Comparative anatomy of otomorphan epibranchial organs

Certain microphagous fishes possess an epibranchial organ (EBO), a paired muscular pocket-like structure in the posterior pharynx, that facilitates the aggregation of small food items entering the oropharyngeal cavity. Morphologically complex and phylogenetically diverse, the anatomy of EBOs has been described in a small number of taxa that possess this structure, in many cases without a thorough investigation at the microscopic and ultrastructural level. Additionally, the evolution of EBOs has not been rigorously examined within a phylogenetic comparative context, leaving many unanswered questions about how the morphological diversity of EBOs relates to historical patterns and ecology. We characterized the anatomy, histological architecture, and structural patterns of EBOs in 13 otomorphan species belonging to the orders Clupeiformes, Gonorynchiformes, and Characiformes; this sampling includes Cetengraulis edentulus, Nematalosa come, and Tenualosa thibaudeaui, in which the presence of an EBO has not been previously documented. We then conducted a preliminary investigation of relationships between otomorphan EBO anatomy, phylogeny, and diet. Patterns of anatomical diversity were best explained by variation along five morphological axes: shape, size, associated gill rakers, muscularity, and adiposity. EBOs consisted of bilaterally paired diverticula surrounded by layers of circumferential and longitudinal muscle and varying amounts of adipose tissue. Papillae were found in the epithelium lining the diverticulum of each organ; they varied in length and width along the proximodistal axis of the diverticulum and were studded with mucus-producing cells. We found that EBO anatomy was not strongly correlated with phylogenetic relatedness but was moderately correlated with diet in some instances. We hypothesize that EBOs have independently evolved in Otomorpha multiple times via a conserved developmental pathway that makes use of the same underlying tissue types to construct morphologically diverse structures. These findings suggest that there are multiple ways to build an EBO using the same basic anatomical components to achieve the same functional goal-the aggregation of small prey.