Functional significance of intramandibular bending in Poeciliid fishes

Substrate-feeding teleosts show multiple, independent evolutionary acquisitions of intramandibular bending (bending within the lower jaw)-a behavior that likely enhances performance when feeding on attached or encrusting food items. However, intramandibular bending has only been quantified for marine teleosts. Here, we examine substrate feeding in eight species from the order Cyprinodontiformes and quantify movements produced by the anterior jaws of four target species selected from the family Poeciliidae to represent a variety of trophic strategies. Intramandibular bending, defined here as bending between the dentary and angular-articular bones of the lower jaw, is not present in some poeciliids (i.e. Gambusia affinis), nor is it present in outgroup cyprinodontiforms (i.e. Fundulus rubrifrons). However, intramandibular bending is present in certain poeciliids (i.e. Poecilia sphenops), and can exceed 90 degrees. Such jaw bending enables the production of a gape angle that approaches 120 degrees, which likely allows the fish to maximize contact between the toothed tips of the jaws and the substrate during the bite. Intramandibular bending in poeciliid species is associated with specific trophic shifts: the greater the intramandibular bending in a given species, the more attached algae (periphyton) reported in its diet. This result supports the hypothesis that intramandibular bending enhances performance when feeding on encrusting food items. We predict that additional examples of functional convergence are likely to be documented in freshwater teleosts as more herbivorous species are examined, and we propose that intramandibular bending represents an excellent model system in which to examine the functional processes that underlie convergent evolution.