SEASONAL ABUNDANCE OF CLAM LARVAE IN RHODE ISLAND WATERS, 1950-52 



Since 19i|8, the Fish and Wildlife Service has carried on a biological 

 study of the soft clam, Mya arenaria , and the hard clam, Venus mercenaria , 

 of the Atlantic coast of the United States. Much of the research was done 

 in Narragansett Bay in Rhode Island, where there is a considerable commer- 

 cial fishery for the hard clam or quahaug. From collections of larvae of 

 soft and hard clams at two locations in Narragansett Bay, quantitative data 

 on seasonal abundance were compiled. This report presents these data. 



Quantitative data on seasonal abundance of clam larvae may help explain 

 the success or failure of particular year classes and thus help in finding 

 reasons for fluctuations in abundance and in developing ways to control the 

 fluctuations. Review of the literature on the early life histories of soft 

 and hard clams reveals a lack of quantitative data, though there is con- 

 siderable general information on the early life history of each of these 

 clams j summarized as follows: 



Mya . --Meade and Barnes (190U) , discussing the spawning season of Mya 

 in Rhode Island waters, state that on the basis of direct evidence (presence 

 of swimming young) and indirect evidence (mature sex products) the season 

 extends from the latter part of May through the whole of June and sometimes 

 into July. They conclude also, from the presence of very small clams in 

 early spring, that spawning must take place to some extent all summer and 

 probably until late fall. 



Stafford (1912) gives the spawning season in Malpeque, Prince Edward 

 Island, as June, July, and August, with the peak in July and August. Sizes 

 of larvae range from 90 microns at the earliest straight-hinge stage to 

 approximately liOO microns at a stage not defined, 



Belding (1931) states that the spawning season on the south side of 

 Cape Cod, Mass., is June and July, and north of Boston, Mass., is July and 

 August. Spawning takes place when the water temperature is suitable for 

 development of the larvae, which in Massachusetts is 70 c to 7h° F. Clams 

 near high-water mark spawn first. On the south side of Cape Cod there are 

 frequently two sets a year. 



Thorson (19U6) states that this larva begins to appear in the Sound 

 (0resund, Denmark) in June and may be met with as late as October but 

 "usually it is not particularly numerous." He gives the size at metamor- 

 phosis as extremely variable. A definite trend toward larger larvae in 

 deeper water is apparent 5 the range is from 210 microns (average of 10 

 measurements) in up to 30 cm. of water to 225 microns to 300 microns in 

 15 to 16 mm. 



Sullivan (I9U8) gives the spawning season in Malpeque as late May to 

 late August, with the bulk of the spawning in June. Water temperature 



