Ampelisca amphipod tube mats may enhance abundance of northern quahogs Mercenaria mercenaria in muddy sediments.

ABSTRACT Field surveys in southeastern Raritan Bay and laboratory studies from 1999 to 2005 were conducted to compare the characteristics of mud and sand habitats in relation to the abundance of the northern quahog (Mercenaria mercenaria). In 2000, the population density of quahogs was about 15 times higher in the mud habitat than in the sand habitat. In addition, the mud habitat also had a dense population of the amphipod Ampelisca abdita (about 24,000 [m.sup.-2]) associated with it. This species produces mats of tubes over the bottom. The sediment surface of the mud was comprised mostly of fecal pellets, the majority of which was produced by A. abdita. In contrast, the sand habitat did not have A. abdita tubes or much erect surface structure; its sediments were comprised of medium grain sand ([phi] = 1.17-1.4). In southeastern Raritan Bay, the principal quahog predators are the longwrist hermit crab (Pagurus longicarpus), Atlantic oyster drill (Urosalpinx cinerea), and xanthid mud crabs. Collectively, they were >7 times more abundant in the sand habitat than in the mud habitat. We suggest that quahogs are abundant in the mud habitat because the presence of the tube mats probably reduces water siltation, encourages settlement of larval quahogs and deters predation on the quahogs.

KEY WORDS: Mercenaria, Ampelisca, habitat, pellets, sand, tube mats, predators

INTRODUCTION

Northern quahogs (Mercenaria mercenaria) can be found in estuaries along the Atlantic coast of North America from the Gulf of St. Lawrence to the Florida keys (Harte 2001). In 2004, the United States commercial landings of 9.4 million pounds (meats) of northern quahogs exceeded landings of any other estuarine bivalve; their ex-vessel value was $37.8 million (Pritchard 2005). Fishermen typically harvest quahogs in sand or muddy-sand sediments, but quahogs are nearly always scarce in soft mud habitats (Rhoads & Young 1970, Rhoads 1974, Fegley 2001. Mann et al. 2005). This scarcity is apparently caused by high silt concentrations in the water close to the bottom (Rhoads & Young 1970, Murphy 1985). The silt clogs the digestive tract of larval quahogs and slows the growth of sedentary quahogs (Davis 1960, Bricelj et al. 1984). Most field studies of quahogs have emphasized the suppressing effects of predators on quahog abundance as summarized by MacKenzie (1977), Kraeuter (2001) and MacKenzie et al. (2002).

In southeastern Raritan Bay, New Jersey, in contrast to the other locations outside of Raritan Bay, quahogs are abundant in mud areas located in the bay's broad central area, and they are far more abundant there than in its sand areas that extend from its shores to the mud area (Celestino, 2003). A major difference between the mud and sand habitats in Raritan Bay is the presence of dense tube mats of the ampeliscid amphipod, Ampelisca abdita. A. abdita ranges from Maine to at least Florida, and produces dense masses of tubes (Stickney & Stringer 1957, Mills 1967, 1969) (Table 1). For this study, from 1999 to 2003, we examined several features of the mud and sand habitats in southeastern Raritan Bay that may explain the difference in quahog abundances between the two habitats. Particular emphasis was placed on describing the antecology of A. abdita, including its distribution, its persistence in the area through time, its number of generations per year and the temporal appearance of its tubes. In the Discussion, we compare our findings with those in the literature on the autecology of the quahog and suggest why the presence of A. abdita may be the reason the quahog abundance is high in the mud area of southeastern Raritan Bay.

Characteristics of Southeastern Raritan Bay

The deeper areas of southeastern Raritan Bay primarily consist of mud sediments that extend over an area of about 30 [km.sup.2] (Fig. 1). The water depth averages about 7 m (range, 3.4-8 m) at low tide. The shallower areas that extend from the shorelines offshore to the mud areas consist primarily of sand sediments and have a total area of about 18 [km.sup.2]. At their midpoints from the southern and eastern shores to the edges of the mud areas, water depths over the sand areas range from 3-4 m and 1-2.5 m, respectively, at low tide. Salinities in this part of the bay are about 26 [per thousand] during summer and 21 [per thousand] to 24 [per thousand] during winter (Cerrato et al. 1989). Surface water temperatures average 21[degrees]C to 24[degrees]C during summer, 0.4[degrees]C in early February, and 6[degrees]C to 7[degrees]C during March.

