Supramedullary Neurons in Teleost Fishes: Distribution and Putative Functions

Supramedullary neurons (SMNs) were described almost 150 years ago as large, unipolar cells whose somata are located on the dorsal aspect of the medulla oblongata and spinal cord in adults of teleost fishes. SMNs are either aligned in a single, median longitudinal row, generally extending over the rostral third of the spinal cord, or clustered over the medulla oblongata and rostral spinal cord. We add to the list of species that have SMNs and provide the first description of a novel distribution of SMNs along most of the spinal cord in the oyster toadfish (Opsanus tau) and sea raven (Hemitripterus americanus). The number of SMNs ranges from 27 in the Atlantic butterfish (Peprilus triacanthus) to 3712 in the oyster toadfish (Opsanus tau), and soma diameter ranges from an average of 23 µm in the Atlantic butterfish to 232 µm in the Southern pufferfish (Sphoeroides nephelus). SMNs could not be identified in 11 species, including seven freshwater fishes. We discuss factors that may affect the presence/absence of SMNs including age, length of time in captivity, and habitat salinity. Proposed functions of SMNs include neurosecretion and mucous secretion, and we suggest approaches that may aid in the discovery of the role of these fascinating neurons.