Studies on the reproductive and nervous systems of the protochordate Styela plicata (Lesueur, 1823) to correlate the nervous activity on reproduction

Abstract

Abu Kir Bay was found to be more suitable area for ascidian collection in Alexandria waters, especially the locality known, in public, the Dead Sea. Specimens of Styela plicata were collected monthly during 1 year. S. partita is usually found attached to the basal part of the other Styela. Both were carefully identified according to Vasseur (1967) and Millar (1970) . Adult specimens of S. plicata with suitable and same size were dissected to examine both nervous and reproductive systems in situ . Others were prepared technically for histological examination. Results of both evidenced each other. The observed changes in size and colour of the gonads, especially those of the oocytes, were used as an essential factor to divide the study into the following: Gonads of March–June; gonads of July–August; gonads of September–October; gonads of November–January; and gonads of February. Those observed changes, in addition to the differentiation of the affinity of the oocyte to the different applied stains led to the investigation of both: the growth of the oocyte and the breeding of the animal. During the growth of the oocyte, (a) it undergoes deutoplasmogenesis, (b) it attains its follicle layers and hence its membranes especially the chorion, (c) it ovulates if it becomes full ripe with optimum size and deep reddish orange colour. Ovulation occurs twice during the breeding season: first occurs during September, which is the peak of the activity, second during following February. Breeding occurs in early autumn and winter, while early spring, especially, March is the inter-breeding period. The testis, on the other hand, matures all the time. The reproductive cycle differs from breeding season since it begins from early spring (April) till the following February. This reproductive cycle of S. plicata is peculiar since it was investigated on histological basis. The neural complex undergoes cyclic changes in size synchronized with the changes of the gonads mentioned above. Only the neural gland changes in width and not in length. It is smallest during March, largest during September, decreases gradually but slightly in size to be almost large during February. The glandular nature and secretory activity of this gland are old observations. The present work gives reasonable histological investigation that it may have hormonal role regulating the reproductive cycle of, at least, S. plicata .