Significance of stingrays (Chondrichthyes: Myliobatiformes) as prey of crocodilians (Reptilia: Crocodylia) in non-marine environments

Abstract

Stingrays (Myliobatiformes) co-occur with many crocodilians (alligators, caimans and crocodiles) in non-marineenvironments of tropical and subtropical regions. However, predation by crocodilians on stingrays is rare. Here,we report two predation attempts (one unsuccessful, one unknown) on Potamotrygon motoro, by a southernspectacled caiman, Caiman yacare, in Iberá Lake, Argentina. Based on these and other observations, we discussthe hypothesis that crocodilian predation is more common on sharks and shark-like batoids than on depressedbatoids, due to the fusiform body shape of the former two and the flattened shape of the latter. Within the literature,all recorded predation events on sharks were successful; one out of two observed predation attempts onshark-like batoids resulted in consumption. No successful predation attempts on depressed batoids were recorded,although only one outcome was known out of the seven events observed. The prey/predator size ratio of oneof our observed events (0.263) was similar to both the mean of all predatory events (0.312 ± 0.060) and only successfulones (0.298 ± 0.104), indicating that size was less important than prey shape in determining the outcomeof the interaction. We reviewed the scientific literature on diet of South American crocodilians to quantify theprevalence of stingrays as prey. No stingray remains were found in any of the 7 species studied, based on 1339samples. The low success rate of crocodilians preying on depressed batoids, the lack of stingray remains in dietstudies of South American crocodilians, and the lack of rake-like marks consistent with caiman bites on livingNeotropical freshwater stingrays indicate that predation by crocodilians on stingrays is very low. This should betaken as a working hypothesis in future assessments of predation on obligate freshwater elasmobranchs.