[FIGURE 1 OMITTED]

In the late 1980s and early 1990s, the mud area of southeastern Raritan Bay had relatively few northern quahogs and a large number of starfish (Asterias forbesi) (Cerrato et al. 1989, MacKenzie & Pikanowski 1999). By the mid 1990s and thereafter, the starfish were no longer present and the quahogs became abundant (MacKenzie & Pikanowski 1999). In 2000, the State of New Jersey Department of Environmental Protection surveyed the quahog abundance in southeastern Raritan Bay. Its population density was about 15 times higher in the mud than in the sand habitats: 14.9 quahogs [m.sup.-2] versus 1.0 quahogs [m.sup.-2] (Fig. 1) (Celestino 2003). Since the mid 1990s, southeastern Raritan Bay has had a year-round commercial fishery for quahogs. It consists of 20-100 boats, each with one fisherman using a bull rake. The fishermen harvest quahogs almost exclusively from the mud area.

In 1974, A. abdita was scarce in southeastern Raritan Bay (Steimle & Caracciolo-Ward, 1989), but it was abundant and widespread in 1986 in the mud area (Cerrato et al. 1989). It is not known when A. abdita became abundant between 1974 and 1986. It is assumed that it was abundant between 1986 and 1999, but there are no data to prove this. A. abdita occupy tubes about 3.5 cm long and 2.5-3.5 mm wide at the mouth end. The tubes are flattened laterally, and are composed of nonchitinous, pliable organic material. Each A. abdita forms a tube by secreting mucus around its body. This mucus collects surface sediments on the tube's outer surface as the amphipod forms it. The Raritan Bay tubes are covered with a continuous layer of brown fecal pellets and finer particles held in place by the mucus. The tube lining is smooth and parchment-like, and lacks attached sediments. The top opening of the tube becomes its mouth and the bottom opening is gradually attenuated. About two-thirds of the tube length extends vertically into the water; the remainder is anchored in the sediment (Fig. 2).

[FIGURE 2 OMITTED]

Mills (1967, 1969) has made the only previous extensive observations of A. abdita in its natural habitat, studying them on an intertidal sandy flat in Barnstable, MA. He found that the abundance of A. abdita was irregular. The substrate sediments were stable when the mats were present, but unstable when absent. The abundances of associated macroinvertebrate species fluctuated accordingly.

A. abdita feeds at the tube mouth collecting diatoms, probably flagellates, amorphous organic material and clay-silt grains. It digests the living and organic matter, and forms the clay-silt grains and diatom cases into pellets to be cast onto the surface substrate below them (Mills 1967, Redmond et al. 1994).

Each generation of A. abdita is short-lived, the males and females producing one brood of juveniles. Adults mate in the water column (Mills 1967, Borowsky & Aitken-Ander 1991) and die shortly afterward. After about 2 wk in brood pouches of the females, the juveniles are about 1.5 mm long, and they are released from their tubes. The females then die shortly afterward. The juveniles construct a new mass of tubes (Mills 1967).

The planktonic invertebrates in Raritan Bay include copepods (mainly, Acartia tonsa in summer and A. clause in winter), polychaete and nemertean larvae, amphipod zoeae, barnacle nauplii, mysid larvae of shrimp, cyprids, crab zoeae, gastropod veligers and fish larvae (Yamazi 1962, Jeffries 1964, Croker 1965). The plankters presumably produce large quantities of fecal pellets. Recent studies have shown that copepods break down most fecal pellets distributed through the water (Hofmann et al. 1981, Bathmann et al. 1987, Lampitt et al. 1990, Noji 1991, Noji et al. 1991, Viitasalo et al. 1999). As they descend onto the bottom, the fragmented pellets likely mix with settled phytoplankton and become a component of the sediment that accumulates with the A. abdita pellets in the mud habitat. At least eight fish species are present seasonally; some prey on A. abdita (Wilk & Silverman 1976, Collette & Klein-MacPhee 2002).

MATERIALS AND METHODS

Benthic Sampling

Fifty-four stations located in the mud habitat were sampled to determine the extent and the characteristics of the A. abdita tubes. At each station, two samples were taken with a Petite Ponar hand grab with an opening of 15 cm x 15 cm. The samples were brought aboard the vessel, the hand grab was opened, and then a shallow sample was scraped from the sediment surface into a beaker, and later viewed under a dissecting microscope, as proposed by Watling (1991). The pellets were measured and photographed using a scanning electron microscope at the EM Facilities, Nelson Biological Laboratories, Rutgers University, Piscataway, NJ. Sediment was collected from the sand areas to determine their grain sizes ([phi] value). This was done by passing them through a stack of sorting screens.

