Variation in sexual size differentiation in North American diaptomids (Copepoda: Calanoida): Does variation in the degree of dimorphism have ecological significance?

There is considerable intraspecific variation in the degree of sexual dimorphism in species of Diaptomus. The ratio of female length to male length in 42 species of Diaptomus varied only from 1.02 to 1.23 and did not reflect ecological differences between species. It is concluded that North American diaptomids do not show patterns of dimorphism like those of freshwater centropagids in Australia. Bayly (1978) showed that, despite some intraspecific variation, significant differences exist in the degree of sexual dimorphism in length between some nonmarine Calanoid copepods in Australia. He found that the degree of dimorphism is greater in species that occur exclusively in temporary water than it is in those from permanent waters. He suggested that this variation may have ecological significance in that in temporary environments, where competition and predation are assumed to be low, a large degree of dimorphism would allow the consumption of food of an exceptionally wide range of particle sizes. We apply these principles to species of Diaptomus in North America in the form of the following questions: What is the degree of intraspecific length dimorphism within species of Diaptomus in North America? Can Diaptomus species be classified as “temporary water species” or “permanent water species” as has been done for many of the Australian Centropagidae by Bayly (1978)? Is there a relationship between degree of sexual dimorphism and ecology, especially as regards those species restricted to temporary water? To investigate intraspecific variation in length dimorphism we calculated the length ratio (mean 0 length : mean 8 length) for several populations of four species of Diaptomus from Arizona collected by G.A. C. in summer and fall 1963 and 1964, and for two further species for which literature data were available. The means of these ratios, the standard deviations, and the maxima and minima for the various species are shown in Table 1. The same statistics are shown for single populations of D. albuquerquensis, D. leptopus, D. clavipes, and D. ashlandi from which length ratios were calculated at various times of the year. There is considerable variation in sexual size difference, both between populations sampled at about the same time and seasonally within populations. The variation in length dimorphism is less in D. nudus and D. minutus, which occupy less varied habitats and show less variability in adult length, than it is in D. siciloides, D. clavipes, and D. albuquerquensis, which are known from permanent and temporary habitats and which show considerable variability in adult length (Cole 1966). The relationship between degree of sexual dimorphism and size of individuals in the population was analyzed by calculating correlation coefficients for female length and dimorphism ratio (Table 2). It appears that, when animals attain a greater length, the females are larger relative to the males than is the case in populations of small individuals. One consequence of this, and of the fact that individuals in populations from temporary localities are generally larger than their conspecific representatives in permanent localities (Cole 1966), is that the length ratio may be higher in populations from temporary localities. The data in Table 3, although limited, support this 367