Comparing the diet of two sympatric urolophid elasmobranchs (Trygonoptera testacea Müller & Henle and Urolophus kapalensis Yearsley & Last): evidence of ontogenetic shifts and possible resource partitioning

Abstract

The diets of two urolophids, the common stingaree Trygonoptera testacea and the kapala stingaree Urolophus kapalensis were analysed and compared to examine resource partitioning between these two morphologically similar sympatric Australian batoids. The diet of T. testacea was polychaete‐based, while that of U. kapalensis was dominated by crustaceans (mostly carid shrimps and amphipods). Intraspecific dietary compositions were examined amongst size classes within each of the two species to identify ontogenetic shifts in diet. Differences in the dietary compositions of the smaller total‐length classes of T. testacea suggest that their diet shifts as they increase in size, from one dominated by carids to one almost entirely comprising polychaetes. Significant ontogenetic dietary shifts were not identified in U. kapalensis. Although the two species shared eight broad dietary categories, their overall dietary compositions were found to be significantly different. The limited overlap in the dietary compositions of these two sympatric stingarees suggests the possibility of resource partitioning, with interspecific competition being implicated as a possible cue. Possible mechanisms for the partitioning of resources within and between these two species are discussed.