ABSTRACT
Carotid labyrinth of land vertebrates is known for the last two centuries but that of fishes for the last few decades only. It has been found to occur in catfishes (Siluroidei) alone and there is much in common between carotid labyrinth offish and amphibian in points of basic structure, topography and innervation. The structure, homology and functions are discussed in detail. The presence of peculiar paraneuronal cells in close vicinity of carotid labyrinth is a significant feature. The fish carotid labyrinth and associated pseudobranchial neurosecretory system, comprising of paraneuronal cells have been described to throw light on the possible function of these structures in the biology of these fishes. In its structure the carotid labyrinth seems to be more elaborate then that of higher vertebrates and its association with neurosecretory cells in these groups of fishes suggest a possible chemosensory function for the former and a paracrine role for the latter. Although an in depth investigation is needed to reach any conclusion, these cells seem to play a role in conditions of hypoxia, signalling the surfacing behaviour of these fishes, which is a common behavioural feature of fishes of this group. Nothing can be said with certainty at present, but the presence of pseudobranchial neurosecretory cells in close proximity of carotid labyrinth and efferent branchial vessel suggests that respiration in these fishes is a much more complicated process than was thought earlier.
More in depth investigations on the carotid labyrinth and associated pseudobranchial neurosecretory are likely to reveal more facts about the biology of catfishes that will help in understanding the nature of its derivatives in higher vertebrates, including carotid body of mammals.
