Abstract
The availability of prey in an environment does not ensure that a predator will consume it: prey must also be detected, captured, and successfully handled. The morphology of the predator and prey imposes limitations on prey selection due to biomechanical constraints, making some prey functionally inaccessible. Morphological factors, including but not limited to tooth shape, body size, and mouth gape, therefore impose constraints on predator trophic niches. We assessed how two important components of trophic morphology (tooth shape and body length) may influence prey selectivity and trophic niche in two large-bodied sympatric sharks with contrasting foraging strategies. The first species captures prey using spear-shaped, grasping teeth (grey nurse/sand tiger/raggedtooth shark, Carcharias taurus), while the second has multi-cuspid cutting teeth used to serrate larger prey (sevengill shark, Notorynchus cepedianus). Stomach content analysis and isotopic values of δ(13)C, δ(15)N, and δ(34)S from muscle and liver were used to characterize isotopic niche and prey selection. As gape-limited grey nurse sharks grew, their consumption of teleosts decreased inversely to chondrichthyans. By contrast, non-gape limited sevengill sharks consumed teleosts and chondrichthyans in similar proportions, along with marine mammals, but with no clear relationship to body size. As body length increased, both species consumed prey from higher trophic levels (higher δ(15)N values), but sevengill sharks accessed prey at relatively higher trophic levels. Values of δ(13)C and δ(34)S remained relatively unchanged with body length presumably because mouth gape and dentition do not limit access to pelagic or benthic food webs. Although many other morphological factors, such as swim performance, biomechanics, or behavior, could also drive the results we observed, it is clear that morphological characteristics play an important role in prey selection and may be the primary mechanism facilitating resource partitioning in large sympatric predators. Their inclusion in ecological studies will help understand prey choice and how it shapes trophodynamics in marine ecosystems.