Resource partitioning among sympatric elasmobranchs in the central-western Mediterranean continental shelf

Abstract

Knowledge of the mechanisms that allow coexistence among sympatric species is fundamental to understand ecosystem functioning. Resource partitioning among seven elasmobranchs inhabiting the Sardinian continental shelf (40°07′N, 9°00′E): Dasyatis pastinaca; Raja asterias; R. brachyura; R. clavata; R. miraletus; R. polystigma and Scyliorhinus canicula, was investigated through stomach content analysis. Data from 1680 samples collected between 2005 and 2014, in 26–200 m depth, were analysed with respect to population, sex, season (winter and summer) and size groups. Species living in shallower waters (characterized by a narrower bathymetric range) had the most specialized diets. All species appeared to be mesopredators, feeding mainly on Crustacea, Actinopterygii, Mollusca and Polychaeta. Despite shared common morphological features, from the high ecological diversity of prey items, we hypothesized the presence of different predatory behavior among the species studied: some species were able to feed on endobenthic and/or epibenthic organisms, while others had made limited movements in the water column. Non-parametric Multi-Dimensional Scaling analysis highlighted the presence of five predator groups, confirming strong resource partitioning, as also demonstrated by low levels of interspecific niche overlap. The observed variations in feeding habits could be ascribed only to size and not to sex or season. Generally, diet changed from small Crustacean prey, to larger prey, like Actinopterygii and Mollusca. Some species became more generalist during development, others restricted their prey range. Shifts in feeding habits affected species’ roles in the food web, with different species occupying different functional trophic groups over the course of their life cycles.