The caudal neurosecretory system of the isospondylous teleost, Albula vulpes, from different habitats

Abstract

The caudal neurosecretory system of the isospondylous teleost, Albula vulpes , was studied by light and electron microscopy. The fish were taken in Kaneohe Bay, Hawaii, from two habitats: oceanic ponds with wide variations in salinity, and the open sea. The caudal system of Albula shows some primitive characteristics, including extensive extraurophysial neurohemal regions resembling those in elasmobranchs, and some advanced characteristics including a well-developed lobate urophysis. Two types of caudal neurosecretory cells were distinguished, based on staining reactions and on the presence of elementary neurosecretory granules of two different sizes in axon terminals. The elementary granules seem to be formed not only in the perikaryon by the Golgi complex but also by transformation of Golgi vesicles into granules. In the neurosecretory terminals, vesicles and granules may originate locally from a tubular system. Depletion of neurosecretory material from the urophysis appears to involve the loss of electron-dense material from the granules and their transformation into vesicles. Some neurosecretory neurons send a process into the central canal of the spinal cord, as well as to the urophysis. Fish from the two habitats could be distinguished on the following bases: (1) The amount of secretory material in the neurohemal areas is much greater in the pond fish, as indicated by the opalescence of the freshly-dissected urophysis, the elaborately-developed stainable areas in the urophysis, and the presence in terminals of many more electron-dense granules. (2) A much larger proportion of caudal neurosecretory cells in the pond fish show prominent intranuclear cytoplasmic invaginations, which appear to be active centers for elementary granule production. In general, the observations are interpreted in support of an osmo(iono)-regulatory role for the urophysial system.