvantage, Cadulus aberrans has been found burrowing as deep as 30 em in its natural environment (Shimek, 1989, 1990), was found near the bottom of and throughout a 46 em deep box core sample (from Monterey Bay, California, 60 m water depth), and frequently burrowed to and from the bottoms of 20 em deep aquaria in the laboratory (personnel observation) . Here I describe the gadilid foot as observed in Cadulus aberrans and the mechanism of locomotion that allows this scaphopod to burrow to these depths.

METHODS cadulus aberrans and the sediments in which they were found were collected using a Smith-Mcintyre grab in Monterey Bay, California (three kilometers off-shore from the Pajaro River in 60 m water depth). Other infauna were removed by washing the sample over a 1 mm mesh screen then sorting under a dissecting microscope. Both scaphopods and sediment were maintained in the laboratory where behavioral observations were made.

Scaphopod movement was observed in finger bowls containing natural seawater. Back lighting, from beneath the bowl, allowed simultaneous observation of events within the translucent shell and at each aperture. A thin layer of sediment placed in the bowl (approximately 5 mm thick) allowed recording of burrowing initiation. McCormick blue 3 food coloring (McCormick Research & Development Labs, Hunt Valley, MD) mixed with seawater was injected next to active scaphopods to look for fluid flow between the mantle cavity and hemocoel . After each collection, the newly collected scaphopods were placed in experimental "ant-farms" (described in detail in chapter 2). Animals placed on the sediment immediately burrowed. Behavior was observed on the surface of and within sediment in these narrow, ant-farm style enclosures, placed in aquaria with continuous flowing sea water. All behavior was studied both by direct observation and video. The video equipment consisted of a Panasonic TV camera (model WV-lOOOA) and a Betamax video cassette recorder (model SL-2700). The image was observed simultaneously on a Panasonic video monitor (model TR-930). Burrowing is usually described in steps (Trueman, 1975) and the procession of steps constitutes a 'digging cycle.' 'Digging period' refers to the duration of burrowing activity from the initiation of activity until a stable position is reached and burrowing ceases. Pedal hemocoel volume (within the shell) and the volume of the fully extended foot were calculated to determine the volume of fluid needed to evert the foot and to determine if the volume of the pedal hemocoel is sufficient to supply this fluid. Although not perfectly cylindrical, the equation for the volume of a cylinder (Tir2L) was used to estimate its

Volume (Where r and L indicate the hemocoel radius and 4 length, respectively). Hemocoel dimensions were estimated from measurements of shell length and diameter (measurement LWm, measurement Wm in Shimek, 1989). Foot length and diameter were determined by measurements of projected slides, scaled to life size.

RESULTS Movement across the finger bowl was initiated by protrusion of the foot through the ventral aperture. The tubular foot is elongated by eversion, analogous to proboscis eversion in priapulids and sipunculids (Barnes,1987). As the tube is elongated a white ball appears to move outward, down the tube. This is the terminal disc and upon reaching the end of the tube it flairs outward then curls backwards (Figure 1). The disc is crenelated and looks like an open umbrella. On the distal surface of the disc is a small filament of unknown function. Probing was often observed prior to complete foot expansion. Probing is basically incomplete eversion; the pedal tube elongates but the terminal disk does not emerge. When the foot is completely everted it is slightly longer than the shell. Average adult shell length is 9.97 mm (Shimek, 1989). Contraction of longitudinal pedal muscles, which are continuous with the mantle longitudinal muscles (Steiner, 1990) causes foot retraction (Figure 2). The inverted foot lays loosely coiled in a fluid filled, pedal hemocoel. wnether the hemocoelic fluid is held within the pedal 5 hemocoel by the equivalent of a one-way Kebers valve, as described in bivalves (Barnes, 1987), or if pressure changes in the hemocoel accompany foot movement were not ascertained. The mean duration of a complete cycle of foot eversion and retraction was 3.03 (± 1.29) seconds (n [timed observations] =55). 6

Figure 1. Foot eversion. Between burrowing cycles the foot is retracted (A). The pedal tube protrudes through ventral aperture (B) and elongates via eversion. A white ball (shown as solid black) moves distally (C,D) gliding outward along the infolded epidermis. The ball (pedal disk) emerges from the tube (E) then flairs outward and backwards (F). on expansion the disk is umbrella shaped and a small filament is ~isible on the distal surface. Probing movement by the foot ~nvolves incomplete eversion and length is variable (B,C, or D). Pedal probing often precedes complete eversion. 7

