Feeding ecology of the cephalopod Octopus vulgaris illustrated by a stable-isotope approach

Cephalopods play a key role in the marine environment, but studies of their feeding ecology are still limited. Stomach content analysis have been the main method used for studying the diet of cephalopods, but despite being time-consuming, the prey is often difficult to determine. This technique allows the determination of species eaten, using hard parts that tend to resist digestion and remain in the stomach for longer period of time or hard parts which resist digestion, such as crustacean exoskeletons, cephalopod beaks, fish otoliths, and bones. A stable isotope study was carried out seasonally to investigate the feeding ecology of the cephalopod Octopus vulgaris, and its potential prey species were identified according to the species that appeared in pots collected by fisherman. The food items identified were the fan mussel (Atrina fragilis), Algarve volute (Cymbium olla), Henslow's swimming crab (Polybius henslowii), and the red-band fish (Cepola macrophthalma). The stable isotope mixing model SIAR indicated that O. vulgaris is predating mainly on the bivalve A. fragilis (mean value: 70%). P. henslowii could have a contribution of 18% to the cephalopod diet, whereas the gastropod C. olla and the fish C. macrophthalma were much less important to the consumer diet (7 and 5%, respectively).