REPRODUCTION AND RECRUITMENT OF 



THE BRACKISH WATER CLAM RANGIA CUNEATA IN 



THE JAMES RIVER, VIRGINIA'^ 



Thomas D. Cain^ 



ABSTRACT 



Reproduction and recruitment of the brackish water clam Rangia cuneata were investigated in the 

 James River, Va., from February 1970 to January 1972. Histological examinations of gonads were 

 made, newly set clams were collected, and temperature and salinity measurements were taken from 

 three populations living in different salinity regimes. 



Gametogenesis began in April and ripe gonads were found from May to late November with no 

 inactive period. From observations of set abundance, two periods of spawning were determined: one in 

 early through midsummer, coinciding with the beginning of spawning as determined from gonadal 

 examinations; and a second and longer period in late fall and early winter, with an increased percen- 

 tage of partially spawned and spent clams. Gametogenesis ceased in December through March as 

 residual gametes were cytolyzed. Sex was not detected during this last phase. More females than males 

 were found in the upstream (lower salinity) populations. Temperature was important in initiating 

 gametogenesis in the spring and midsummer. Spawning correlated best with changes in salinity to 

 approximately 5 "/oo. 



Over its estuarine range, salinity has a controlling effect on Rangia spawning and recruitment. 

 Seasonal reduction in input of freshwater (increased seawater intrusion) is needed to induce spawning 

 and recruitment in upstream populations. Best recruitment occurred to the middle of the habitat range 

 which has an annual salinity change from fresh to 5"/oo. 



A southern species of clam, Rangia cuneata (Gray) 

 has in the last 15 yr extended its range into 

 Chesapeake Bay estuaries (Hopkins and Andrews 

 1970). This clam occupies an otherwise "open 

 niche" in the oligohaline region of these estuaries. 

 Although species diversity is usually low, there 

 may be large numbers of individuals of species 

 adapted to this environment. In the upper James 

 River estuary, Rangia accounts for nearly 95% of 

 the benthic biomass. 



Rangia is important both ecologically and com- 

 mercially (Hopkins 1970). It provides a substantial 

 food source for several species of fish and crabs 

 (Darnell 1958), and waterfowl (Wass and Wright 

 1969). It is ecologically significant because it con- 

 verts detritus into biomass that can be utilized by 

 these organisms (Odum and Copeland 1969). 



Not only is R. cuneata a species for which low 



'Contribution No. 582 from the Virginia Institute of Marine 

 Science, Gloucester Point, VA 23062. 



■This paper is part of a dissertation submitted to the Univer- 

 sity of Virginia at Charlottesville, in partial fulfillment of the 

 requirements for a Ph.D. degree. The research was funded by the 

 Virginia Electric and Power Company. 



'Division of Technical Review, U.S. Nuclear Regulatory Com- 

 mission, Washington, D.C. 20555. 



salinity, l-15''/oo, is optimal; it is also a species 

 which evidently cannot maintain a population 

 outside this range (Hopkins 1970). That Rangia 

 thrives in a zone unfavorable for most animals 

 indicates it has some unusual adaptations. Despite 

 its abundance in favorable environments and long 

 history on the Gulf Coast, this clam has received 

 little attention. 



The study reported here concerns the reproduc- 

 tive cycle and recruitment of Rangia cuneata in 

 the James River. The major objectives were to: (a) 

 study the gametogenic cycle of Rangia from his- 

 tological sections; (b) determine differences in 

 gametogenesis or spawning of clams over the 

 species range in the estuary; (c) investigate, from 

 analysis of field data, the influence of temperature 

 and salinity on initiation of gametogenesis and 

 spawning; (d) corroborate gametogenic findings 

 by collecting newly set clams; and (e) determine 

 the duration of the larval period and differences in 

 set abundance in the estuary. 



Fairbanks (1963) studied the spawning cycle of 

 Rangia in Lake Pontchartrain, La., but it is known 

 that physiologically different races of bivalves can 

 occur at different latitudes (Loosanof f 1969). Cain 



Manuscript accepted May 1974. 



FISHERY BULLETIN: VOL. 73, NO. 2, 1975. 



412 



