Organisms associated with oysters cultured in floating systems in Virginia, USA.

ABSTRACT The number and abundance of macro-faunal taxa was estimated from six floating structures (floats) used to culture the Eastern oyster (Crassostrea virginica) near Chincoteague Island, Virginia, USA. After a 10-too grow-out period, all organisms found among and attached to the cultured oysters were counted. The final mean size of oysters was 80.5 (14.7 SD) mm. Overall, 45 species of macrofauna were recorded with the number of species in the floats ranging from 24 to 36. There was no relationship between the number of taxa and the density of oysters in the floats. Total abundances of associated organisms were estimated at 12,746/float to 92,602/float. These findings highlight the diverse (taxonomic and trophic) and abundant nature of communities associated with cultured oysters. They also provide a baseline set of information that may help more clearly define the interactions between oyster culture and the environment.

KEY WORDS: Crassostrea, virginica, oysters, aquaculture, epifauna, floats

INTRODUCTION

A consequence of declining wild Eastern oyster (Crassosotrea virginica) stocks throughout the eastern seaboard of the United States (MacKenzie et al. 1997) has been concerted efforts focused on oyster restoration (see Luckenbach et al. 1999a). Shellfish restoration throughout the United States is driven by a number of motives (e.g., ecological and fishery value) and typically involves a wide range of user groups (da Silvo Pinho 2000, Breitburg et al. 2000, Brumbaugh et al. 2000, Mann 2000). One approach in support of shellfish restoration is the promotion of aquaculture (mostly of oysters) with the ultimate goal of reducing fishing pressure on wild stocks. During the culture process, oysters are subject to colonization by a variety of organisms. However, the number and relative abundances of taxa associated with the cultured oysters has rarely been assessed, as is the influence of the culture activity on the surrounding habitat. The primary goals of this study are, to identify and enumerate the number of taxa (macro-fauna >2 mm in size) associated with oysters (under culture conditions in Virginia) and to quantify the relative proportions of each taxa. Furthermore, it is hoped that these data will help define any ecological and environmental associations of oysters under culture conditions.

MATERIALS AND METHODS

Oyster culture in Virginia has three distinct stages. The first involves broodstock conditioning, spawning, larval culture, settlement, and an early nursery phase that is effected in a land-based hatchery. Upon removal from the hatchery, a field nursery phase follows where the oysters (3-mm shell height) are placed in mesh bags (2500 oysters per 61 cm x 61 cm bag, 1.5 mm mesh size). As the oysters increase in size, they are stocked in bags with increasingly larger mesh sizes (e.g., 1.5 mm to 3 mm to 9 mm bags) with a concomitant decrease in densities (2500 to 1200 to 600 or less oysters per bag, respectively). The majority of this nursery phase is conducted in off-bottom floating structures called oyster floats (Luckenbach et al. 1999b). The third phase or grow-out phase is conducted either in tray structures located on or near the bottom of the leased area or in floats. The floats most commonly used are mesh baskets (mesh size = 2.54 cm) that are 2.5 mL x 0.6 m W x 0.3 m D. A polyvinyl chloride (PVC, 10 cm in diameter) collar is secured around the open part of the basket providing floatation for the system. A shade cloth cover is stretched over the top of the float to reduce algal fouling and predation of the single oysters in the floats from birds and mammals (e.g., seagulls and otters). Typically, the oysters are emptied directly into the float for final grow-out (1200-1400 oysters per float). This reduces maintenance associated with tending bags and reduces potential growth restrictions on the oysters, a consequence of confinement within bags. Grow-out typically extends from 8-14 mo. During this period, although the aquaculturist may remove the floats from the water to remove dead oysters, the oysters remain undisturbed until they are harvested.

Oysters for this study were cultured at a polyhaline site south (1 km) of Chincoteague Island, Virginia. In May 1998, oysters were removed from the hatchery at a mean shell height of 3 mm and as part of a field nursery phase were grown as outlined earlier. The grow-out phase commenced early September 1998. As part of another study, examining the effect of differential stocking densities on growth in the floats, three floats were initially stocked with 1750 oysters per float and three with 1000 oysters per float. Dead oysters were removed from the floats twice during the grow-out period (November 1998 and April 1999). After approximately 10 mo (mid June 1999), when it was expected that many of the oysters would be harvestable (at 76-mm shell height), sampling of the oysters and associated fauna took place.

