The Epibranchial Organ And Its Anatomical Environment In The Gonorynchiformes, With Functional Discussions

All extant gonorynchiforms except for species within the genus Gonorynchus are provided with an epibranchial organ (EBO), which consists in a blind sac opening through a canal into the buccopharyngeal cavity just above and behind the last branchial slit. The proximal part of the EBO is supported by specialized elements of the last two branchial arches. The gill rakers rows of the same arches continue in the canal part of the EBO, but not in the sac. The epithelium and tunicae of the EBO are similar to those of the surrounding buccopharynx; it is plicate and rich in mucous cells. The gill rakers of all the branchial arches are particularly long and bear microbranchiospines (except in Gonorynchus and Grasseichthys). They constitute a very efficient filter complicated by bridges between the branchial arches and between the opposite gill rakers rows. All gonorynchiforms are considered to be efficient filter feeders and the EBO plays an important part in that function. A hypothesis is presented as to the water currents in the buccopharyngeal, opercular and EBO cavities, and as to the trapping of food particles by mucus and their transport into the esophagus. In Gonorynchus, the EBO is functionally replaced by a dorsally elongated fifth branchial slit provided with very long gill rakers. This configuration is puzzling, regarding the phylogenetic position in which the genus has been placed in recent cladistic studies.