CAIN: REPRODUCTION AND RECRUITMENT OF RANGIA CUNEATA 



northward limit of Rangia is not controlled by low 

 temperature effects on reproduction or larval 

 tolerance. Reports on populations in upper 

 Chesapeake Bay (Gallagher and Wells 1969; Pfit- 

 zenmeyer 1970) infer large adult mortalities from 

 low temperature and low salinity. The only mor- 

 tality of adults (freshly gaped clams) in this study 

 was seen in the dredge hauls during the winter 

 and early spring. Long periods of low temperature 

 and freshwater may combine to increase the mor- 

 tality of adults and thereby limit the northward 

 range. 



The possibility that widely separated popula- 

 tions may belong to different physiological races 

 cannot be excluded (Loosanoff 1969). Populations 

 of marine animals exposed to different environ- 

 ments within their geographical range may have 

 different physiological properties (Sastry 1970), 

 suggesting that more research should be conduct- 

 ed on the tolerances of embryos, larvae, and adults 

 from different geographical areas. 



The key to the welfare of a Rangia population 

 over its normal distribution is probably not the 

 physiology of the adult individual but successful 

 reproduction and recruitment. Adults may live for 

 years in habitats where reproduction is impossible. 

 Spawning will not occur unless salinity changes, 

 up from low salinity or down from high salinity. If 

 spawning does occur, embryos and early larvae 

 will have poor survival unless salinity is between 2 

 and lOy 00 (Cain 1973). Once the larvae have 

 developed past the swimming stage and settled to 

 the bottom as juvenile clams, salinity is probably 

 not as critical (except in combination with low 

 temperatures). 



The influence of salinity on reproduction and 

 recruitment indicate that some changes in its en- 

 vironment may restrict the distribution of Rangia 

 in the estuary. Rangia's estuarine distribution is 

 maintained by changes in salinity related to 

 variations in the freshwater input. Any overall 

 reduction in freshwater input or reductions in the 

 seasonal variations of salinity will limit its range. 

 These characteristics of this species should be 

 considered before dams are constructed, fresh- 

 water is diverted for other uses, or other changes 

 in the hydrography of the estuary are approved. 



ACKNOWLEDGMENTS 



For critically reviewing the manuscript, I wish 

 to thank the members of my graduate committee, 

 Morris L. Brehmer, Dexter Haven, Joseph G. 



Loesch, and Clinton E. Parker. I am indebted to 

 my major adviser, Marvin L. Wass, for his help 

 during this study and for his careful review of the 

 manuscript. I am especially grateful to my 

 colleague, Richard Peddicord, for his helpful 

 suggestions and assistance with the field work. 

 The work done by the Histology Department of 

 the Virginia Institute of Marine Science, 

 especially Patsy Berry, on the gonadal sections is 

 appreciated. I also thank Samuel Rivkin, for his 

 advice and the Algae Department for providing 

 algae food when needed. I am especially indebted 

 to my wife, Diane, for her encouragement and 

 help. 



LITERATURE CITED 



Anonymous. 



1966. Water Resources Data for Virginia - Part I Surface 



Water Records. U.S. Geol. Surv. 

 1970a. James River Basin, Comprehensive Water Resources 

 Plan. Va. Dep. Conserv. Econ. Dev., Div. Water Resour., 

 Planning Bull. 215, Vol. III. 

 1970b. Surface Water Supply of the U.S. 1961-1965: Part II 

 South Atlantic slope and eastern Gulf of Mexico 

 basins. Vol. I Basins from James River to Savannah 

 River. U.S. Geol. Surv. Pap. No. 1905. 

 Brehmer, M. L., and S. Haltiw anger. 



1966. A biological and chemical study of the tidal James 

 River. Va. Inst. Mar. Sci., Final Rep. Fed. Water Pollut. 

 Contr. Admin., Contract PH86-65-86, Nov. 1966, 32 p. 

 Bureau of Sport Fisheries and Wildlife, North Carolina 

 Wildlife Resources Commission, and Virginia Com- 

 mission OF Game and Inland Fisheries. 



1965. Back Bay - Currituck Sound data report. Vol. 1, 84 p. 

 Cain, T. D. 



1973. The combined effects of temperature and salinity on 

 embryos and larvae of the clam Rangia cuneata. Mar. 

 Biol. (Berl.) 21:1-6. 



1974. Combined effects of changes in temperature and 

 salinity on early stages of Rangia cuneata. Va. J. Sci. 

 25:30-31. 



Calabrese, a. 



1970. Reproductive cycle of the Coot clam, Mulinia lateralis 

 (Say) in Long Island Sound. Veliger 12:265-269. 

 Carriker, M. R. 



1961. Interrelation of functional morphology, behavior, and 

 autecology in early stages of the bivalve Mercenaria 

 mercenaria. J. Elisha Mitchell Sci. Soc. 77:168-241. 

 Chanley, p. 



1965. Larval development of the brackish water mactrid 

 clam, Rangia cuneata. Chesapeake Sci. 6:209-213. 

 Coffin, G. W., and W. R. Welch. 



1964. A technique for separating small mollusks from bot- 

 tom sediments. Proc. Natl. Shellfish Assoc. 53:175-180. 

 Darnell, R. M. 



1958. Food habits of fishes and larger invertebrates of Lake 

 Pontchartrain, Louisiana, an estuarine communi- 

 ty. Publ. Inst. Mar. Sci. Univ. Tex. 5:353-416. 

 Fairbanks, L. D. 



1963. Biodemographic studies of the clam Rangia cuneata 

 Gray. Tulane Stud. Zool. 10:3-47. 



429 



