Local consumers are the first line to control biological invasions: a case of study with the whelk Stramonita haemastoma (Gastropoda: Muricidae)

The increasing spread of invasive species in the Mediterranean Sea determines several alterations in local food webs, changing the feeding habits of native organisms. The whelk Stramonita haemastoma is a widespread Mediterranean gastropod that consumes bivalves, barnacles and limpets. Previous studies showed a shift in its diet from the bivalve Mytilaster minimus to the invasive mussel Brachidontes pharaonis, presumably due to a higher energy gain. Here we tested whelks' preference among natives and a novel prey, calculating the profitability ratio, and integrating those results with biochemical analysis on prey tissues and the routine metabolism of the whelks. Further, we used the scaled functional response as a theoretical tool to describe whelk ability to obtain energy from their environment by using four different prey species: B. pharaonis, Mytilus galloprovincialis, M. minimus and Patella caerulea. Whelks evidenced a Type II functional response for all prey, while Brachidontes displayed a lower attack rate and a higher handling time. Stramonita showed a greater preference for Brachidontes, that resulted as the prey with the higher energetic content, and the second most profitable after Patella. This suggests that the higher energy gain is behind the change in the predator's diet, with possible effects on its energy budget.