Cave amphipods reveal co‐variation between morphology and trophic niche in a low‐productivity environment

Abstract

Abstract Identifying the relationships between morphology and trophic niche is at the core of functional morphology. Low resource diversity and fluxes of organic carbon are expected to constrain trophic specialisation of morphological structures because food resources are too scarce to promote trophic differentiation. However, species from low‐productivity habitats often exhibit specialised biological traits such as resistance to starvation and high food‐finding abilities, which may in turn release constraints on trophic and morphological differentiation among species. Groundwaters are food resource‐limited because of the lack of photosynthetic production and limited inputs of organic carbon from surface ecosystems. We tested for co‐variation between morphology and trophic habits in co‐occurring Niphargus amphipods from five groundwater caves of the Dinaric Karst, Europe. We predicted that the size of gnathopods—the accessory feeding appendages—would positively co‐vary with trophic position: species with larger gnathopods should more easily grab and immobilise prey. We quantified gnathopod size and shape by means of morphometric measurements and assessed isotopic niche, trophic position, and carbon signatures using nitrogen (δ15N) and carbon (δ13C) stable isotopes. We tested for correlation between morphological traits and trophic position and δ13C signatures while accounting for phylogenetic relationships among species. All co‐occurring species differed morphologically in at least one gnathopod measurement and all of them differed in their isotopic niches. As predicted, gnathopod size increased with the increasing trophic position. This co‐variation probably reflects differences in detritivorous and predatory habits among species: amphipods with larger gnathopods, hence larger muscle and more powerful grip, could more easily subdue prey. Moreover, we found a significant correlation between gnathopod shape and the normalised δ13C values, indicating that shape of the gnathopods may be related to exploitation of different food resources. We show that low‐productivity subterranean habitat species can exhibit strong trophic specialisation of morphological structures. Gnathopod size and shape of Niphargus amphipods are functional traits that co‐vary with trophic habits. Our findings pave the way for investigating how co‐variation of morphological and trophic traits may control energy flow and species’ coexistence at lower bounds of habitat productivity.