Meeting Abstract
Novel functions typically arise through the co-option of existing anatomy. This creates a close coupling between the new and existing functions of the associated structures, which is thought to constrain morphological evolution. We examine the anatomy and evolution such a musculoskeletal system — the flatfish urohyal. Flatfishes (Pleuronectiformes) are a diverse group of laterally compressed fishes with extreme cranial asymmetry. They bury within the sediment using body undulations and fin flicking, and preliminary evidence shows that, like many other fishes, they use jets of water from their gill openings to help fluidize sediment. Because flatfishes lay on their side, only one gill opening is in contact with the substrate, which could reduce their potential jetting capacity. However, flatfishes have a highly modified urohyal that may provide a shunt between the left and right gill cavities. The urohyal is a sesamoid bone that develops within the tendon of the sternohyoideus, a major suction feeding muscle. In most acanthomorph fishes, the urohyal is broad and flat, but in flatfishes, it is sickle-shaped. Our dissections revealed that the sternohyoideus attaches dorsally, medially, and posteriorly in three distinct segments. Manipulation showed that the dorsal segment (encased by a large tendon) rotates the urohyal posterodorsally (possibly to pull the urohyal into shunting orientation), and the medial and posterior segments pull the urohyal posteriorly (for hyoid retraction during suction feeding and ventilation). We characterize the considerable variation in urohyal and sternohyoideus shape across 8 species from 3 families, and we discuss the implications of the multifunctionality of this structure for the evolution and ecology of flatfishes.
