GROSS MORPHOLOGY AND FINE-STRUCTURE OF THE OLFACTORY ORGAN OF RAINBOW FISH (ATHERINIFORMES, MELANOTAENIIDAE)

The olfactory organs of 6 rainbow fish species were studied, and were compared with those of closely related cyprinodonts. It has been found that, in contrast to cyprinodonts, the olfactory mucosa of rainbow fish is more conspicuously thrown up into folds. Their arrangement in parallel is considered a relatively primitive character of the organ. Electron microscopic investigations on Nematocentris maccullochi have shown that the sensory epithelium consists of basal, supporting, and two types of receptor cells, ciliated and microvillous. As in cyprinodonts the receptor cell cilia exhibit a basic 9 + 0 microtubular pattern. In Nematocentris the frequency of receptor cells, between 200,000 and 250,000 per mm2, is lower than in cyprinodonts. It may be suggested that the lower density is compensated for by enlargement of the sensory area due to the folding. In addition to broad‐surfaced dendritic membrane appositions found in both Nematocentris and cyprinodonts, contacts by means of dendritic spines are found only in the rainbow fish. The olfactory supporting cells of the latter are distinguished from those in cyprinodonts by more prominent lateral interdigitations, centrioles, and the higher number of vesicles associated with extensive Golgi complexes. 1979 The Royal Swedish Academy of Sciences