Gastropod predation on egg cases of skates (Chondrichthyes,Rajidae) in the southwestern Atlantic: quantification and life history implications

Mean predation rates (± SD) on egg cases of the skates Bathyraja macloviana, B. albomaculata, Amblyraja doellojuradoi, and Psammobatis spp. from the southwestern Atlantic were estimated to be 0.151 (±0.230), 0.423 (±0.344), 0.254 (±0.390), and 0.150 (±0.288), respectively. These estimates are within the ranges reported earlier (14–40%). Egg cases of B. albomaculata were preyed on in higher proportion than expected from their abundance and suffered a heavier predation rate where the snail Trophon acanthodes was present. Predation rates were not correlated with the thickness of the egg case wall, which indicates that other factors (ecological or chemical) could explain this pattern. Five types of boreholes were found in the egg cases: one was attributable to muricid gastropods, one to naticid gastropods, and a third type to an unknown gastropod (probably Fusitriton magellanicus); the remaining were of unknown origin. Cladistic analyses showed that skates are secondarily oviparous and have maximized adaptations for living in deep water. We suggest that oviparity in skates appeared as an adaptation to maximize fecundity (40–160 eggs per year, as compared to 2–18 pups annually or biannually in viviparous guitarfishes, the plesiomorphic sister clade of skates). If a predation rate of 24% (the mean of predation rates of all skate species studied to date) is applied to the range of fecundities reported for skates, the result is that 18–114 viable pups are produced annually per female skate. Even with a high mortality rate of 64% (the only direct estimate of natural mortality for any elasmobranch), each female skate produces 17–54 eggs annually. These values are higher than most elasmobranch fecundities. This maximization of fecundity is possible mainly because the fecundity of oviparous species is not limited by body size, as in viviparity. The protracted egg-laying season (4–12 months) of most skates (as in many other deep-sea fishes) maximizes the number of eggs laid.Communicated by P.W. Sammarco, Chauvin