Gonad formation, sex differentiation and gonad maturation processes in artificially produced juveniles of the abalone, Haliotis discus hannai

Abstract

The gonad formation, sex differentiation and gonad maturation processes of the abalone, Haliotis discus hannai, are clarified through histological observations on gonads of juveniles artificially produced and reared in tanks. Juveniles were produced in March 1998 and March 1999 and collected from April 1999 to March 2000 periodically (from 1 to 20 months after settlement (AS)), which resulted in observations of 480 abalones with various shell lengths (SL; <2–42 mm). The gonad formation and sex differentiation processes could be divided into four successive phases depending on the appearance of the following histological structures: gonad cavity, primordial germ cells (PGC), chromatin nucleolus stage oocytes (CNO) and maturing germ cells. The gonad cavity started to develop in juveniles over 7 mm SL at 2 months AS. It appeared in the surface connective tissue layer of the growing conical appendage. At 3 months AS, PGC appeared in the gonad cavity of juveniles larger than 7 mm SL. At 7 months AS, in November, sex differentiation was confirmed by the appearance of CNO in females larger than 21 mm SL, but differentiation into males could not be confirmed. From 15 to 18 months AS, from early summer to early autumn, yolk accumulation and spermatogenesis proceeded. Juveniles of both sexes became fully mature in November at about 20 months AS. Besides the germ cell development, several types of non-germ cells were also observed in the gonad cavity: acidophilic granule cells and nuclei strongly stained with hematoxylin in association with the CNO, follicle cells on the stalk of the oocyte at the oil droplet or yolk globule stage, and cells with an elongated nucleus adjacent to spermatogonia in the mature testis. These results on abalone sex differentiation and gonad maturation processes will form the basis for future studies on the effects of endocrine disrupters on such processes.