WILSON ET AL.: EFFECT OF SNAIL ON OYSTERS 



were produced by increases in concentration. Soniat 

 and Koenig (1982) observed significant changes in 

 the free ammo acid pool due to P. marinus, par- 

 ticularly in taurine concentration. We noted changes 

 in taurine during the treatment period and hypo- 

 taurine during recovery in the adductor muscle but 

 these were related to snail parasitism. 



Biochemical components, though rarely signifi- 

 cantly affected, were affected not just in the mantle 

 tissue but also in the adductor muscle. One possi- 

 bility, that the snail's effect is localized at the point 

 of feeding, is not supported by the data. Snail para- 

 sitism produces systemic effects. 



CONCLUSIONS 



Complementary results of White et al. (1984, 

 1988, in press) and this study permit a general 

 description of the impact of snail parasitism on 

 oysters at normal field levels. Both growth rate and 

 reproductive development slow significantly, but 

 recover rapidly once the snails are removed. Hence, 

 the temporal stability of snail patches must deter- 

 mine the cumulative effect on field populations of 

 oysters. The prevalence and intensity of infection 

 by Perkinsus marinus is significantly increased, but 

 recovery does not occur. That is, Boonea impressa 

 probably facilitates and encourages the normal 

 spread and intensification of P. marinus from which 

 oysters, if they recover, only do so the following 

 winter when low temperatures typically reduce in- 

 fection levels (Hewatt and Andrews 1956; Burrell 

 et al. 1984; Soniat 1985). This effect, then, is long 

 term. Most changes in biochemical components were 

 due to infection by P. marinus. Snail feeding 

 reduces net productivity but, at normal field levels, 

 starvation is an unlikely result. Increased infection 

 by P. marinus typically raises glycogen and lipid 

 levels, at least in light to moderate infections. Of 

 the free amino acids, taurine and hypotaurine have 

 been shown to be affected by P. marinus and B. im- 

 pressa. Little change in the remaining FAA or the 

 total pool has been observed singly or in concert. 

 Feng et al. (1970) noted increased taurine levels in 

 oysters parasitized by Bucephalus sp. and Minchinia 

 nelsoni. Hence, an increase in taurine and hypo- 

 taurine levels apparently is a general response to 

 parasitism in oysters. 



Yuill (1987) emphasized the importance of subtle 

 effects produced by parasites on host populations. 

 Perkinsus marinus is an important source of mor- 

 tality in oyster populations (Mackin and Sparks 

 1962; Mackin 1962; Hofstetter 1977). Data suggest 

 that one of the most important aspects of parasitism 



by B. impressa is to encourage this second parasitic 

 organism. To this extent, over the year, B. impressa 

 at normal field densities could be responsible for a 

 substantial amount of mortality in oyster popula- 

 tions. 



ACKNOWLEDGMENTS 



We would like to thank M. White, D. Davies, and 

 L. Priest for laboratory and statistical assistance. 

 T. J. McDonald graciously provided his expertise in 

 GC analysis, and M. C. Kennicutt and A. Vastano 

 ran the fatty acid analyses. Suggestions by J. Par- 

 rack, T. Bright, and an anonymous reviewer im- 

 proved the manuscript. We thank R. Covington for 

 typing the manuscript and tables. R. Pratt, care- 

 taker of the Aransas Pass Lighthouse, provided 

 space for the field portion of the study. This research 

 was supported by part of an institutional grant 

 NA85AA-D-FG128 to Texas A&M University by the 

 National Sea Grant Program, National Oceanic and 

 Atmospheric Administration, U.S. Department of 

 Commerce to E. N. Powell and S. M. Ray. We ap- 

 preciate this support. 



LITERATURE CITED 



Allen, J. F. 



1958. Feeding habits of two species of Odostomia. Nautilus 

 72:11-15. 

 Andrews, J. D. 



1961. Measurement of shell growth in oysters by weighing 



in water. Proc. Natl. Shellfish Assoc. 52:1-12. 

 1965. Infection experiments in nature with Dermocystidium 

 marinum in Chesapeake Bay. Chesapeake Sci. 6:60-67. 

 Andrews, J. D., and W. G. Hewatt. 



1957. Oyster mortality studies in Virginia. II. The fungus 

 disease caused by Dermocystidium Tnarinum in oysters of 

 Chesapeake Bay. Ecol. Monogr. 27:1-25. 

 Beninger, p. G., and a. Lucas. 



1984. Seasonal variations in condition, reproductive activity, 

 and gross biochemical composition of two species of adult 

 clam reared in a common habitat: Tapes decussatus L. 

 (Jeffreys) and Tapes phillippinarum (Adams & Reeve). 

 J. Exp. Mar. Biol. Ecol. 79:19-37. 

 Brockelman, C. R. 



1978. Effects of parasitism and stress on hemolymph protein 

 of the African giant snail, Achatinafulica. Z. Parasitenkd. 

 57:137-144. 

 Brown, C. R., and M. B. Brown. 



1986. Ectoparasitism as a cost of coloniality in cliff swallows 

 (Hirundo pyrrhonota). Ecology 67:1206-1218. 

 Burrell, V. G., M. Y. Bobo, and J. J. Manzl 



1984. A comparison of seasonal incidence and intensity of 

 Perkinsus marinus between subtidal and intertidal oyster 

 populations in South Carolina. J. World Mariculture Soc. 

 15:301-309. 

 Carr, R. S., and J. M. Neff. 



1984. Quantitative semi-automated enzymatic assay for tissue 



563 