0 l 2 '"'"" Figure 2. Foot retraction. Retraction involves contraction of longitudinal muscles within the pedal tube. The retractors are continuous with the body wall musculature. The tube is shortened (G-I) and the pedal disk folds inward (J,K) as it is pulled through the ventral aperture (K). After retraction the foot is again infolded completely within the pedal hemocoel. 8 When a thin layer of sediment was placed on the bottom of the finger bowl Cadulus aberrans buried themselves as completely as possible. When the sediment was thinner then the shell's length (i.e. complete burial impossible) burial with the convex side of the shell upward or with the concave side of the shell upward was observed. Some scaphopods also dragged the shell obliquely into the sediment. Food coloring moved with respiratory currents through the mantle cavity, into and out of both apertures. Dye was never seen within the body or pedal hemocoels. Foot eversion mechanics were the same when observed in "ant-farms," and complete burial was possible because of increased sediment depth (Figure 3). When the foot was partially everted, as the disc first emerged from the shell, the foot-tube was angled downward and elongation continued into the sediment. As the foot was fully extended the fluid pumped into the foot caused it to straighten. This pulled the shell into an erect position. Often, the shell spun in an arc around the foot as it erected. This cork screw motion allowed deeper penetration of the foot and initial penetration of the shell. The terminal disc, the radius of which is larger than the shell aperture radius, anchored the foot. Because the foot was slightly longer than the shell, the first digging cycle resulted in complete burial. Penetration was nearly perpendicular to the sediment surface. Usually only one 9

Figure 3. Burrowing sequence. The pedal tube elongates via eversion then angles downward to penetrate the sediment. The shell acts as a penetration anchor allowing the foot to insert without pushing the backwards. Elongation continues until the pedal disk emerges on the distal end establishing a pedal anchorage. Hemocoelic fluid pumped into the foot causes it to straighten. Often the shell revolves around the foot as it becomes perpendicular to the sediment surface. Contraction of longitudinal retractor muscles pulls the shell down through the sediment over the foot. 10 digging cycle occurred within each digging period. Time between digging periods ranged from 40 seconds to longer than 20 minutes. Foot movement was not used exclusively for burial. Cadulus aberrans occasionally pull themselves along the sediment surface, for periods exceeding 10 seconds, using foot eversion. Horizontal movement was also observed when an animal was completely buried. They appeared to move with ease in all three dimensions upon and within the sediment.When moving downward or horizontally they move foot first. When moving upward they push the shell up first in a manor analogous to reverse burrowing described for Dentalium

(Gainey, 1972). The volume of the everted foot (v = 8.2 mm3) is less than the volume of the pedal hemocoel (v = 11.7 mm3). The measurements used were: foot length= 10.0 mm; foot radius= 0.51 mm; hemocoel length= 2.27 mm; hemocoel radius= 1.28 mm. Thus, although equal to the entire body in length, the foot requires no additional fluid, beyond that in the pedal hemocoel for eversion. A small decrease in diameter of the hemocoel would cause a large increase in foot length (see Kier, 1988, Fig.1).

DISCUSSION The burrowing mechanisms of Dentalium spp., Cadulus aberrans and the burrowing Bivalvia are functionally similar,

Possibly as a result of convergent evolution of a 1 1 longitudinally enlarged foot (reviewed by Trueman, 1975). The burrowing cycle in each group consists of corresponding coordinated muscular and hydraulic activities. Burrowing is initiated by a series of pedal probing movements. Initial anchorage is provided by the shell; this prevents the animal from being pushed backward as it thrusts the foot into the substrate. The foot is inserted into the substrate and a pedal anchor is then formed. Pedal retraction results in dislodgment of the penetration anchor as the shell is pulled down over the foot. A post-burrowing recovery period during which the shell anchor replaces the pedal anchor follows retraction. At this time pedal probing may occur but does not necessarily lead to burrowing. The only muscular similarity found in all three taxa is longitudinal retractor muscles extending from the tip of the pedal hemocoel and inserting into the mantle within the shell. Despite the similarities in gross mechanisms, fine comparison of the mechanics of burrowing for each group (dentaliids, gadilids and bivalves) indicates unique anatomical and behavioral solutions to substrate penetration. The scaphopods must rely on shell weight to establish the shell anchor whereas bivalves open their valves for shell anchorage. In Dentaliumthe pedal anchor is established by erection of the epipodial lobes (Morton, 1959; Dinamani, 1964). In bivalves high pressure in the pedal hemocoel causes Pedal dilation anchoring the foot (Trueman, 1967) In Cadulus 12 eversion of a disk on the distal end of the foot is used to establish an anchor. In Dentaliumthree sets of muscles are responsible for foot movement: the longitudinal pedal retractors, and antagonistic circular and transverse muscles (Trueman, 1968c). In bivalves four muscle sets regulate foot movement: the shell adductor muscles increase fluid pressure in the foot by closing the shells; retractor muscles are functionally similar to Dentalium; transverse and protractor muscles are antagonistic to the longitudinal retractors and regulate foot diameter and formation of pedal anchors (Trueman, 1968a). The musculature is much simpler in Cadulus, two pairs of longitudinal pedal retractors are responsible for foot retraction (Petitte-Fischer and Franc, 1968; Steiner, 1990). Because other musculature is lacking, protraction is likely accomplished by hemocoelic fluid flow into the foot, either pumped in by contraction of longitudinal muscles of the general body hemocoel or only those longitudinal muscles in the pedal hemocoel, accompanied by relaxation of the pedal retractors. The dentaliid and bivalve foot can be withdrawn, protruded and moved from side to side by concerted control of the three pedal muscle layers (Trueman, 1968a). The foot of Cadulus being considerably less muscular lacks the latter