Prior to removal of the floats from the water, a mesh basket (capture basket: 2-mm mesh size) was carefully placed in the water surrounding each float and the capture basket and float were loaded onto the support vessel. The purpose of the exterior basket was to retain any motile fauna that passed through the larger mesh of the float. All motile organisms captured on the exterior basket and located within the float were identified and enumerated. Sampling was effected by blindly selecting 50 oysters from all oysters (within each float) laid out in trays on the deck of the vessel. The shell height and width was measured for each randomly collected oyster. In addition, all attached faunal organisms were identified and enumerated. Colonial species were enumerated as a single representative of that organism. Total counts of live oysters in the floats were also calculated, thus allowing us to standardize the abundance of organisms in the float.

RESULTS

The final density of oysters cultured in the floats varied considerably and ranged from 488-1381 oysters per float, with an overall mean of 974 oysters per float (Table 1). The density disparity was most likely due to a combination of error associated with original stocking density and differential mortality of oysters in the floats. However, there was little relationship ([r.sup.2] = 0.182, P > 0.05) between oyster density and the number of taxa found in the floats (Fig. IA). There was a slightly stronger relationship ([r.sup.2] = 0.238, P > 0.05) between the oyster density and the abundance of organisms in the float (see Fig. 1B). Shell height of oysters in the floats ranged from 76.17 to 83.52 mm, with an overall mean of 80.50 mm (see Table 1). The number of macrofaunal taxa in each float ranged from 24 to 36 with an overall mean of 29. In total, 45 faunal taxa were found in the oyster floats (Tables 1 and 2). These represented 11 broad taxonomic groups (see Table 2). The most abundant taxonomic group was annelids that comprised from 47.44% to 69.37% of organisms in the six floats. Mollusks comprised the next most abundant group (11.45% to 27.93%) followed by crustaceans (11.49% to 15.73%). Overall, in terms of abundance, annelids comprised 57.64% of all organisms, mollusks 20.01% and crustaceans 13.76% (see Table 2). These groups comprised 91.4% of all organisms sampled from the floats.

[FIGURE 1 OMITTED]

DISCUSSION

The total number of taxa (45) found among the oysters in this study was relatively high and compare favorably with the findings of previous studies of oyster communities (Wells 1961, Babr 1974, Dame 1979, Larsen 1985, Stanley & Sellers 1986, Zimmerman et al. 1989). Meyer and Townsend (2000) recorded 41 species associated with newly constructed oyster reefs in North Carolina. However, many of these authors report on open reef structures relying on natural recruitment of oysters in mesohaline environments. Given the polyhaline character of the Chincoteague site used in this study it might be expected that the number of taxa recorded would be higher. That greater numbers of taxa were not recorded is likely a consequence of the fact that the assessment of associated organisms was carried out after only 10 months of potential colonization time in an enclosed structure suspended in the water column. In addition, this limited colonization time may have some bearing on the fact that certain species, typically associated with oysters in similar environments (e.g., the boring sponge, Cliona celata) were not recorded in this study.

Palmer et al. (1998) observed that little baseline data are available to serve as guidelines for restoring aquatic habitats. Not the least of these is species composition, from which community structure and trophic interactions may be evaluated. From our findings, a number of trophic groups are apparent in the culture systems. While filter feeders dominated (bivalves), others represented were detritivores (e.g., Polydora websteri, Palaemonetes vulgaris, omnivores (e.g., Gobiosoma bosci) and carnivores (e.g., Callinectes sapidus). The diversity of feeding types suggests the potential for various trophic interactions within the culture system and between the system and the surrounding habitat. Of course, limitations are imposed on the interactions by the meshing that comprises the floats. For example, larger predators (e.g., mature blue crabs, fishes, etc.) may be excluded. A potential limitation in interpreting the results of this study, imposed by the sampling methodology, is that smaller motile species (e.g., amphipods) may not have been retained by the 2-mm sampling mesh.

The dominance (in terms of abundance) of relatively few taxa is not unusual for shellfish assemblages. In this study, 3 groups of organisms (polychaetes, crustaceans, and mollusks) comprised 91.4% of all of the macrofauna sampled. Tenore and Gonzalez. (1975) observed that dense epifaunal assemblages (dominated by few taxa) were associated with cultured mussels in Spain. For wild oyster populations, O'Beirn (unpublished data) documented that numerically 94.6% of all faunal organisms found on a constructed oyster reef in Virginia were represented by four taxa. Seed and Suchanek (1992) and Lintas and Seed (1994) had similar findings for faunal communities associated with the blue mussel, Mytilus edulis. It must be noted that, whereas numerical dominance (as assessed in this study) may be a useful indicator of community composition, species biomass (not assessed in this study) may yield different conclusions regarding community structure that may be equally as important.

