Generalist and specialist feeding crabs maintain discrete trophic niches within and among estuarine locations

Intertidal crabs are abundant, key components of tropical estuaries whose trophic interactions provide a direct and identifiable link within the ecosystem. Our study investigated spatial variability in food resource use of intertidal crabs, using stable isotope (δ13C and δ15N) values. This was done for two genera with contrasting feeding strategies (specialist Uca vs. generalist Metopograpsus) within and among locations in North Queensland, Australia. Uca coarctata, Uca seismella, Metopograpsus frontalis, and Metopograpsus latifrons occupied distinct isotopic niches, as quantified by standard ellipse areas. Bayesian mixing models suggested a diet supported by microphytobenhos for Uca species and a more depleted source for Metopograpsus species. Evidence for opportunistic feeding at higher trophic levels by Metopograpsus spp. was reflected by higher δ15N values compared to Uca species. Differences in diet between Uca and Metopograpsus, based on isotopic data, were maintained among ten locations across five estuaries. Food resource use was more variable for Metopograpsus spp. compared to Uca spp. among locations, reflecting the opportunistic feeding of the former. Sewage pollution was echoed in augmented δ15N values of all species. Results revealed separate trophic niches for both generalist and specialist feeding intertidal crab species across the estuarine landscape. The isotopic patterns of Uca spp. and Metopograpsus spp. fitted within the wider intertidal crab community, with generally low overlap among species within individual habitats. The greater flexibility in food resource use by generalists among locations could potentially provide a buffer against changes in food availability. We argue that patterns in food-resource use need to be considered in response to anthropogenic changes in the estuarine landscape