Sex Dimorphism and Skewed Sex Ratios in the Prawn Species Palaemon Adspersus and P. Squilla

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Abstract
The average size difference between syntopic P. adspersus Rathke and P. squilla (L.), 2 closely related prawn species, was large enough to allow coexistence, as it agreed with Hutchinson''s rule of thumb. In both species, intersexual differences were also large, reducing intraspecific competition. Females gain from large body size, since this is associated with high fecundity and males from small body size since this reduces predation pressure and their costs of locomotion. The relatively large size of females makes them vulnerable to starvation and predation, and they probably suffer a higher rate of mortality than males. This leads to a sex ratio skewed in favor of males. P. squilla had greater sexual size dimorphism and more skewed sex ratio possibly because it exists in more open habitats where predation effects are strong and where small size in the male is a particularly advantageous trait. In the environments where this species occurs male competition for females is apparently weak.