Assessing the diet of octopuses: traditional techniques and the stable isotopes approach

Most studies of the feeding habits of octopuses have focused on the use of a single technique, often selected on the basis of its familiarity or minimal impact on study animals. Although over time new methods have been developed for dietary assessment, no comparative analysis has ever been done. Using Octopus insularis as a model, we examined the results of three methods of diet analysis to understand how these techniques can influence study conclusions. Octopus diets were assessed through the analysis of (1) fresh prey remains deposited in midden piles, (2) digestive contents from the octopus crop and stomach, and (3) stable isotope data on the octopus and its potential food sources. The research was conducted at Rocas Atoll, a pristine oceanic reef off the northeastern coast of Brazil, with data from a coastal environment with substantial human impact (i.e. Rio do Fogo, northeastern Brazil) being evaluated as well. Overall, the three methods differed in the type of data they produced and the timescale represented, reflecting their own biases, the biology and behaviour of the octopus, and even the local oceanographic and other environmental conditions. We therefore provide a conceptual diagram to help researchers to consider the inherent aspects of each method when planning and performing studies of octopus diet, and also when interpreting study results.