A stable isotopic study of the diet of Potamonautes sidneyi (Brachyura: Potamonautidae) in two coastal lakes of the iSimangaliso Wetland Park, South Africa

Abstract

Potamonautes sidneyi Rathbun 1904, is a dominant freshwater crab in KwaZulu-Natal, South Africa. Recent flood events in the iSimangaliso Wetland Park have allowed a substantial range expansion of this species, including previously hypersaline and desiccated areas. A stable isotope study was conducted to examine the feeding habits of the populations from Lake Sibaya and Mpophomeni Stream, two contrasting sites in the iSimangaliso Wetland Park. Juveniles from Mpophomeni Stream were more depleted in δ¹³C and more enriched in δ¹⁵N compared to adults, indicating a more carnivorous diet and higher trophic position. A general shift in diet was observed at both sites, with consumption of aquatic invertebrates and sedimentary organic matter more prevalent in the wet summer months, while greater proportions of detritus and microphytobenthos were consumed in autumn/winter. No significant difference was observed between adults from the two sites despite the abiotic variations. The species appears to play a complex role in the trophic web, by acting as an intermediate consumer facilitating the flow of nutrients across levels and by breaking down decomposing organic matter, allowing for rapid recycling of nutrients within its ecosystem.

Keywords: diet, trophic role, stable isotopes, freshwater, brachyurans, opportunistic feeders