LIGHT AND ELECTRON-MICROSCOPIC STUDIES ON THE DEVELOPMENT OF THE INNER-EAR AND OTOLITH OF THE AYU PLECOGLOSSUS-ALTIVELIS

The development of the inner ear, especially sensory maculae and otoli ths, was studied in the ayu Plecoglossus altivelis, using light and el ectron microscopy. Light microscopy indicated: 1) The auditory vesicle first appears by 3 days after fertilization, The saccular macula over lain by a round otolith appears and possesses apical hairs by 4 days, while the utricular macula appears by 5 days after fertilization. 2) 9 days after fertilization (hatching day), both the saccular and utricu lar maculae are well differentiated: composed of hair cells and suppor ting cells, and overlain by saccular and utricular otoliths, respectiv ely. Electron microscopy revealed: 1) By 5 days after fertilization, s ensory hair bundles and bud-like microvilli protrude from the apical s urface of the saccular and utricular maculae into the endolymphatic lu men. 2) Secretory materials, such as electron dense cytoplasmic extrus ions, vesicles containing spherules, and dense granules, seem to occur from the apical surface of sensory epithelium, probably supporting ce lls, and to contribute to the formation of the otolith and/or otolith increments. 3) By 9 days after fertilization, the saccular otolith pos sesses an inner incremental layer around the central nucleus and a thi ck belt of peripheral incremental layer. Consequently, it was suggeste d that the hearing mechanism in the ayu may be established earlier com pared with other fishes.