Isotopic niche analysis of sympatric hermit crabs (Decapoda: Anomura) reveals different trends among generalist consumers

Abstract

Hermit crabs are generalist consumers capable of obtaining food from various sources, showing resource overlap within and among species. In this study, we used stable isotopes of carbon (δ13C) and nitrogen (δ15N) to determine the species isotopic niche size, overlap, and trophic positions of four sympatric hermit crab species in Ubatuba Bay, southeast Brazil. We chose two different approaches to measure the size of the isotopic niche. Isotopic niche size was defined as (1) the occupation of individuals in a stable isotope bi-plot (δ13C and δ15N) and (2) as a probability region in a n-dimensional space and reduced elliptical regions (dimensions = 2). While some pairs of species showed overlapping isotopic signatures, others were more distinct, exhibiting isotopic partitioning. Overall, isotope analysis indicated that hermit crabs varied in their niche size and occupy different trophic positions based on two distinct isotopic baselines. This suggests that even generalist consumers with omnivorous diets exhibit distinct foraging behaviors, which may facilitate the coexistence of different species and help avoid competitive exclusion. This means that hermit crabs not only compete for empty shells but also feed on different food sources, or that the contribution of each item varies, as reflected by their isotopic values.