Abstract:
Reproduction of the holothurian Stichopus mollis from the north-east coast of New Zealand was investigated through histological examination of the gonads. Changes in the gametogenic condition of the gonads are documented by the maturity index and oocyte size-frequency methods. The ovary or testis is located in the anterior coelom, and consists of two bilaterally symmetrical tufts of gonad tubules. S. mollis has an annual reproductive cycle, spawning in the austral summer. Males and females show synchronous development. Growth of the gonads begins in early spring, and involves the regrowth of the gonad basis, with subsequent branching and lengthening of the gonad tubules. In female sea cucumbers, oocytes develop from 5 µm to a maximum of 180 µm. Growth is characterised by the formation of larger oocytes surrounded by folJicular epithelium in the centre of the tubules, with a ring of smaller oocytes along the tubule wall. The testis retains a large spermatocyte layer throughout most of gonad growth. Few gonads were found to contain large numbers of spermatozoa. After spawning, the gonad tubules and basis are completely resorbed in both sexes so that no gonad tissue exists for 5–6 months. Observations from the South Island suggest that individuals do not undergo complete resorption, but maintain a resting phase gonad during the winter. Stichopus mollis, therefore, shows a different reproductive pattern with respect to latitude. Although the “tubule recruitment model” of Smiley (1988) may be representative of gonadal growth in S. mollis in southern New Zealand, in the warmer waters of north-east New Zealand the gonad is completely resorbed and the progressive recruitment of tubules cannot occur. These results emphasise the importance of location in studying reproduction in widely distributed holothurian species.