The occurrence of some species in the floats is of particular interest. The portunid crab, Cronius ruber, was considered rare in Virginia (Van Engel & Sandifer 1972). However, in this study, five specimens, ranging in size from 24.9-34.9 ram, were located within the floats. These findings allied with previous and subsequent observations (O'Beirn personal observation), suggest that they are more than just occasional or rarely occurring species. The numerous fish species sampled highlight the importance of oysters as habitat for these species. Not only is the interstitial space among the oysters important, but dead articulated shells also provide useful habitat (in the form of refuge or nesting sites) for the blennies and gobies (Breitburg 1999).

The increase in aquaculture activities throughout the world has initiated much discussion and research on the interaction between the activity and the environment (Freeman 1988, Hickey & Hurst 1989, Iwama 1991, Hastings & Heinle 1995). Oysters may be cultured in areas that were previously devoid of the species. Hence, they could be regarded as an introduced species into a system. Simenstad and Fresh (1995) recommend that resource managers should consider the effects of aquaculture and attendant activities on ecosystem and community levels. The effects of introducing a large-scale monospecific culture operation into a particular system can have a cascading effect and ultimately affect production and the species composition inherent within the system (Simenstad & Fresh 1995). A shift in the ecological balance may affect the ability of the communities within the system to withstand perturbations of one form or another.

It has been suggested that shellfish culture activities benefit the environment by attracting numerous other organisms to the area (Dewey 2000). Whether they are truly an ecological or environmental benefit or not has yet to be specifically determined. The benefits (i.e., habitat and production) afforded by oyster culture activities may be minimal given the ephemeral nature of these communities (mediated by harvest and handling schedules). Many organisms constituting these communities may not actually mature to reproduce. Consequently, these systems may be regarded as sink populations. In addition, certain associated species may actually compete with the culture organism for food resources (e.g., blue mussels and wild oysters setting on the cultured oysters; Adams et al. 1991), and increase the organic loading emanating from the culture systems (Nugues et al. 1996). Finally, as urbanization and human activities lead to an increase in habitat alteration in coastal areas (Connell 2000), suites of potentially competing organisms may be introduced into an area. Such a phenomenon may upset the ecologic balance of the area by changing the constituent organisms and species interactions within the system. As the scale of anthropogenic activities increase, it will be increasingly important to further elucidate these and other ecologic interactions.

A major impediment to successful restoration of impacted shellfish habitats is defining success of a particular venture (Coen & Luckenbach 2000). Defining goals and establishing success criteria has proven difficult, given that in many regions natural shellfish habitats (to use as reference locations) are lacking (Luckenbach et al. 1999a, and references therein). Anecdotal accounts of shellfish population structure and associated organisms in particular regions most often are insufficient upon which to base an expensive and long-term restoration program. Therefore, these data may give an indication (in terms of community assemblage and/or trophic interactions) of what may be expected when initiating an oyster restoration program.

TABLE 1.

Final individual and overall parameter values calculated for oyster
and associated organisms in the culture floats.

           Oyster       Height          Width        Number
          Density       Mean-mm        Mean-mm         of    Estimated
                         (SD)           (SD)          Taxa   Abundance

             806     80.98 (14.15)   52.23 (6.29)      29     20,501
             488     80.43 (11.62)   52.33 (8.03)      30     12,746
             749     79.48 (12.20)   51.46 (8.03)      24     28,132
            1363     76.17 (19.56)   47.29 (10.18)     36     92,602
            1004     82.42 (14.54)   51.97 (5.25)      25     32,449
            1381     83.52 (16.55)   51.66 (7.63)      29     36,044
Overall
  means      974     80.50 (14.65)   51.15 (7.62)     ~29     37,079

TABLE 2.

Relative proportions of organisms sampled in oyster culture floats used
in this study and overall means of proportions for each taxa.

