Dietary niche differentiation in a mesopredatory dasyatid assemblage

Abstract

Most batoids are mesopredators and are often hypothesised to play important ecological roles. However, a comprehensive understanding into these roles remains limited given the paucity of information of their trophic habits. St. Joseph Atoll ($5.43^{\circ }\hbox { S}$, $53.35^{\circ }\hbox { E}$) is a remote ecosystem that hosts a resident assemblage of dasyatids (Pastinachus ater, Urogymnus granulatus, and U. asperrimus). Both stomach contents (SC) and stable isotope samples (SI) ($\delta ^{15}\hbox {N}$ and $\delta ^{13}\hbox {C}$) were collected in 2015 and used to investigate the trophic niches of this dasyatid assemblage and whether these niches differed inter- and intra-specifically. Dasyatid muscle samples as well as baseline samples of potential prey species and primary producers were collected for SI analyses. SC data highlighted significant inter-specific differences in diet, U. granulatus juveniles mostly consumed decapod crustaceans (Callianassidae and Portunidae) and P. ater juveniles mostly consumed bivalve molluscs. The mean species trophic positions for juveniles of all three species ranged from 2.9 to 3.6 when calculated using $\delta ^{15}\hbox {N}$ and from 3.4 to 3.6 when calculated using stomach content data. Analysis of $\delta ^{13}\hbox {C}$ showed that all juveniles were reliant upon the same carbon resources (primarily derived from seagrass beds) at the base of the food web. Diet appeared to change with size as larger individuals displayed lower $\delta ^{13}\hbox {C}$ and higher $\delta ^{15}\hbox {N}$ compared to smaller juveniles. Additionally, isotope values of the smallest individuals were similar to the largest individuals, suggesting maternal meddling. The identified patterns of inter- and intra-specific trophic niche differentiation may be indicative of competitive effects and contributes to the understanding of mesopredators in community trophic dynamics.