CAIN: REPRODUCTION AND RECRUITMENT OF RANGIA CUNEATA 



of partially spawned and spent clams, as identified 

 by histological preparations. The second peak of 

 spawning appeared to be the major one for the 

 normal reproductive period (Table 1). Fairbanks 

 (1963) found set (>0.375Aim) from October to 

 April. A longer and more intense setting period 

 was found in the area with the more variable 

 salinity. Tenore (1970) collected set in bottom 

 grabs only during the fall and winter months. The 

 spring and summer spawning was either so light 

 in these areas that no set were found or an abun- 

 dance of predators at this time quickly consumed 

 the sparse set. 



Sex Ratio 



There are at least two possible explanations for 

 the unusual sex ratios in Rangia. Rangia may be 

 protandric with a higher ratio of females to males 

 in the older stages. If so, the clams at stations B, D, 

 and C would have had to be older than those at 

 station A; however, there were no consistent 

 differences in the lengths of the clams at the 

 various stations. This does not preclude the pos- 

 sibility that the ages of clams at the stations are 

 different, but masked by varying growth rates 

 resulting from substrate effects or nutrient levels. 



The second possibility is that the environmental 

 conditions upriver differ enough from those 

 downriver at station A to affect the sex ratios. 

 Changes in environmental factors could affect 

 either juveniles or adults during the undifferen- 

 tiated period. There is no proof of this type of sex 

 alteration. None of the other mactrids studied, M. 

 lateralis, S. solidissima, or T. capax have been 

 found to have a ratio other than 1:1. 



Relationship of Larval 

 Studies to Setting 



Cain (1973), on the basis of laboratory results, 

 indicated that best survival and growth of larvae 

 would be expected in the summer. Higher 

 temperatures and generally high salinities are 

 expected to provide for rapid growth to setting. 

 But setting is very poor in summer (Table 1). A 

 number of factors may account for this: clams are 

 in all stages of gametogenesis, the gonads of ripe 

 clams are not as full, and even though the stimulus 

 to spawn may exist there is probably poor 

 synchrony of spawning. 



In the James River, fall and winter were the 

 times of greatest setting activity. Fall spawning 



occurred as the temperature dropped from 29° to 

 15° C. This temperature range would provide good 

 survival of eggs to straight-hinge larvae, but the 

 larvae are exposed to declining fall temperatures. 

 Larvae at low temperatures (and low salinity) 

 should survive well, but grow slowly. Con- 

 sequently, set in the late fall and winter come from 

 a fall spawning after the delay in growth and meta- 

 morphosis expected from low temperatures. 

 Some set in the jars could have come from 

 previously set, slow-growing clams washed in by 

 turbulence. This set, which is fairly active, tends to 

 crawl over the bottom by use of the muscular foot 

 and therefore is affected by currents (Carriker 

 1961). 



Distribution and Recruitment of 

 Rangia in the James River 



Rangia extends to nautical mile 60 in the James 

 River. In the upper reaches of its distribution it 

 has been in freshwater for the last 4 or 5 yr. Since 

 the embryos and early straight-hinge larvae can- 

 not tolerate freshwater (Cain 1973) and a salinity 

 rise is needed to induce spawning, there must be 

 little recruitment to this population. No set or 

 small clams have been found in this area, which 

 raises the similar question of how Rangia spread 

 into this region initially. The upriver population 

 consisted of clams ranging in length from 53 to 63 

 mm in the spring of 1971. Using the von Ber- 

 talanffy growth curve constructed for Rangia by 

 Wolfe and Petteway (1968), these clams were es- 

 timated to be from 5 to 7 yr old. The water records 

 for the James River basin were analyzed from 

 1963 to 1966 (Anonymous 1966, 1970b). These 

 records show yearly lows in the late summer and 

 fall when the input dropped below 22.66 mVs (800 

 cf s), and in 1966 the input dropped to an average of 

 13.59 mVs (480 cfs) during the first half of Sep- 

 tember. These very low flow periods allowed 

 measurable-salinity water to extend 63.5 miles 

 upstream in December 1965 (Brehmer and Hal- 

 tiwanger 1966). 



To reach these upstream areas larvae must be 

 transported in the more saline bottom water which 

 has a net upstream movement (Pritchard 1952; 

 Nichols 1972). Although the mechanism of such 

 transport has not been deduced for Rangia, work 

 done on the eastern oyster may indicate some 

 possible mechanisms. Wood and Hargis (1971), 

 who studied the lower James estuary, found a 

 definite upstream transport of oyster larvae and 



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