                                                           Float

                                                   Overall Mean    #1
Porifera
  Cliona celata             Boring sponge              0.15
  Halichondria bowerbanki   Bread sponge               0.81        0.08
  Lissodendoryx sp.         Garlic sponge              0.06
  Microciona prolifera      Red beard sponge           0.07
Cnidaria
  Actinaria spp.            Anemones                   2.50        0.31
  Hydroida                                             3.16        2.59
Platyhelminthes
  Stylochus ellipticus      Flatworm                  <0.01
Annelida
  Hydroides dianthus        Fan-worm                  25.99       24.69
  Lepidonotus sp.           Scale worm                <0.01        0.01
  Polvdora websteri         Spionid-worm              <0.01        0.01
  Sabella micropthalma      Fan worm                   1.32        0.39
  Sabellaria sp.            Mason worm                 0.26
  Spirorbis sp.             Spirobid worms            29.88       34.44
Crustacea
  Balanus eburneus          Barnacle                  11.46       12.74
  Callinectes sapidus       Blue crab                  0.01        0.01
  Cronius ruber             Red crab                  <0.01        0.01
  Dyspanopeus sayi          Mud crab                   0.16        0.20
  Eurypanopeus depressus    Mud crab                   0.02       <0.01
  Hemigrapsus sanguineus    Asian shore crab          <0.01       <0.01
  Lysmata wurdermanni       Peppermint shrimp         <0.01
  Palaemonetes vulgaris     Grass shrimp               1.73        0.27
  Xanthidae spp.            Xanthid crabs              0.12
  Panopeus herbstii         Mud crab                   0.26        0.21
Mollusca
  Anadara ovalis            Blood ark                  0.01
  Anadara transversa        transversa ark             0.02
  Anomia simplex            Common jingle              3.77        2.75
  Crassostrea virginica     Eastern oyster            15.47       18.95
  Crepidula fornicata       Slipper shell              0.20        0.31
  Crepidula plana           Slipper shell              0.49        0.08
  Doris verrucosa           Nudibranch                <0.01
  Mytilus edulis            Blue mussel                0.04
  Urosalpinx cinerea        Oyster drill              <0.01
Bryozoa
  Membranipora tenuis       Encrusting bryozoan        1.30        1.10
Echinodermata
  Arbacia punctulata        Purple sea urchin         <0.01
Tunicata
  Aplidium stellatum        Sea pork                   0.06        0.08
  Didemnum sp.              Paintsplash tunicate       0.05        0.08
  Mogula manhattensis       Sea squirt                 0.09        0.08
  Styela plicata            Rough sea squirt           0.16        0.16
Pisces
  Chasmodes bosquianus      Striped blenny            <0.01        0.02
  Fundulus heteroclitus     Mummichog                  0.13        0.09
  Gobiosoma bosci           Naked goby                 0.02        0.03
  Gobiesox strumosus        Skilletfish                0.01        0.02
  Opsanus tau               Oyster toad fish          <0.01
  Hypsoblennius hentzi      Feather blenny             0.18        0.28
  Tautoga onitis            Tautog                    <0.01