coordination but can be directionally angled by unequal contraction of the longitudinal retractors. Lacking the hydraulic force to push the foot through the sediment, generated by contraction of the circular and transverse 13 muscles in Dentalium, pedal eversion allows gliding

penetration as the foot tip encounters less surface area of sediment thus less sheering stress as the infolded epidermis glides outward along the everted portion of the foot (Figure

1 ) . Appendage eversibility is a feature usually associated with feeding organs of lower metazoans (e.g. nemerteans, acanthocephalans, priapulids, sipunculids) and some annelids and molluscs. Some invertebrates use an eversible probosis

for locomotion through sediment. For example in the polychaete annelids Nepthys and Arenicola, successive

eversions and retractions of the proboscis are mechanically similar to Cadulus pedal movement. High pressures are produced in the coelom of Arenicola, the lugworm, by

contraction of longitudinal muscles in the trunk segments (Trueman, 1975). While no other mollusc uses an eversible foot for burrowing, eversible probosci used for feeding are not uncommon, (e.g. Bullia digitalis in Trueman and Brown, 1987)

Many burrowing bivalves have the ability to eject water from the mantle cavity into the sediment while burrowing (Trueman, 1968a). This mechanism produces a fluid-sand mixture analogous to quick-sand, increasing the speed and ease of penetration. This fluid ejection was not found in Dentalium (Trueman, 1968c). Local currents were observed around the ventral aperture of cadulus aberrans, during 14 studies with dye, but lacked the velocity to 'liquify' sediment. The burrowing rate observed in Cadulus aberrans is faster than that observed for Dentalium inaequicostatum (Trueman, 1968c) or the bivalves Donax vittatus (Trueman, 1983) or Ensis arcuatus (Trueman, 1967), the latter considered one of the fastest burrowing molluscs. Unlike these other molluscs Cadulus can burrow completely, in one digging cycle with an average duration of 3 seconds (rate = 0.33 cmjs). Dentaliumtook 30 seconds and eight digging cycles to penetrate 1 em into sand (rate= 0.03 cmjs) (Trueman, 1968c). u. vittatus took greater than 60 seconds to penetrate 1 em (rate= 0.02 cmjs) (Trueman, 1983). Ensis burrowed completely (body length 7 em) in over 40 seconds (rate= 0.18 cmjs) (Trueman, 1967). Burrowing rates may be most appropriately compared between Cadulus and Ensis because both have thin, elongate shells, adaptations for rapid vertical burrowing (Stanley, 1970). 15 Chapter 2 BURROWING RHYTHMICITY

Introduction Differences in foot morphology, shell structure and the size ratio between shell and foot are used to define and distinguish the two scaphopod orders (Palmer, 1974). Dentaliid scaphopods have a large, thick shell, formed from three structural layers, that is often sculptured or ornamented. The dentaliid foot is muscular and short, relative to shell length, and has two lateral, epipodal lobes. Gadilid scaphopods have a thin, often translucent shell that is composed of only two prism layers. In many gadilid the shell appears highly polished (personal observation). This group is characterized by a long, eversible foot with a distal disk. Besides morphologic differences the two taxa appear to utilize the benthic habitat differently (Shimek,l990). The dentaliids are shallow burrowers that drag their large shell into the sand at an oblique angle, the dorsal end of the shell usually remaining exposed (Morton, 1959; Dinamani, 1964). This facilitates respiration and the release of gametes through the dorsal aperture into the water column (Yonge, 1937). The gadilids burrow completely and deeply into the sediment (Shimek, 1990; Chapter 1). This strategy likely Poses respiratory and reproductive constraints. Additionally, because there is no access to the water column, the mantle cavity can not be flushed for removal of wastes. Because this 16 behavior does not fit into the simplified scenario of dentaliid behavior, and in fact, appears to contradict much that is assumed about scaphopod behavior, laboratory experiments were conducted to describe the burrowing behavior of Cadulus aberrans. This gadilid species is abundant, accessible, found along the Pacific Coast of North America, and recently the focus of several studies (Hebert, 1983; Shimek, 1988, 1989, 1990).

METHODS ANT FARMS Cadulus aberrans and sediments (collected simultaneously) were placed in 20 X 30 X 1 em glass enclosures (ant-farms). Three enclosures were placed in each aquarium on an open seawater system. Two aquaria were used for a total of six enclosures. The narrow enclosures allowed observation of burrowing when the scaphopods moved near the glass plates. The enclosures were built of two sheets of glass resting in grooves on a wooden base and sealed along the sides with aquarium caulking (silicon). The aquaria were covered by black plastic sheets to prevent algal growth on the enclosures and to simulate their original habitat (at a depth of 60 m).