The total organic carbon (TOC) in the A. abdita tubes was determined by collecting them at several mud stations. The TOC in the sand sediments was determined by taking cores of bottom sediments on January 17, 2003. The tubes and sediments were spooned separately into small glass jars aboard the vessel. The samples later were placed in a refrigerator overnight, and the tubes afterward were washed of loose particles under a freshwater faucet. The tubes and sand were put into separate aluminum cups and dried at 110[degrees]C for at least 24 h, weighed and then held in a muffle furnace at 510[degrees]C for 4 h and reweighed. The weight loss in the furnace was considered to be the percentage of TOC present.

The A. abdita tubes were sampled on 37 dates from August 22, 2001 through October 17, 2005. Most but not all stations were sampled on every survey date. Upon retrieval, the grab cover was opened and the extent of coverage of amphipod tubes, from 0% to 100%, was estimated visually. A surface with 0% coverage had no tubes, whereas a surface with 100% tubes was entirely covered with a dense mass of erect tubes (Fig. 3A). In addition, the age of the tubes at each station was recorded, as new (recently formed), middle-aged (upright but obviously not new or disintegrating), or old (disintegrating) (Fig. 3B). The contents of the grab were emptied into a 0.5-mm mesh sieve, spooned into labeled jars, and later in the day buffered formalin was added to the jars to preserve the sample. After 4 to 5 days, the formalin was replaced with 70% alcohol. The A. abdita and their tubes were counted using a dissecting microscope.

[FIGURE 3 OMITTED]

The relative abundance of predators was determined by sampling four fixed stations spaced across each of the mud and sand habitats with a specially designed dredge. The dredge had a mouth opening 60-cm wide. The blade at its bottom was 7-cm wide and angled at 45[degrees] so the predators in its path would slide up and into the fine-meshed bag (2-mm mesh openings) of the dredge. A planing board, 12.5-cm wide, was fastened 60 cm ahead of the mouth and positioned at the same level as the mouth to kick up predators from the bottom to collect in the bag. The dredge was towed along the bottom at a speed of 1.5 knots for 4 min at each station. When retrieved, its collected material was bagged and the predators later were identified and counted. The eight stations were sampled every 4-6 wk on 10 dates during 1999 (April 12 to October 22), on 8 dates in 2000 (January 26 to August 22), and on 2 dates in 2001 (May 9 and September 20).

Laboratory Observations

A. abdita were observed constructing its tubes in laboratory pans to aid in explaining its distribution in the mud and sand sediments. Three types of sediments were tested in plastic pans, 20 x 35 cm and 13-cm deep. Each pan received a different sediment: (1) medium-sized sand; (2) a collection of mud from the surface of the mud habitat and (3) an inorganic paste consisting of fullers earth (clay) and raw seawater. To prepare the mud, it was spray-washed with seawater through a 0.63 [micro]m sieve to break up any A. abdita pellets. The three sediments were spread in a layer 5 mm thick over the pan bottoms. Trays were then filled with raw seawater and air stones added. Finally, about 500 A. abdita were scattered across the bottom of each pan.

RESULTS

The entire mud area was covered with dense mats of A. abdita tubes (Figs. 2, 3a, 3b); its sediment surface consisted of a layer of fecal pellets mixed with a relatively small mixture of silt, clay, fecal fragments, and other organic matter. The pellets were of 2 distinct sizes. The A. abdita pellets are 110-140 [micro]m long (Fig. 4); the others are about 450 [micro]m long. They are rod-shaped with rounded ends. As viewed under a dissecting microscope and a scanning electron microscope, they consisted of clay, silt and diatom shells probably held together by mucus. The A. abdita pellets were far more numerous than the larger pellets and they usually occupied about 90% of the space. The pellets were whole for a vertical distance of about 1 cm below the sediment surface. At distances between 1 and 2 cm below the surface, they were partially eroded (the larger pellets disintegrated more slowly than the A. abdita pellets), and below them the sediment consisted of anaerobic black silt. The TOC concentrations in A. abdita tubes were 10.8% and 13.2% in two determinations on January 17, 2003.

