Meeting Abstract
Epibranchial organs are paired food-aggregating structures thought to have evolved at least six times within actinopterygian fishes. Ranging in complexity from small slits on the ventral pharyngeal roof to complex spiraling structures, epibranchial organs are morphologically diverse. Despite this morphological diversity and broad distribution, little is known of the development and evolution of epibranchial organs. Here we investigate the origin of the epibranchial organ by comparing ontogeny within three different species. Anchoa mitchilli (Engraulidae) represents a more basal species with a simple epibranchial organ, Brevoortia tyrannus (Clupeidae) has a more anatomically complex epibranchial organ, and Hypophthalmichthys molitrix (Cyprinidae) represents the most complex epibranchial organ described. Despite the final adult morphology all epibranchial organs initially develop from an epithelial involution that is subsequently surrounded by muscle dorsal to the fourth and fifth pharyngeal arches. The epibranchials and infrapharyngeobranchials are hypertrophied to support the growing structure. There was a strong correlation between ossification rates of the branchial arches and the development and overall size of the epibranchial organ. Thus, in H. molitrix the fourth and fifth arches develop and ossify before the more anterior arches. The final structure is composed of all five arches whereas in both Brevoortia and Anchoa ossification of the arches begins later in development and only the fourth and/or fifth arches are included in the epibranchial organ. We hypothesize that the epibranchial organ originated from an involution into the pharyngeal tissue supported by a modified posterior branchial arch.