Before these enclosures were built, I built enclosures of many different widths, from 2 mm to 2 em, to determine the minimum width necessary for burrowing and the maximum width through which scaphopods could be observed. Scaphopods would not burrow in enclosures narrower then 1 em. This is because 17 as a scaphopod pulls itself upward, from a horizontal to a vertical position, the shell revolves in an arc around the foot. In the narrow enclosures, the shell would knock against the glass and the scaphopod would fall. Unable to erect itself, the shell could not be inserted into the sediment. In enclosures wider than 1 em burrowing scaphopods were seldom visible. The height was the maximum which would be completely submerged in the aquaria. The original aquaria were partially destroyed in the october 1989 earthquake and enclosures were rebuilt using plexiglass (earthquake proof) of the same dimensions. These sheets were held together by plexiglass bases and spacers. The new enclosures were placed in a long fiberglass tank, at Hopkins Marine Station, Pacific Grove, California, with running seawater, and this was covered with black plastic. Three new enclosures and three glass enclosures were used in November 1989. In January and February 1990 six plexiglass enclosures were used. The plexiglass enclosures may have allowed less aeration of sediment then did the glass enclosures because the seals along the sides were tighter. To investigate the effects of sediment aeration on scaphopod burrowing six additional plexiglass enclosures were built with aquarium bubblers or capillary tubing supplying oxygen Placed on the bottom. In all trials the gas blew the sediment out of the enclosures.

It was not possible to see all scaphopods through the ant farms so animals were removed and replaced by fresh 18 animals at monthly intervals. Fourteen times between April 1988 and March 1990, scaphopods and sediment were collected and placed in experimental enclosures then removed one month later in 4 em increments (i.e .. 0-4, 4-8, 8-12, 12-16, & 16- 20 em) using water pressure. The scaphopods were washed from ant farms onto a 0.5 mm screen to remove sediment. They were then placed in finger bowls for counting. Dead scaphopods which were on the surface (i.e. probably never burrowed) were not counted. Buried, dead scaphopods were counted. Removal always took place in the late afternoon. Survival varied between months and enclosures making it impossible to maintain equal numbers of animals in each replicate. Thirty to 50 scaphopods were placed in each enclosure. Because the number of scaphopods per enclosure varied, the results are given as percent found at each depth. This was determined by dividing the number found at each depth by the total number found in each enclosure. The percentages from six replicates were averaged. Histograms from each month represent the mean percent (±standard deviation) found in each depth interval. Some original data were not available after the earthquake. However all percentages were available and were used for graphs.

TIME-LAPSE

Six scaphopods burrowing in an experimental enclosure Were observed continuously for 4 days (March 30- April 3, 199 0) by a blaclc & white video camera which fed into a 19 panasonic time-lapse video recorder. The camera was obliquely aimed at the enclosure surface which was continuously illuminated by a red light for night-time camera exposure. The enclosure was always exposed to natural light. Time of emergence on the sediment surface, duration of surface stay, and time of reburial into the sediment were noted.

BOX CORE A box core (46 em deep X 20 X 30 em surface area) was deployed from the R/V Pt. Sur in March 1990 to look at natural burrowing depth at a 60 m deep collection site. After retrieval, the box was removed from the frame and returned upright to the laboratory. The sediments were partitioned and then sliced into 4 em thick sections from the surface downward by removing one side of the box and inserting plexiglass sheets at measured intervals. Each section was sieved over a 1 mm mesh screen and fauna were identified and enumerated under a dissecting scope. The sediment surface appeared to be intact, that is the surface layer was not blown away by the coring device. Only one core was taken.

RESULTS

ANT FARMS

In the experimental enclosures Cadulus aberrans frequently burrowed to a depth of 20 em (the bottom of ant farms). The Percent of Q. aberrans found in any depth stratum in the exp · er~mental enclosures varied throughout the course of 20 study. There were two trends in burrowing distribution in the enclosures: either shallow where most (greater than 70 percent) animals were in the upper strata (0-4 em) with little variation between replicates, or non-shallow where the scaphopods were found throughout the enclosures with large variation between replicates (Figure 4, Table 1). In the shallow months there was little variability (error bars on histograms) within each depth within each month signifying uniform distribution between the enclosures. Throughout the rest of the year inter-replicate variability increased (i.e. standard deviations approached, were equal to, or exceed the means). This can be noted by the error bars on histograms which are larger for other months indicating uniform or random burial within any enclosure (Fig. 4). Darkened sediment appeared in the bottom of each ant farm after several weeks, and in areas where scaphopods could be seen burrowing after several days. 21 Figure 4. scaphopod distribution within ant farms for each month. Six replicate ant farms were sampled by removing animals and sediment in 4 em increments. Bars are mean percentages of animals in each depth strata, error bars are 1 standard deviation. One of two trends is visible: either burrowing predominantly in the upper strata (with small standard deviations), or burrowing throughout the depth strata (with larger variation between replicates) in each histogram.

