Suction feeding turned on its head: a functional novelty facilitates lower jaw protrusion

Functional novelties play important roles in creating new ways for
organisms to access resources. In fishes, jaw protrusion has been
attributed to the massive diversity of suction-based feeding systems,
facilitating the dominant mode of prey capture in this group. Nearly all
fishes that feed by suction use upper jaw protrusion, achieved by rotation
of the mandible at its base, which then transmits forward motion to
independently mobile upper jaw bones. In this study, by contrast, we
explore an unusual form of lower jaw protrusion in the freshwater
invertivore, Nannocharax fasciatus, enabled by a novel intramandibular
joint (IMJ). We combine morphological, kinematic, and biomechanical data
to show that the added mobility created by the IMJ influences the pattern
of suction-based prey capture movements and contributes to lower jaw
protrusion (increasing it by 25%, based on biomechanical modeling).
Interestingly, the upper jaw bones are fused in N. fasciatus and rotate
about a single fixed joint, like the lower jaws of most other suction
feeding fishes. We suggest that this vertical inversion of the jaw
protrusion mechanism for ventrally directed suction-feeding on benthic
prey is a likely exaptation, as the IMJ is used for biting in related
taxa. This work highlights the ability of novelties to facilitate
ecological specialization by enabling new functional capabilities.