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

Abstract

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.

Life History of the Arctic Squid Gonatus fabricii (Cephalopoda: Oegopsida) Reconstructed by Analysis of Individual Ontogenetic Stable Isotopic Trajectories

“…Reported results on diet are generally in accordance with previous studies using stomach contents [ 36 , 43 , 44 , 45 , 46 , 115 , 116 ] that showed crustaceans are major prey for G. fabricii with the increasing importance of fish and cephalopods in individuals larger than ML 54–70 mm [ 44 , 45 , 46 ]. However, assimilated diet predicted by SIA data often does not entirely coincide with stomach contents analysis results in cephalopods [ 28 , 30 , 117 ]. Moreover, food spectra changes in G. fabricii were expected to happen due to the hook appearance at its arms and tentacular clubs [ 43 , 44 , 45 , 46 ], which exactly fits the predictions from this study.…”

The inter‐annual fishing variability in Octopus insularis (Leite & Haimovici 2008) as a result of oceanographic factors

“…In a recent study using stable isotopes to evaluate the impact of O . insularis at a shallow water tropical food web, it was demonstrated that this species, due to its fast metabolism, exerts great predatory pressure on bottom‐associated organisms (de Souza Dantas et al, 2020), while serving as a relevant food source for top and mesopredators (Pinheiro et al, 2017).…”