0-4

-e ... 111111 April 89 -:;::: S-12 1:;: w 0 12-16

16-20

0 20 40 60 eo 100 PERCENTAGE

0-4

..... ~~ E e II May aa ~ 8-12 Q.w Q 12-16

16-20

0 20 40 60 80 100 PERCENTAGE 22 Figure 0-4

4~ -..,E ...... IIIII Oct 88 :I: 8-12 1- Q. w Q 12-16

16-<0

0 20 40 60 80 100 PERCENTAGE

0-4

4~ -E ...... II Nov 88 :I: 8-12 1- Q. w Q 12-16

16-<0

0 20 40 60 80 100 PERCENTAGE

0-4 ! 4~ i:. 8-12 Ill D•c 88 c..w Q 12-16

16-

20 40 60 80 100 PERCENTAGE 23 Figure 4 continued

0-4

-e 4~ ... Ill Jan 89 --:X: 8-12 1- Q. UJ Q 12-16

16-20

0 20 40 60 80 100 PERCENT N:i E

0-4

4~ -E ... 1111 r~b 89. :X:-- 8-12 1- Q. UJ Q 12-16

16-20

0 20 40 60 80 100 PERCENTN:iE

0-4

..... 4~ e. ~ 1111 April 89 8-12 i!:Q. 11.1 Q 12-16

16-20

0 20 40 60 80 100 PERCENTN:iE 24 Figure 4 continued

0-4

IIIII May e9 ::z:: 8-12 I;: LIJ Q 12-16

16~0

0 20 40 60 eo 100 PERCENT~E

0-4

-e .!:, Ill Jun~ e9 ::z:: 8-12 1- Q. LIJ 0 12-16

16~0

0 20 40 60 eo 100 PERCENT~E

0-4

...... e ~.a .!:, 8-12 Ill July e9 i!:Q. LU 0 12-16

16~0

0 40 60 eo 100 PERCENT~E Figure 4 continued

0-4

..... 4-8 E ...... 1111111 Nov 89 :::t: 8-12 Q..... w 0 12-16

16~0

0 20 40 60 80 100 PERCENTAGE

0-4 e ~ 1111111 Jan 90 :::t: 8-12 c..... w Q 12-16

16~0

0 20 40 60 80 100 PERCENTAGE

0-4

..... 4-8 e ~ 1111111 Feb 90 =·.... S-12 c. UJ Q 12-16

16~0

0 20 40 60 80 100 PERCENTAGE 26 Table 1. Summary of Burrowing Behavior in Ant Farms.Shallow denotes greater than or equal to 70% of animals in upper 4 em strata. Non-shallow denotes months when animals were found throughout the depth strata. Note that not all months were sampled in each year.

Month Behavior

April 1988 Shallow

May 1988 Shallow

October 1988 Non-shallow

November 1988 Non-shallow

December 1988 Non-shallow

January 1989 Non-shallow

February 1989 Non-shallow

April 1989 Shallow

May 1989 Shallow

June 1989 Shallow

July 1989 Non-shallow

November 1989 Non-shallow

January 1990 Shallow

February 1990 Shallow 27

TIME-LAPSE h total of 27 emergences and burials were observed during the 96 hours of video monitoring. Twenty-two emergences occurred during the night time, which was defined from 1831 to 0629 hours. Two emerged during dusk which was the hour preceding night, two emerged during the daylight hours, 0731 to 1729 hours, and one surfaced at dawn, the hour preceding daylight (Figure 5). Some emergences were complete, the scaphopods literally popped out of the sediment and fell onto their sides, others just exposed the dorsal tip of the shell. Horizontal movement over the sediment surface occurred every night. The surface-stay duration ranged from less than 1 minute to 4 hours and 20 minutes. Most surface intervals (15) were less than 49 minutes, five were between 50-99 minutes, two were between 100-149 minutes, one was between 200-249 minutes, and one was greater than 250 minutes (Figure 6). Dividing the number of surface emergences (27) by the number of scaphopods burrowing ( 6) , each scaphopod could have emerged slightly greater than four times during the 4 days of observation or, once per day. However, because individuals could not be distinguished I could not determined how many of the six scaphopods emerged.

Although time-lapse equipment was not available at other times, observations of scaphopod emergence were made most 28 frequently at night. They appear to be nocturnally active throughout the year. ~~------~ 16:31-6:29

20 (I) <.1"' ~ 15 w~ 10 0 0 t::>. -~ ,.... I 0 5 t::' i .;;

0 Dawn

Figure 5. Number of scaphopod emergences per time of day. Times are given in military hours (n=6). 29

15 Q) "'u l 10

! 5

0 1-49 50-99 100-149 150-199 200-249 250-300 Sl.RF ACE DlflA TION (mil)

Figure 6. Surface-stay durations for emerged scaphopods observed by time-lapse video over four days (n=6 scaphopods in ant farm). 30 BOX CORE DATA Cadulus aberrans were found throughout the box core.