[FIGURE 4 OMITTED]

Ampelisca Abdita Tube-pellet mud habitat

The A. abdita tube mats were consistently present over the entire mud area. The tubes usually covered the mud surface so completely that it was not visible. This was especially true after a new formation of tubes by the juvenile A. abdita. Year-round sampling, which included observations of their appearances (new, middle-aged or old), showed that A. abdita has three breeding cycles per year. New generations settled onto the bottom and constructed new tubes in May-June, September-October and December-January. The new tubes usually were spaced densely, and so the settling of A. abdita juveniles must compete strongly for available space. Several weeks after the new tubes were constructed, they slowly began to disintegrate and lay flat on the bottom. At this time, the sediment surface was visible between the tubes (Fig. 3B).

The next generation of juvenile A. abdita and their tubes did not settle and construct its tubes over the entire 30 [km.sup.2] all at once, but rather large sections, as much as a third of the area was settled at a time, and then another large section was settled. The time to settle the entire area was three or more weeks.

The tube coverage of the mud habitat averaged 62% (range, 36% to 96%)/sampling day from August 2001 through October 2005 (Fig. 5). Some of the lower values were recorded during the period when the tubes were decaying. While examining the collections in the grabs, no small quahogs of any size were observed among or on the surfaces of the masses of tubes. A small number of dwarf surfclams Mulinia lateralis sometimes were present on the tubes and in the sediment beside them.

[FIGURE 5 OMITTED]

The numerical abundance of A. abdita was determined on 6 dates in 2001, 2002 and 2003 by counting their numbers in single samples from 2-8 sampling stations (34 determinations). As projected to a square meter, its density averaged 23,700 [m.sup.-2] (range, 989-57,185 [m.sup.-2]) (Table 1).

On October 23, 2001, 7 stations with 100% coverage had from 29,000-98,000 tubes [m.sup.-2]; a grab sample with 75% coverage had 25,000 tubes [m.sup.-2] and a grab sample with 45% coverage had 14,000 tubes [m.sup.-2]. On this date, the number of tubes present was about the same as the number of A. abdita present: an average of 23,290 tubes and 23,536 A. abdita [m.sup.-2] at the 7 stations.

Sand Habitat

The sediment in the sand habitats was medium sand ([phi] = 1.17-1.4). In the broad area off the south coast of the bay, the sand surface was essentially free of any emergent structures. In contrast, the narrower sand habitat that extends westward from Sandy Hook had scattered quahog shells covered with bryozoans and sponges and chains (stacks) of the common Atlantic slipper-snail (Crepidula fornicata). On January 17, 2003, the TOC was 0.29% at two areas in the sand sediments.

Predator Abundances

Quahog predators had a much lower abundance in the mud habitat than in the sand habitat. The longwrist hermit crab (Pagurus longicarpus), Atlantic oyster drill (Urosalpinx cinerea) and xanthid mud crabs were the most numerous quahog predators collected in our dredge. For the 20 sampling dates over three years, 1999 to 2001, their combined numbers at the four mud stations were 27 P. longicarpus, 11 U. cinerea and 175 xanthid mud crabs. The four sand stations had a combined total of 415 P. longicarpus, 131 U. cinerea and 899 mud crabs; or 15 times the number of P. longicarpus, 12 times the number of U. Cinerea and 5 times the number of mud crabs in the mud stations.

All years and stations were combined to test the main hypothesis, Ho: predator density is the same in mud and sand stations. The untransformed and log-transformed sampling distributions of all three species failed normality. Mann-Whitney rank sum tests found the lower mud densities were significantly different from the higher median sand densities of all three species with P < 0.001. Mean values of predators were lower in the mud collection also, but were not tested (Table 2). The mud samples showed a lower predator density than the sand samples.

The sand habitat also had some predaceous northern moon snails (Euspira heros) (Greene 1978, Haskin 1951); their sand collars were found in the dredge collections in May and June each year. The dredge also collected some lady crabs (Ovalipes ocellatus), Atlantic rock crabs (Cancer irroratus) and blue crabs (Callinectes sapidus). The mud stations had a total of 7 of these larger crabs, whereas the sand stations had 25.

Laboratory Observations

In the three laboratory trays, the 500 introduced A. abdita dug immediately into each sediment. The A. abdita released in the tray with a layer of medium sand did not construct tubes.

In the tray with mud, more than 100 A. abdita had formed short tubes within an hour. After 56 h, the tubes were about 14-mm long, less than half the length of the tubes in the mud substrate in Raritan Bay. All were upright, and they were covered with the fine organic particles and silt that comprised the sediment. The particles collected on sticky mucus on the outside of the tubes as they were being formed in the midst of the sediment. The tubes did not grow further.