                                            Float

                             #2      #3      #4      #5      #6
Porifera
  Cliona celata              0.22    0.18    0.05    0.17    0.62
  Halichondria bowerbanki    0.33    1.07    1.06    0.59    1.09
  Lissodendoryx sp.          0.33                            0.31
  Microciona prolifera               0.18            0.08    0.31
Cnidaria
  Actinaria spp.             0.89    0.09    4.55    0.08    4.52
  Hydroida                   4.12    3.28    2.90    3.29    3.90
Platyhelminthes
  Stylochus ellipticus       0.01
Annelida
  Hydroides dianthus        20.28   28.84   24.19   33.96   22.93
  Lepidonotus sp.            0.01            0.01
  Polvdora websteri          0.01                    0.01    0.01
  Sabella micropthalma       0.22    0.71    2.71    0.25    0.31
  Sabellaria sp.                     0.18                    2.03
  Spirorbis sp.             32.53   39.76   26.58   31.93   22.15
Crustacea
  Balanus eburneus          10.58    8.08   12.83   12.50    7.64
  Callinectes sapidus        0.01    0.01    0.01    0.01    0.01
  Cronius ruber              0.01            0.01
  Dyspanopeus sayi           0.21    0.30    0.10    0.16    0.16
  Eurypanopeus depressus     0.01    0.01    0.03    0.01    0.01
  Hemigrapsus sanguineus
  Lysmata wurdermanni                       <0.01
  Palaemonetes vulgaris      4.33    3.61    1.43    0.52    2.99
  Xanthidae spp.             0.23    0.20    0.04    0.28    0.18
  Panopeus herbstii          0.35    0.33    0.17    0.28    0.50
Mollusca
  Anadara ovalis             0.11
  Anadara transversa                         0.05
  Anomia simplex             4.90    1.60    3.77    2.53    8.11
  Crassostrea virginica     16.49    9.76   16.83   11.49   17.16
  Crepidula fornicata        0.11    0.09    0.28    0.08    0.16
  Crepidula plana            0.56            0.41            2.34
  Doris verrucosa            0.01           <0.01
  Mytilus edulis             0.11            0.05            0.16
  Urosalpinx cinerea                                <0.01
Bryozoa
  Membranipora tenuis        1.45    1.24    1.38    0.93    1.72
Echinodermata
  Arbacia punctulata                        <0.01
Tunicata
  Aplidium stellatum         0.56                            0.16
  Didemnum sp.                               0.05            0.16
  Mogula manhattensis        0.67    0.18    0.05
  Styela plicata             0.33            0.14    0.17    0.31
Pisces
  Chasmodes bosquianus                      <0.01
  Fundulus heteroclitus              0.16    0.07    0.43    0.05
  Gobiosoma bosci                            0.04    0.01
  Gobiesox strumosus                 0.01    0.01   <0.01
  Opsanus tau                               <0.01
  Hypsoblennius hentzi       0.01    0.14    0.23    0.22    0.01
  Tautoga onitis                            <0.01

ACKNOWLEDGMENTS

The authors thank the staff and students of the VIMS Eastern Shore Laboratory who helped in this study. Helpful reviews of two anonymous reviewers are also duly acknowledged. This paper is Contribution No. 2619 of the Virginia Institute of Marine Science, The College of William and Mary.

LITERATURE CITED

Adams, M. P., R. L. Walker, P. B. Heffernan & R. E. Reinart. 1991. Eliminating spat settlement on cultured oysters in coastal Georgia: a feasibility study. J. Shellfish Res. 10:207-213.

Bahr, L. M. 1974. Aspects of the structure and function of the intertidal oyster reef community in Georgia. Ph.D. thesis. Athens, GA: University of Georgia. 151 pp.

Breitburg, D. L. 1999. Are three-dimensional structure and healthy oyster populations the keys to an ecologically interesting and important fish community? In: M. W. Luckenbach, R. Mann & J. A. Wesson, editors. Oyster reef habitat restoration: a synopsis and synthesis of approaches. Gloucester Point, VA: Virginia Institute of Marine Science Press. pp. 239-250.

Breithurg, D. L., L. D. Coen, M. W. Luckenbach, R. Mann, M. Posey & J. A. Wesson. 2000. Oyster reef restoration: Convergence of harvest and conservation strategies. J. Shellfish Res. 19:371-377.

Brumbaugh, R. D., L. A. Sorabella, C. Oliveras Garcia, W. J. Goldsborough & J. A. Wesson. 2000. Making a case for community-based oyster restoration: an example from Hampton Roads, Virginia, USA. J. Shellfish Res. 19:467-472.

Coen, L. D. & M. W. Luckenbach. 2000. Developing success criteria and goals for evaluating shellfish habitat restoration: ecological function or resource exploitation? Ecol. Engineer. 15:323-334.

Connell, S. D. 2000. Floating pontoons create novel habitats for subtidal epibiota. J. Exp. Mar. Biol. Ecol 247:183-194.

Dame, R. F. 1979. The abundance, diversity and biomass of macrobenthos on North Inlet, South Carolina, intertidal oyster reefs. Proc. Nat. Shellfish. Assoc. 68:6-10.

da Silvo Pinho, O. 2000. Community involvement in projects to reduce non-point source pollution. J. Shellfish Res. 19:445-447.

Dewey, W. F. 2000. Maintaining the shellfish niche in the new millennium: the Pacific shellfish growers approach. J. Shellfish Res. 19:449-454.

Freeman, K. 1988. Ecology and aquaculture: Shall the twain meet? Bull. Aqua. Assoc. Canada 88-2:82-87.

Hastings, R. W. & D. R. Heinle (editors). 1995. The effects of aquaculture in estuarine environments. Estuaries 18.

