A comparative study on morphological differences in the olfactory system of red sea bream ( Pagrus major) and black sea bream ( Acanthopagrus schlegeli) from wild and cultured stocks

Abstract

The improving technology in artificial propagation of aquatic animals is having a direct effect on increasing aquaculture fishery production of many aquatic food species throughout the world. High-valued fish species are being produced en masse and released into the wild to augment populations depleted due to over-fishing and habitat degradation. This study was conducted to determine the effect of procedural conditions in mariculture hatcheries on the olfactory systems of fish in Kagoshima Prefecture, Japan, and to compare the results with fish from wild stocks. Red sea bream ( Pagrus major) and black sea bream ( Acanthopagrus schlegeli) were suitable candidates for this study because of their economic importance and history in mariculture production, especially ongoing restocking of natural populations via stock enhancement programs in Japan. An extensive survey revealed a high percentage of fish with an abnormal single olfactory nasal opening. The single abnormal nasal opening contrasted strongly with a pair of natural nasal openings, an anterior and a posterior nares, in normal fish of both species whereby olfactory water is sampled when the fish swims. Ultrastructural observation of lamellar topography demonstrated not only a substantial variation in sensory epithelial distribution patterns in red sea bream of cultured stock but also showed that the sensory epithelial area in abnormal cultured fish was significantly smaller ( P<0.005) than the lamellar area in wild fish. The densities/mm 2 of microvillous and ciliated olfactory receptor neurons in both species were not significantly different when normal fish were compared with abnormal fish. Nonetheless, there were some notable irregularities in distribution patterns of the two receptor neurons on the lamellar surface. In abnormal red sea bream, the density of ciliated nonsensory cells was relatively higher than the normal fish. The kinocilia in cultured fish (normal red sea bream with anterior and posterior nares; CNR, 6.45±0.83 and abnormal red sea bream with a single nasal opening; CAR, 8.05±1.39 μm SD) were significantly longer ( P<0.05) than the ones observed in wild fish (5.46±1.39 μm SD). Further, ciliary beat cycle and metachrony were evident in normal fish olfactory epithelia but were usually not evident in abnormal fish. The results indicated that gross and micromorphological abnormalities in the olfactory systems of fish are a consequence of certain procedural condition(s) in mariculture facilities. The physiological and behavioral implications of morphological deformities in fish olfactory systems are discussed, especially in the context of ensuring production of healthy, normal fish for marine ranching programs and ecological preservation.