Feeding kinematics and morphology of the alligator gar (Atractosteus spatula, Lacepede, 1803)

Abstract

Modern (lepisosteid) gars are a small clade of seven species and two genera that occupy an important position on the actinopterygian phylogenetic tree as members of the Holostei (Amia + gars), sister-group of the teleost radiation. Often referred to as "living fossils," these taxa preserve many plesiomorphic characteristics used to interpret and reconstruct early osteichthyan feeding conditions. Less attention, however, has been paid to the functional implications of gar-specific morphology, thought to be related to an exclusively ram-based, lateral-snapping mode of prey capture. Previous studies of feeding kinematics in gars have focused solely on members of the narrow-snouted Lepisosteus genus, and here we expand that dataset to include a member of the broad-snouted sister-genus and largest species of gar, the alligator gar (Atractosteus spatula, Lacepede, 1803). High-speed videography reveals that the feeding system of alligator gars is capable of rapid expansion from anterior-to-posterior, precisely timed in a way that appears to counteract the effects of a bow-wave during ram-feeding and generate a unidirectional flow of water through the feeding system. Reconstructed cranial anatomy based on contrast-enhanced micro-CT data show that a lateral-sliding palatoquadrate, flexible intrasuspensorial joint, pivoting interhyal, and retractable pectoral girdle are all responsible for increasing the range of motion and expansive capabilities of the gar cranial linkage system. Muscular reconstructions and manipulation experiments show that, while the sternohyoideus is the primary input to the feeding system (similar to other "basal" actinopterygians), additional input from the hyoid constrictors and hypaxials play an important role in decoupling and modulating between the dual roles of the sternohyoideus: hyoid retraction (jaw opening) and hyoid rotation (pharyngeal expansion) respectively. The data presented here demonstrate an intricate feeding mechanism, capable of precise control with plesiomorphic muscles, that represents one of the many ways the ancestral osteichthyan feeding mechanism has been modified for prey capture. RESEARCH HIGHLIGHTSAlligator gars use a surprisingly expansive cranial linkage system for prey capture that relies on specialized joints for increased mobility and is capable of precise modulation from anterior to posterior using plesiomorphic osteichthyan musculature.