The Fish Bites Back: A Developmental Analysis of Feeding Biomechanics in Danionin Minnows

Ecological roles are often closely related to the food organisms consume, and investigating the developmental control of feeding apparatus morphology and biomechanics contributes to a broader understanding of how ecological roles evolve. Changes in thyroid hormone signaling are thought to be involved in the evolutionary diversification of feeding mechanics in many fish lineages, including danionin minnows such as zebrafish (Danio rerio), a common model organism. Recent work suggests that changes in thyroid hormone signaling may have influenced the trophic divergence of fishes in the genus Danio and those in the genus Devario, including the giant danio (Devario aequipinnatus). We examined developmental series of wild-type zebrafish, transgenic zebrafish rendered incapable of producing thyroid hormone (hypothyroid zebrafish), and giant danio. To test whether eliminating thyroid hormone would cause the form and functional abilities of zebrafish skulls to converge toward those of the giant danio, we tracked development of form and function in all three types from early larval stages when exogenous feeding begins to the point of normal sexual maturity. Clearing and staining techniques were used to visualize the cranial skeleton so that changes in skull form could be analyzed using geometric morphometrics to statistically compare developmental trajectories in morphology. We also used high speed video to record and kinematically compare fish feeding strikes at multiple developmental stages. We found that hypothyroid zebrafish do not converge in overall head morphology with giant danio, and that while hypothyroid zebrafish and giant danio are closer in some kinematic parameters, timing of events during the feeding strike is generally consistent between hypothyroid and wild-type zebrafish. This suggests that while changes in thyroid hormone signaling are likely involved in the evolution of different feeding modes among these fishes, thyroid hormone is not sufficient on its own to account for these differences.