Stomach content analysis in cephalopods: past research, current challenges, and future directions

Dietary descriptions based on stomach content are pervasive regardless of the biases concerning its methodology, statistics and descriptors. We here review different biases on the stomach content analysis, sample size, and indices used for cephalopods. These mollusks are key components in marine food webs and most are characterized as active predators, thus, knowledge of their feeding ecology based on stomach content analysis is of paramount importance. We suggest alternatives to describe dietary data using unbiased estimators. These are based on re-sampling, using published data of cephalopod stomach content, and conducting estimations using intervals. In our review, we detected that several studies found a high proportion of empty stomachs and highly digested preys, which influences the sample size with contents. In turn, sample size is positively associated with the number of prey species in the stomach contents. Moreover, many studies confuse the terms opportunistic vs. generalist predator and selective vs. specialist predator. Therefore, to improve the understanding of these concepts, we calculated niche-width indices and compared them with prey richness and evenness at individual and population levels. We also show that although most cephalopods exhibit generalized diet and wide trophic niche, their hunting strategy is selective and specialized at individual level. It is worth noting that the diet of over 90% of cephalopod species is not yet known, and thus, the number of trophic studies should increase to fill this gap of knowledge. We provide a series of recommendations to improve and correctly conduct and interpret the studies of stomach content for cephalopods.