Factors Governing Clutch Size in Two Species of Diaptomus (Copepoda: Calanoida)

We conducted a synoptic survey of 11 nonstratifying takes in eastern Quebec to investigate the relative importance of water temperature, food concentration ([Chl a]), female prosome length, copepod density (population and community densities), and mate availability (sex ratio and density of males) in determining the clutch size of two freshwater copepods, Diaptomus minutus and D. oregonensis. Multiple regression analyses indicated that 60% of the variation in clutch size of D. minutus could be accounted for by temperature and [Chl a] (n = 33; P = 0.0001), while 80% of the variation in the D. oregonensis data (n = 11; P = 0.0017) was explained by temperature and prosome length. Addition of other variables to either model did not significantly reduce the residual variation. We also investigated the effect of interspecific interaction on the egg-bearing potential of D. minutus and found some evidence that animals in sympatry may have lower clutch size than those in allopatry when temperature and food concentration were held constant. By using an independent measure of food and mate availability, we confirmed that clutch size decreased significantly in relation to food limitation for both species, but was unaffected by mate limitation for either.