The largest number (13) were found in the surface (o-4 em) layer. The numbers in the next 11 depth strata were: 3, 3 , 1 , 0, 0, 1, 0, 4, 0, 0, 1, respectively (Figure 7). The deepest stratum was only 2 em thick, that is 44-46 em from the surface. The sediment was aerobic throughout the core. Isolated pockets of blackened sediment were found below the sixth strata (20-24 em). These anaerobic areas were approximately 1-2 em in diameter. The deeper poclcets contained a fibrous mucus which appeared to be degrading polychaete tubes. The sediment contained large quantities of wood chips and various terrestrial plant debris. A green juniper leaf was found in the eleventh strata (40-44 em), indicating that these sediments had been recently turbated or deposited. No scaphopods were in the anaerobic pockets. Several storms passed through Monterey Bay just before this core was taken and may explain the juniper leaf, the large woody debris and aerobic sediment at this depth. This may also indicate that these scaphopods were passively buried by moving sediment. However, given the ease with which this species moves through the sediment, upward burrowing through accumulated sediment would probably not be a problem. 31

0-4 4-8 8-12 - 12-16 ...E 16-20 ; 20-24 f- 24-28 c.. w 28-32 c 32-36 36-40 40-44 44-46

0 10 20 30 40 50 60 PERCENTAGE

Figure 7. Percentages of scaphopods found in each depth interval in a box core taken in 60 m water depth in Monterey Bay. The core was partitioned then sliced in 4 em increments after retrieval (N=26). Data are shown by percentages for comparison with ant farm burrowing. In the box core when three or more scaphopods were found in the same stratum they were usually close together. However, because replicate samples were not taken distribution patterns can not be assessed.

Discussion Differences in behavior between the two scaphopod orders, Dentaliida and Gadilida, were predicted based on gross morphological and microstructural differences. This prediction was confirmed in at least two species: burial of Cadulus aberrans is variable while Dentalium entalis consistently burrows shallowly. The former burrows deeply or shallowly depending on time of day and perhaps time of year, and does not maintain continuous contact with the water column. The behavior of Dentalium entalis was summarized by

Morton (1959) as consisting of repeated shallow burrowing events interspersed with browsing within the sediments for food. Emphasizing the necessity of these scaphopods to stay near the water column, Gainey (1972) described reverse burrowing and reburrowing. Dentaliids maintain continuous contact with the water column for respiration, elimination of feces, and gamete dispersal (Morton, 1959; Shimek, 1990; Yonge, 1937). None of the studies of dentaliid burrowing were conducted through time. However, Shimek (1990) reported direct observations, while SCUBA diving, of wholly or Partially exposed animals, suggesting that shallow burrowing 33 by the dentaliids is not an artifact of laboratory conditions. The behavior of cadulus aberrans is not as easily summarized. Complexities arise from behavior patterns which appear to be regulated by circadian periodicity compounded by possibly random burrowing depth. Alternately, burrowing depth could be correlated to any of the numerous variables, including proximity of other scaphopods or prey, time of day, small and large scale sediment size variations, or any combination of these and other variables, which were not controlled during these experiments. And in fact the ability to bury and remain buried for extended periods, possibly longer then days, leaves unanswered the questions of how these organisms respire while buried, reproduce, and flush wastes from their mantle cavity. Dentaliumspecies are broadcast spawners (Lacaze­ Duthiers, 1857) and it has been assumed that this method of reproduction is ubiquitous throughout the class Scaphopoda (Morton, 1959). In a year-long study of Cadulus aberrens reproductive activity, Hebert (1983) found some gametogenic activity in most months of the year with almost all individuals active in the spring (April, May and June). This period of highest reproductive activity coincides with several months when shallow burrowing was observed (both in 1988 and 1989) possibly indicating a correlation between peak reproductive periods and shallow burrowing. If related, the shallow burial would facilitate frequent contact with the 34 water column for ejection of gametes. Shallow burrowing would also facilitate reproductive activity indirectly by decreasing the energy expended in burrowing through the deeper, more compacted sediment strata. The shallow burrowing observed during January and February 1990 either contradict this hypothesis or could indicate some unusually timed reproductive activity. But, more likely, these data resulted from some experimental artifact, perhaps due to the change of experimental aquaria (see methods). During the time-lapse observations, nocturnal emergence was seen more frequently then diurnal or crepuscular activity. Whether this rhythm results from endogenous processes or exogenous influences, such as lunar period or daylight, was not determined. The activity is characterized by short periods of exposure in the water column. A similar nocturnal behavior pattern, the first report of 24-hour rhythms, was described in earthworms, Lumbricus terrestris, under laboratory conditions by Darwin (1881) and in natural conditions by Baldwin (1917). This worm crawled about on the soil surface at night then retreated into the sediment during the daytime. Bennett (1976) reported that the light­ withdrawal reflex, the locomotor patterns and the rate of oxygen consumption vary throughout the day in L· terrestris. Solar-day oxygen utilization and locomotor activity rhythms have been described in many invertebrates and vertebrates (see Brown et.al., 1958). The possibility of rhythmic oxygen consumption could have important implications for the 35 mechanism of respiration of cadulus and may, in part, explain the ability of this scaphopod to remain buried for extended periods without access to the water column. There appears to be a relationship between circadian and annual rhythmicity, and examples can be found among most living organisms including plants, invertebrates and vertebrates from both the marine and terrestrial habitats (Sollberger, 1965). Palmer (1976) suggested that photoperiodism is important in the control of seasonal cycles and oscillation with exogenous cycles results in entrainment (development of synchronized, endogenous rhythms). Some data, for example from April and May 1988 and 1989, are similar enough to indicate that this type of rhythmicity, both circadian and annual may be controlling the burrowing behavior of Cadulus aberrans. Other data, January and

