Date Awarded

1972

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Marvin Wass

Committee Member

Morris Brehmer

Committee Member

Dexter S. Haven

Committee Member

Joseph G. Loesch

Committee Member

Clinton E. Parker

Abstract

The autecology of the brackish water clam Rangia cuneata was investigated in the James River, Virginia, from February 1970 to January 1972. The reproductive cycle, combined salinity and temperature effects on embryos and larvae, and setting were studied. Histological slides were made, set collected, and temperature and salinity measurements taken from three populations living in different salinity regimes. Phases of gonad development are described. Gametogenesis started in April and from May through September clams were found in all gonadal phases. Ripe gonads were found from May to late November with no summer or inactive period. It was determined by the abundance of set that two periods of spawning occurred. The first took place in early and mid-summer, coinciding with the beginning of spawning as determined from histological sections. The second period was longer with a greater number of set and .took place in late.fall and winter, coinciding with the increased percentage of partially spawned and spent clams in the sections. Spent specimens were common in December and were present through March. Residual gametes were cytolyzed. Sex was not discernible during this last phase. A sex ratio of more females than males was found in the upstream (lower salinity) populations. The reproductive cycle was correlated to the salinity and temperature data. Temperature was important in initiating gametogenesis in the spring and mid-summer. Spawning was best correlated to changes in salinity. A salinity of 5%, was found to stimulate spawning in the laboratory. Embryos and larvae were reared at a number of salinity temperature conditions within the ranges of 0-20%, salinity and 8-32C. Salinities near 0%, were lethal in all cases. Response surfaces were computed to analyze the combined effects on survival of embryos and on survival and growth of the larvae. Optimum conditions for embryos (85% survival) were 6-10%, and 18-29C. Salinity had more effect than temperature on early development. Optimum conditions for the larvae were broader, being 2-20%,, salinity and 8-32C. However, temperature and salinity interacted to reduce survival at low salinity-high temperature and high salinity-low temperature combinations. Growth of larvae was best at high salinities and high temperatures. Several inferences are drawn from this field and laboratory study regarding Rangia's distribution and recruitment in the James River. Over its estuarine range, salinity had the greatest effect. Seasonal reductions in freshwater input (increased salinity) are needed for spawning and recruitment of the upstream populations. The amount of gametes produced and the degree of spawning were related to the total weight of the clams at the various stations. This dissertation is from the Joint Program Degree from the College of William & Mary and University of Virginia and awarded by the University of Virginia.

DOI

https://doi.org/10.25773/R1V6-RX65

Rights

© The Author

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