SEASONAL REPRODUCTIVE ACTIVITY IN BIVALVES OF THE GENUS MARCIA

Abstract

The preparation of histological sections from fresh gonads of Marcia hiantina and Marcia marmorata was effective in preserving important details regarding the distribution and relative abundance of the various types of cells. The use of Meyer's Haemalum and Schiff Reagent enabled fast and effective staining of these sections, which can then be analysed using a Digitizer Tablet to yield data on the gametogenic cycle of these bivalves. The Developing, Ripe and Spawned Stages of the gametogenic cycle are present throughout the year in the species of Marcia studied. Initiation and maturation of the gonads appear to be strongly influenced by the number of sunshine hours. Exogenous factors such as depressed salinities and temperature fluctuations may act together with endogenous factors to affect the timing and duration of spawning activity. This is indicated by spawning being timed such that the subsequent settling of larvae occurs under favourable conditions of low sunshine hours and a high degree of cloud cover. Marcia hiantina and Marcia marmorata are both dioecious, spawn continuously throughout the year but have two peak spawning periods a year. Males spawn over a more protracted period but at a lower intensity compared to females. Microgrowth increment patterns are found in tr e shells of Marcia marmorata but the periodicity of the patterns is not correlated with tidal cycles and reproductive activities.