February 1989 and 1990, would indicate not. The lack of data for all months makes interpretation of this trend impossible. Variables acting in concert such as photoperiod changes, changes in water temperature, or sediment grain size could influence rhythmicity, nightly emergence and variations in burrowing depth. One could test for entrainment of these rhythms by observing behavior under conditions of constant light, temperature and grain size. In the monthly burrowing experiments, the experimental animals were removed from enclosures during the daytime. Diurnal rhythms of movement through the sediment column may have determined where any scaphopod was at the time of 36 removal and enumeration. Because the animals were sampled at haphazard times, but always during the daylight hours the distributions (e.g.figure 4) are a measure of vertical daytime location only. Because these animals burrow rapidly (chapter 1) there exists a possibility that the deeper animals burrowed between depth strata while the experimental enclosures were being washed. This possibility exists but is small because 1) the enclosures were washed out quickly, and 2) there is a latent period between burrowing cycles. Scaphopods burrow for one or two cycles (1-2 em) then pause or stop completely. The appearance of darkened sediment in the bottom of the experimental enclosures and in burrows in the ant farms did not seem to affect burrowing. Scaphopods were frequently found in these apparently anaerobic sediments and seemed to facilitate anaerobiosis in their burrows. Morton (1959) described scaphopod feeding burrows as areas excavated by the foot then browsed by the captacula. Cadulus appeared to excavate this type of burrow but the permanence of this location is questionable in light of the time-lapse data and the speed and ease with which this scaphopod can move through the sediment (Chapter 1). Scaphopods were noted daily in anaerobic burrows consecutively for over 1 week . The within month variability (error bars on histograms) may result from intraspecific interactions, specifically aggregation. Data from Hodgens & Nybakken (1973), and Hebert (1983), suggest that Q. aberrans forms aggregations (standard 37 deviations greater than the means) in the same area where these animals were collected and the box core was taken. These aggregations may occur in two and three dimensions. In experimental enclosures, some three-dimensional movement is prevented by the enclosure walls. In essence relatively unhindered movement is possible only in a vertical plane. Therefore using data from this experiment to confirm vertical aggregation is not possible. The presence of scaphopods as deep as 46 em from the sediment-water interface in the box core indicates that deep burrowing was not an experimental artifact of ant-farm enclosure walls and that this behavior occurred in this population in nature. Many questions remain regarding gadilid scaphopod behavior. How and when are gametes released? How do these animals respire and remove wastes while buried? Are circadian rhythms present throughout the year, and if so are they similar to those observed (daily burial with nightly emergence)? Are the monthly rhythms correlated with reproductive behavior?. If aggregatory behavior occurs, is this from recruitment of settling larvae to particular areas, or do adults find one another? How is burrowing behavior affected by other burrowing infauna and predators? How is burrowing behavior affected by seasonal variation in sediment grain size, and storms? 38 LITERATURE CITED Baldwin, F.M. 1917. Diurnal activity of the earthworm. J. Anim. Behav. 7:189-190 Barnes, R.D. 1987. Invertebrate Zoology. CBS College Publ., New York. pp. 304-5 Bennett, M.F. 1976. Possible roles of biological rhythms in the osmoregulation of animals, In: Biological Rhythms in the Marine Environment. Decoursey, P.J. (ed.) Univ. South Carolina Press. pp.137-44 Brown, A.C. 1964. Blood volumes, blood distribution and sea­ water spaces in relation to expansion and retraction of the foot in Bullia (Gastropoda) . Journal of Experimental Biology 41:837-854 Brown, F.A., H.M. Webb & M.F. Bennet 1958. Comparisons of some fluctuations in cosmic radiation and in organismic

activity during 1954, 1955, and 1956. Am. J. Physiol. 195:237-43 Darwin, c. 1881. The Formation of Vegetable Mould Through the Action of Worms. John Murry, London. 326 p. DeFresse, D.S. 1978. Ecology and burrowing behavior of Ascobulla ulla (Opisthobranchia: Ascoglossa). The

Veliger 30 ( 1): 40-45 Dinamani, P. 1964. Burrowing Behavior of Dentalium.

