abundance of early larvae exceed 200 per 35 gallons, and the abundance of 

 late umbone stage larvae never exceeded k0 per 35 gallons. In general, 

 late umbone stage larvae were present in the samples during the latter 

 half of July and the first half of August. Sampling was discontinued by 

 the first part of October. 



DISCUSSION AND CONCLUSIONS 



The problem of the reliability of plankton-sampling techniques has 

 been recognized and investigated in the past. Winsor and Walford (1936) 

 concluded that the variability of their vertical net hauls could be explained 

 on the basis of random distribution of the population, but they also r ealized 

 that nonrandom distribution in other cases had not necessarily been ruled 

 out. Ricker (1937), in studying variability of catches of freshwater plank- 

 ton, found evidence of aggregation of organisms in some cases. Barnes and 

 Marshall (1951) concluded that when population density is low the distribu- 

 tion of catch from replicate plankton samples closely approaches that of a 

 random population. At higher densities, nonrandom distribution or clumping 

 is indicated. These findings suggest that the abundance values in this paper 

 are subject to some error. However, since only the general t rends of abun- 

 dance are inferred from the results, an attempt to estimate variability would 

 be superfluous. 



Mya arenaria and Venus mercenaria have different geographical ranges 

 along the Atlantic Coast of the United States. Mya is abundant in more 

 northerly latitudes than Venus , and Venus occurs abundantly in southerly 

 latitudes where Mya is rare or nonexistent. Because of this it is to be 

 expected that their spawning patterns in Rhode Island, waters will differ. 

 Mya begins to spawn in late April or early May in Narragansett Bay, when 

 according to the literature the water temperature has risen to about 10° 

 to 12 p C. Spawning increases in intensity to a peak and then decreases 

 steadily until late June or early July, when spawning ceases teinporarily. 

 In late summer or early fall there are one or more waves of spawning 

 smaller than the spring spawning. Venus , on the other hand, doesn't begin 

 to spawn until late May or early June, when according to the literature the 

 water has warmed to about 20° C. Spawning continues for the remainder of 

 the summer, usually decreasing in intensity until it ceases sometime in 

 September. Isolated late umbone stage larvae of both Mya and Venus can 

 occasionally be found well into the winter. 



It is logical to assume that the beginning and duration of spawning 

 of any bivalve will vary from year to year with variations in environmental 

 conditions. A reexamination of the WIckford Harbor data for 1950, 1951, and 

 1952 indicates that this is true. VJhen either the Mya or the Venus seasonal 

 larval abundance data for the 3 years are studied, it becomes apparent that 

 the pattern for 1950 differs from those for 195l and 195?, which are similar. 

 In the case of Mya , larvae did not appear until the week of May 21 in L950, 



25 