Hickey, M. & J. W. Hurst, Jr. 1989. Molluscan shellfish introductions--concerns of states. J. Shellfish Res. 8:465-466.

Iwama, G. K. 1991. Interactions between aquaculture and the environment. Crit. Rev. Environ. Cont. 21:177-216.

Larsen, P. F. 1985. The benthic macrofauna associated with oyster reefs of the James River estuary, Virginia, USA. Inter. Rev. Hydrobiol. 70:797-814.

Lintas, C. & R. Seed. 1994. Spatial variation in the fauna associated with Mytilus edulis on a wave-exposed rocky shore. J. Molluscan Stud. 60:165-174.

Luckenbach, M. W., R. Mann & J. A. Wesson (editors). 1999a. Oyster reef habitat restoration: a synopsis and synthesis of approaches. Gloucester Point, VA: Virginia Institute of Marine Science Press. 366 pp.

Luckenbach, M. W., F. X. O'Beirn & J. Taylor. 1999b. An introduction to culturing oysters in Virginia. Virginia Institute of Marine Science Special Report. 24 pp.

MacKenzie, C. L., Jr., V. G. Burrell, Jr., A. Rosenfield & W. L. Hobart (editors). 1997. The history, present condition, and future of the molluscan fisheries of North and Central America and Europe, vol. 1,

Atlantic and Gulf Coasts. U.S. Dept. of Commerce, NOAA Tech Report NMFS 127: Sep. 1997, Seattle, WA. 234 pp.

Mann, R. 2000. Restoring the oyster reef communities in Chesapeake Bay: a commentary. J. Shellfish Res. 19:335-339.

Meyer, D. L. & E. C. Townsend. 2000. Faunal utilization of created intertidal Eastern oyster (Crassostrea virginica) reefs in the Southeastern United States. Estuaries 23:34-45.

Nugues, M. M., M. J. Kaiser, B. E. Spencer & D. B. Edwards. 1996. Benthic community changes associated with intertidal oyster cultivation. Aquacul. Res. 27:913-924.

Palmer, M. A., R. F. Ambrose & N. L. Poff. 1998. Ecological theory and community restoration ecology. Restor. Ecol. 5:291-300.

Seed, R. & T. H. Suchanek. 1992. Population and community ecology of Mytilus. In: E. Gosling, editor. The mussel Mytilus: ecology, physiology, genetics, and culture. Development in aquaculture and fisheries science, vol. 25. Amsterdam, London, New York, Tokyo: Elsevier. pp. 87-169.

Simenstad, C. A. & K. L. Fresh. 1995. Influence of intertidal aquaculture on benthic communities in Pacific northwest estuaries: Scales of disturbance. Estuaries 18:43-70.

Stanley, D. W. & M. A. Sellers. 1986. Species profile: life histories and environmental requirements of coastal fishes and invertebrates (Gulf of Mexico)-American Oyster. US. Fish Wildl. Serv. Biol. Rep. 82(11.64). US Army Corps of Engineers TEL-82-4:25 pp.

Tenore, K. R. & N. Gonzalez. 1975. Food chain patterns in the Ria de Arosa, Spain: an area of intense mussel aquaculture. 10th European Symposium on Marine Biology, Ostend, Belgium, September 2. 601-619.

Tenore, K. R., J. Corral & N. Gonzalez. 1982. Effects of intense mussel culture on food chain patterns and production in coastal Galicia. NW Spain. Proc. SIUEC. 1:321-328.

Wells, H. W. 1961. The fauna of oyster beds, with special reference to the salinity factor. Ecol. Monogr 31:239-266.

Zimmerman, R., T. J. Minello, T. Baumer & M. Castiglione. 1989. Oyster reef as habitat for estuarine macrofauna. NOAA technical memorandum NMFS-SEFC-2.

Van Engel, W. A. & P. A. Sandifer. 1972. Order decapoda. In: M. L. Wass, editor. Checklist of the biota of the lower Chesapeake Bay. Virginia Institute of Marine Science, Special Scientific Report, 65, ix. pp. 155-164.

F. X. O'BEIRN, * P. G. ROSS AND M. W. LUCKENBACH

College of William and Mary, Virginia Institute of Marine Science, Eastern Shore Laboratory, P.O. Box 350, Wachapreague, Virginia 23480

* Corresponding Author. E-mail: [e-mail]