Reproductive Output and Seasonality of <i>Limnoperna fortunei</i>

Abstract

Young <i>Limnoperna fortunei</i> mature sexually from 5–6 to ~15 mm. The species is generally dioecious, with approximately equal numbers of males and females and very small (< 0.6 %) proportions of hermaphrodites. The gametogenic cycle has been described for both Asian and South American populations, recognizing between four and five reproductive phases. Gonadal cycles based on histological sections yielded somewhat dissimilar results for different areas. In Hong Kong, two yearly peaks in reproductive output were detected. In South America, mature sperm and ova have been recorded year round and several irregularly spaced spawning events have been observed, as well as more or less continuous breeding punctuated by peaks in spring and at the end of the summer. Reproductive studies based on changes in the abundance of larvae in the water column have been carried out in South America and in Japan. In tropical and subtropical South America, larval output is more or less continuous for 6–10 months of the year, often with a major peak in spring–early summer, and a smaller one in the late summer–autumn. In Japan, at considerably lower water temperatures, larval production is limited to 1–2 months centered around summer. Apparent disagreements between results based on histological data and on larval counts stem from the fact that while the latter integrate the reproductive output of extensive mussel beds dispersed over large areas, histological evidence pinpoints with high precision the ripening and spawning of isolated mussel clusters. Aside from water temperature, several other factors (pH, salinity, dissolved oxygen, suspended solids, chlorophyll a, flood–drought cycles) have been proposed as reproductive triggers, but actual associations have not been demonstrated. Peak larval densities can exceed 20,000 ind./m³, but, normally, values range around 6000 ind./m³, showing major fluctuations within short periods, as well as changes as a function of time elapsed post colonization, and availability of substrata suitable for adult occupation. Microcystin-producing cyanobacterial blooms can kill L. <i>fortunei</i> larvae.