The A. abdita released in the tray whose bottom was covered with a paste of fullers earth constructed a small number of tubes--about 20 could be clearly identified. By the second day, the tubes had the overall shape of a typical tube but were much shorter. Their walls were constructed of tiny balls of fullers earth held together by mucus. Numerous fecal pellets that consisted of fullers earth were present on the sediment surface beside the tubes.

DISCUSSION

During the 5-y field study, the entire mud area was consistently covered by dense mats of A. abdita tubes. In contrast, populations of A. abdita and their mats in the shallow sections of Boston Harbor (Gallagher & Keay 1998) and Barnstable Harbor, Massachusetts (Mills 1967), were washed away by waves produced by strong wind storms. In southeastern Raritan Bay, the A. abdita are able to persist during windstorms because of its deeper water (7 m avg. depth), and shorelines that protect the area from wave action. Laboratory observations suggest that A. abdita does not occur in habitats where sediments consist of medium and coarse sand, because they have difficulty constructing their tubes with it. A. abdita construct tubes readily in sediments consisting of their own pellets, silt, and also fine sand. The A. abdita likely are present in the mud habitat because of its grain size rather than its high organic content. The laboratory observations also suggest that juveniles begin constructing tubes as soon as they land on the bottom and the tubes are built rapidly.

The A. abdita tubes were free of attached macrofauna and visible plants throughout the mud habitat at all times, as Mills (1967) had observed for A. abdita tubes in Barnstable Harbor. The cover of the tubes' surface with their pellets and smaller particles apparently prevents macrofauna from attaching and growing. Their absence obviously favors the persistence of the A. abdita. Southeastern Raritan Bay has abundant macrofauna that attach to the hard surfaces of shells and stones in the sand areas and they could be present in the mud habitat if it were suitable. The species include bryozoans, blue mussels (Mytilus edulis), common Atlantic slipper-snails, red beard sponge (Microciona prolifera) and the egg cases of U. cinerea, three lined mudsnail (llyanassa trivitatta) and eastern mudsnail (I. obseleta). In other locations, some benthic invertebrates produce calcareous tubes on which other macro-invertebrates can settle and grow and eventually kill them (Russ 1980, Qian 1999, Zuhlke 2001).

It was observed that for a few weeks before each of the breeding periods, the A. abdita tubes gradually disintegrate, leaving a surface of pellets and fine particles available for settlement and tube building by the next generation of juvenile A. abdita. Can these be the periods, especially the May-June period and lesser so the September-October and least in the December-January periods, when most larvae of benthic invertebrates, including quahog pediveligers, can settle in the substrate?

It is likely that the reasons the quahogs thrive in the A. abdita tube-pellet mud habitat in Raritan Bay are because the water near the bottom probably has little silt, the presence of the tubes favors the settlement of their pediveligers and quahog predators are scarce and have difficulty finding small quahogs among the tubes.

Some studies suggest that mud bottoms that lack surface tubes are marginal habitats for quahogs because of the silty water associated with them (Davis 1960, Rhoads & Young 1970, Rhoads 1974). The A. abdita tube mats and pellets and the amphipods modify the mud surface in Raritan Bay substantially. The mats cover and stabilize it, thereby minimizing the transport of silty sediments into the water during strong currents and the movements of fish and crabs (Hunt 2005). Moreover, the mud surface consists mostly of pellets that produce little turbidity even when swirled into the water. In addition, the A. abdita clear the water by capturing silt and forming pellets while feeding.

The presence of A. abdita tubes and their pellets may provide some positive attributes for settling quahog pediveligers. The tubes serve as baffles to slow water currents, a feature that has been shown to produce higher settlements of quahog larvae (Carriker 1961, Peterson 1986, Wilson 1990). The tubes also provide a shaded zone at their bases, another feature that attracts settling quahog larvae (Carriker 1961). The settling pediveligers, that are about 225-[micro]m long, are more stimulated to affix their byssus to sediment grains when among small grain sizes than when among larger sizes (Carriker 2001). The A. abdita pellets are about half the length of quahog pediveligers whereas medium-sized sand grains are much larger than the pediveligers.

