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Habitat structure and prey size are important determinants in the outcome of predator-prey interactions. We investigated the role of simulated habitat type and density (simulated Zostera marina [hereafter referred to as 'Zostera']: 500 and 1500 shoots m -2 , and simulated Spartina alterniflora [hereafter referred to as 'Spartina']: 97 and 291 shoots m -2 ) in mediating predator-prey interactions. Proportional survival during predation by the piscene predator Fundulus heteroclitus on 2 successive life-history stages (post-larvae and first juvenile instars) of the blue crab Callinectes sapidus (Rathbun), was quantified under laboratory conditions that closely approximated field conditions. We also examined the effects of juvenile crab size (equal biomass and equal numbers of 2 or 3 prey-size categories, respectively) on survival during adult F. heteroclitus predation in the absence of vegetation. Crab size categories included small (first juvenile instars; 2.1 mm carapace width [cw]), medium (fourth and fifth crab stage juveniles; 6.0 to 9.1 mm cw), and large (sixth and seventh stage juveniles; 9.2 to 12.6 mm cw) in the equal numbers experiments and small and medium crabs for the equal biomass experiment. Mean proportional survival was higher for both life-history stages in simulated Zostera, with first-instars exhibiting higher proportional survival than post-larvae at the experimental densities tested (post-larvae: no grass = 0.23, low-density grass = 0.44, high-density grass = 0.57; first-instars: no grass = 0.47, low-density grass = 0.87, high-density grass = 0.87). Mean proportional survival did not differ significantly among life-history stages in the simulated Spartina treatments, although proportional survival between treatments was higher for first-instars than post-larvae (post-larvae: no Spartina = 0.17, low-density Spartina = 0.18, high-density Spartina = 0.19; first-instars: no Spartina = 0.46, low-density Spartina = 0.43, high-density Spartina = 0.48). Finally, size-refuge experiments with equal numbers or biomass of prey demonstrated significant differences between each size or/and biomass category tested. Crabs exhibited increasing proportional survival with increasing crab size (small = 0.34, medium = 0.66, large = 0.97) or weight (small = 0.18, medium = 0.67) in the presence of F. heteroclitus, suggesting that C. sapidus attains a refuge in size from predation by adult F. heteroclitus at approximately 9.2 to 12.6 mm. Our findings suggest that the influence of habitat structure on crab survival (possibly as a function of surface area) varies with simulated habitat type, crab density and crab stage (post-larvae versus first-instar), providing additional evidence of the importance of seagrasses in the early life-history stages of blue crabs.

Habitat quality and prey size as determinants of survival in post-larval and early juvenile instars of the blue crab Callinectes sapidus