Morphological and trophic specialization in a subterranean amphipod assemblage

Sympatric species are expected to exhibit specialisation that reduces interspecific competition in environments with food resources that exhibit little spatial or temporal variation in availability, while sympatric species in more unpredictable environments should exhibit generalised feeding strategies to exploit a wide range of variable resources. We combined stable isotopes and quantitative mouthpart morphometric data to investigate trophic structure among sympatric species in a diverse subterranean amphipod assemblage in the Edwards Aquifer, Texas, U.S.A. Seven amphipod species occupied different regions of isotopic (δ¹³C and δ¹⁵N) space, suggesting the use of different food resources. Trophic position, measured as δ¹⁵N, was negatively correlated with planar area of the mandible and number of molar ridges, while it was positively correlated with incisor width. Reduced molar size and robust incisors are generally associated with predatory feeding strategies in non‐subterranean amphipods. δ¹³C exhibited non‐significant relationships with mouthpart morphology although one species, Texiweckeliopsis insolita, had significantly different mouthpart and δ¹³C values. Another species, Stygobromus russelli, had more generalised mouthparts, but isotope values indicate that it had a unique and unidentified feeding method and food source, illustrating the obfuscating effect of phylogeny on form–function relationships. Intraspecific relationships between body size and isotope values indicate that amphipod species showed little to moderate ontogenetic shifts in trophic position. Furthermore, body size did not predict trophic position when data were combined across species, suggesting that larger amphipods do not necessarily feed at higher trophic levels in this community. Our results indicate that sympatric subterranean amphipod species can exhibit specialised feeding strategies, suggesting that competition among species is driving niche partitioning. These results contradict the assumption that variable resource availability in groundwater habitats selects for trophic generalists among sympatric species.