NOTE Beal et al,: Shell marker for luvenile, hatchery-reared Mya arenana 



385 



Although not specifically tested, at least two com- 

 peting hypotheses may explain the mechanism that 

 creates this shell marker. The first hypothesis is distur- 

 bance, for clams in the hatchery are grown in sediment- 

 free trays with fine-mesh screening (150-1500 \.i), 

 handled (sieved) weekly, fed tropical species of micro- 

 algae such as Isochrysis galbana (Tahitian variety), 

 and are grown at temperatures between 20 and 

 25°C — well above those normally experienced in the 

 wild in eastern Maine. Once they have left the hatch- 

 ery environment, clams are planted on flats and im- 

 mediately burrow into sediments where 1 ) hatchery- 

 produced disturbance ends, 2) the clams begin feed- 

 ing on natural phytoplankton assemblages, and 3) 

 they experience seawater temperatures that typically 

 do not exceed 17°C. The new shell that grows is al- 

 ways distinctly whiter than the older shell. This dif- 

 ference in coloration may indicate a release from com- 

 petition for calcium or aragonite which may be lim- 

 ited under hatchery conditions (Barber-). The sec- 

 ond hypothesis is bacterial damage that may also 

 relate to conditions in the hatchery where elevated 

 levels of marine bacteria such as Vibrio spp. fre- 

 quently occur. Qualitative tests for the presence of 

 Vibrio spp. (using Difco TCBS agar [Elston et al., 

 1981] ) at BIRSH are made regularly and show a gen- 

 eral presence of these bacteria in the seawater within 

 tanks holding juvenile clams, in the cultured algae, 

 and on the valves of juvenile clams. These gi'am-nega- 

 tive, oxidase-positive, fermentive rods have been 

 observed similarly in commercial bivalve hatcheries 

 in the coastal northeastern United States (Elston, 

 1984) where they have been described as coating the 

 shell surface of cultured juvenile northern quahogs, 

 oysters, Crassostrea vi>-ginica (Gmelin), Ostrea editlis 

 L., and bay scallops, Ar^opec^e/! irradians (Lamarck). 

 Bacteria in the family Vibrionaceae can erode and 

 perforate areas on the surface of bivalve shells 

 through the production of a variety of acidic metabo- 

 lites that are inimical to normal deposition of cal- 

 cium carbonate (Elston et al., 1982). The SEM pho- 

 tograph of the valve of the hatchery-reared softshell 

 clam ( Fig. 1 , C and D ) clearly shows pitting and amor- 

 phous grooves that may indicate a bacterial origin. 



During the past decade, clam landings in eastern 

 Maine have declined by nearly 75^r (Wallace, 1997). 

 Communities that manage their clam stocks in this 

 and other regions along the coast are beginning to 

 use cultured softshell clams to enhance clam produc- 

 tion. Testing the biological and economic efficacy of 

 hatch-and-release programs is critical for the devel- 

 opment of sensible management programs. Results 



^Barber, B. 1994. Universityof Maine. Orono, ME. Personal 

 commun. 



presented here demonstrate the ease of distinguish- 

 ing cultured from wild Mya and will allow scientists 

 as well as clam harvesters a rapid assessment of field 

 planting programs. 



Acknowledgments 



Support for this work was provided by a Maine/New 

 Hampshire Sea Grant research grant to B. F. Beal 

 and M. G. Kraus (R/FMD-191; NA89AA-D-SG020). 

 Additional support was provided by the Department 

 of Animal, Veterinary, and Aquatic Sciences at the 

 University of Maine and by the University of Maine 

 at Machias. We thank all those that helped in the 4-H 

 hatchery and in the field, including M. Alley, A. 

 Chapman, J. Chapman, J. Cox Jr., J. Cox Sr., D. 

 Engels, C. Gay R. Look, S. Renshaw, and T. White. 

 We also thank W. Congleton, S. Fegley, R. Hawes, A. 

 Lewis, R. Vadas Sr., K. Vencile, and three anonymous 

 reviewers for their useful comments on previous 

 drafts of this note. 



Literature cited 



Baptist, J. P. 



1955. Burrowing ability of juvenile clams. U.S. Fish Wild. 

 Ser\'. Spec. Sci. Rep. 140:1-13. 

 Beal, B. F. 



1983. Effects of environment, intraspecific density, preda- 

 tion by snapping shrimp and other consumers on the popu- 

 lation biology of Mercenaria mercenaria near Beaufort, 

 North Carolina. M.S. thesis, Univ. North Carolina, Chapel 

 Hill.NC, 180 p. 



1994. Biotic and abiotic factors influencing growth and sur- 

 vival of wild and cultured soft-shell clams, Mya arenaria 

 L., in eastern Maine. Ph.D. diss., Univ. Maine, Orono, 

 ME, 499 p. 



Beal, B. F., C. D. Lithgow, D. P. Shaw, S. Renshaw, and 

 D. Ouellette. 



1995. Overwintering hatchery-reared individuals of the 

 soft -shell clam, Mya arenaria L.: a field test of site, clam 

 size, and intraspecific density. Aquaculture 130:14.5-158. 



Brousseau, D. J. 



1979. Analysis of growth rate in Mya arenaria using the 

 von Bertalanffy equation. Mar. Biol. 51:221-227. 

 Commito, J. A., and W. G. Ambrose Jr. 



1985. Predatory infauna and trophic complexity in soft- 

 bottom communities. In P. E. Gibb (ed.). Proceedings of 

 the nineteenth European marine biology symposium, p. 

 323-333. Cambridge Univ. Press, Cambridge. England. 

 Elston, R. 



1984. Prevention and management of infectious diseases 

 in intensive mollusc husbandry. J. World Marie. Soc. 

 1.5:284-300. 



Elston, R., E. L. Elliot, and R. R. Colwell. 



1982. Conchiolin infection and surface coating Vibrio: shell 

 fragility, growth depression and mortalities in cultured 

 oysters and clams, Crassostrea virginica. Ostrea edulis. and 

 Mercenaria mercenaria. J. Fish Diseases 5:265-284. 