Biological Bulletin 126 ( 1): 28-32 Gainey, L.F. 1972. The use of the foot and the captacula in the feeding of Dentalium. Veliger 15(1):29-34 39 Hebert, A 1986. Reproductive behavior of three Central Californian scaphopods. Masters Thesis. Moss Landing Marine Labs. 7lp. Hodgson A.T.& J.W. Nybakken 1973. A quantative survey of the benthic infauna of Northern Monterey Bay, California. Final summary data report for August 1971- February 1973. Contrib. from the Moss Landing Marine Labs. #40. Technical Publication 73-8. 24lp. Kier,W.M. 1988. The arrangement and function of molluscan muscle In: The , Vol. 11. E.R. Trueman and M.R.

Clarke ,eds. Academic Press, New York. pp.211-251 Lacaze-Duthier, H. 1857. Histoir de la organisation et du developpement du Dentale. Ann. Sci. Nat., Zool. et Biol.

Animale.6:266-281, 7: 5-51, 8:18-44 Morton,J.E. 1959. The habits and feeding organs of Dentalium entalis. Journal of the Marine Biological Association of

the U. K. 38:225-238 Palmer, J.D. 1986. An Introduction to Biological Rhythms. Academic Press, New York. 375p. Petitte-Fischer and Franc. 1968. Classe des Scaphopodes. In: Traite' de Zoologie, Tome v. P.P. Grasse', ed. pp.987-

1017 Poon, P. A. 1987. The diet and feeding behavior of Cadulus tolmiei Dall, 1897 (Scaphopoda: Siphodentalioida). The

Nautilus 101 ( 2): 88-92 Shimek, R.L. 1988. The functional morphology of scaphopod captacula. Veliger 30:213-221 40 Shimek, R.L. 1989. Shell morphometries and systematics: A revision of the slender, shallow-water Cadulus of the

Northeast Pacific. The Veliger 32 ( 3): 233-246 Shimek, R.L. 1990. Diet and habitat utilization in a Northeastern Pacific Ocean scaphopod assemblage. AmericanMalacological Bulletin. 7:147-169 Sollberger, A.l965. Biological Rhythm Research. Elsevier, New York. 46lp. Stanley, S.M. 1970. Relation of shell form to life habits in the Bivalvia. Geological Society of America, Memoir 125:1-296 Steiner, G. 1990. Beitrage zur vergleichenden Anatomie unf Systematik der Scaphopoda (Mollusca). Dissertation UniversityVienna. 174p. Trueman, E. T. 1967. The dynamics of burrowing in Ensis

(Bivalvia). Proceedings of the Royal society, series B 166:459-476 Trueman, E. T. 1968a. The burrowing activities of bivalves. In: Studies in the Structure Physiology and Ecology of Molluscs. Zoological Society London Symposia 22:167-187 Trueman, E. T. 1968b. The mechanism of burrowing of some naticid gastropods in comparison with that of other molluscs. Journal of Experimental Biology 48:663-678 Trueman, E. T. 1968c. The burrowing process of Dentalium

( Scaphopoda). Journal of Zoology 154:19-27 Trueman, E. T. 1969. The fluid dynamics of molluscan locomotion. Malacologia 9(1) :243-248 41 Trueman, E. T. 1975. The Locomotion of Soft-Bodied Animals.

Elsevier, New York. 194 pp. Trueman, E. T. 1983. Locomotion in molluscs, In: The Mollusca, vol. 4, A.S.M. Saleuddin and K.M. Wilbur ,eds.

Academic Press, New York. pp.155-198. Trueman, E. T. and A.C. Brown. 1985. Dynamics of burrowing and pedal extension in Donax serra (Mollusca: Bivalvia).

Journal of Zoology 207(A) :345-355 Trueman, E. T. and A.C. Brown. 1987. Proboscis extrusion in Bullia (Nassariidae): A study of fluid skeletons in Gastropoda. Journal of Zoology 211:505-513 Trueman, E. T., A.R. Brand and P. Davis. 1966. The dynamics of burrowing of some common littoral bivalves. Journal of Experimental Biology 44: 469-492 Whiteaves, J.F. 1887. On some marine invertebrates dredged or otherwise collected by Dr. G. M. Dawson in 1885 in the northern part of the strait of Georgia, in Discovery Passage, Johnstone strait, and Queen Charlotte and Qui tsino Sounds, British Columbia; with a supplementary list of a few land and fresh water shells, fishes, birds, etc., from the same region. Transactions of the Royal Society of Canada 4:124, Fig. 2 Yonge, C. M. (1937) Circulation in the mantle cavity of Dentaliumentalis. Proc. Malac. Soc. Lond. 22:333-338