The tube mats probably are poor habitats for the predators of juvenile quahogs in Raritan Bay. All are scarce on muddy bottoms with tube mats. The long wrist hermit crab is abundant on sandy bottoms that have little structural relief and the Atlantic oyster drill, and xanthid mud crab are most abundant among oysters and shells. Where they are present among the A. abdita tube mats in Raritan Bay, they probably have difficulty finding the juvenile quahogs. Mills' (1967) observation that eastern mudsnails (Ilyanassa obseleta) disappeared from areas that became covered with A. abdita tubes in Buzzards Bay is another example of gastropod numbers being low in substrates densely covered with its tubes. Similarly, earlier studies have shown that habitats containing dense upright structures, such as shells, eelgrass (Zostera marina) and polychaete tubes have higher abdndances of quahogs and other invertebrates in their sediments (MacKenzie 1977, Beal 1983, Peterson 1986, Peterson & Beal 1989, Zuhlke 2001). The structures probably provide them cover from predators, though other factors may also play a role.

In Virginia, quahogs are equally abundant in all water depths from 3-21 m; they appeared to be slightly scarcer at a depth of 2 m (Mann et al. 2005). This suggests that in Raritan Bay, the depth difference between the mud area, 7-m avg., and the wide sand area off the south coast, 3-4 m, and smaller sand area off the east coast, 1-2.5 m, has little influence on the difference in quahog abundance in them.

Our study has described several aspects of the quahog' s habitat in mud and sand bottoms in Raritan Bay. Because we conducted a survey and not an experiment, we did not control treatment; that is mud versus sand was not randomized. Because of the contiguity of the mud and sand sites and to the hydrodynamic processes causing sediment dichotomy, there are unaccounted for differences between the sites, the effects of which may confound the findings. This situation prevails for other similar surveys and cannot be helped.

Many questions remain. Do peliveligers actually settle in higher abundances among A. abdita tubes? Do peliveligers attach similarly to fecal pellets and sand grains? How do infaunal invertebrates, such as nematodes and polychaetes, interact with fecal pellets? After mature A. abdita females breed with males in the water, what is their behavior? How do the tubes affect predation rates on juvenile quahogs? How do finfishes prey on the A. abdita? Do they swallow whole tubes? What caused the starfish to abandon southeastern Raritan Bay? Can starfish inhabit bottoms covered with these tubes? How abundant were quahogs in Southeastern Raritan Bay during periods when the A. abdita were scarce? Will the quahogs become scarcer in Raritan Bay if the A. abdita become scarce again?

ACKNOWLEDGMENTS

The authors thank V. Starovoytov and R. E. Triemer, EM Facilities, Nelson Biological Laboratories, Rutgers University; for taking the scanning electron microscope photograph of the fecal pellet, M. Celestino; who provided the quahog density data used in Figure 1, P. Wilbur; who provided Figure 2, T. Finneran and D. Johnson made Figure 1 and V. G. Burrell, Jr., F. Csulak, D. S. Haven and R. N. Reid, who reviewed earlier drafts of the paper.

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CLYDE L. MACKENZIE, JR., ROBERT PIKANOWSKI AND DONALD G. MCMILLAN

James J. Howard Marine Sciences Laboratory, Northeast Fisheries Science Center, National Marine Fisheries Service, NOAA, 74 Magruder Road, Highlands, New Jersey 07732

E-mail: [e-mail]

TABLE 1.
Abundances of Ampelisca abdita in three northeastern
United States estuaries.

    Location        Number [m.sup.-2]           Reference

Long Island Sound    7,000 to 80,450    Sanders et al. (1962)
Jamaica Bay, NY     18,000 to 80,000    Franz and Tanacredi (1992)
Raritan Bay            989 to 57,185    This study

TABLE 2.
Summary of the relative densities of the quahog predators Pagurus
longicarpus, Urosalpinx cinerea, Xanthid mud crabs, and Crangon
septemspinosa on mud and sand substrates. All years and stations
are combined. Neither the untransformed nor log-transformed
distributions passed normality tests. Mann-Whitney rank sum tests
showed that all three species had significantly larger medians on
sand at the P > 0.001 level, but Crangon was not
significantly different.

    Predator       Substrate   [N.sup.1]   Median   Mean   Significance

P. longicarpus       Mud          75        0.0      0.4     <0.001
                     Sand         61        4.0     11.7
U. cinerea           Mud          75        0.0      0.1     <0.001
                     Sand         62        0.5      2.4
Xanthids             Mud          75        2.0      2.7     <0.001
                     Sand         62        6.0     15.6
C. septemspinosa     Mud          64        1.0      8.7       n.s.
                     Sand         64        4.0     15.8

[N.sup.1]--number of samples taken.