The reproductive ecology of Antarctic free-spawning molluscs

Abstract

This study on the free-spawning Antarctic limpet Nacella concinna and soft-shelled clam Laternula elliptica has revealed new information about reproduction in these species. N. concinna and L. elliptica show long-term and continuous reproductive strategies over their mature lifetime. Gametogenic cycles last �18 to 24 months leading to overlapping cohorts of oocytes within the same ovary. The initiation of gametogenesis coincides with the spring phytoplankton bloom. Vitellogenesis and spermiogenesis take �12 months. Reproductive processes continue over the austral winter fuelled by body reserves. Spawning is annual for N. concinna coinciding with the spring bloom, a strategy expressed by related Nacella spp. inhabiting the Southern Ocean. L. elliptica appears to have multiple spawning events in a year. The timing of the major spawning events in geographically isolated populations of L. elliptica moves from late summer to mid-winter across a latitudinal cline (from higher to lower latitudes). Annual reproductive effort in both species varies in response to the inter-annual variability in environmental conditions.

Antarctic free-spawners require �100 times more sperm to achieve the same levels of fertilization success as temperate and tropical free-spawners. Specific spawning behaviour are required to enhance success (e.g. synchronous release and spawning aggregations). The investment in individual sperm may be balanced by the extreme longevity of the sperm. Temperatures and salinities outside the normal tolerance range of the animals reduces fertilization success and increases abnormal development. The highly stenothermal and stenohalinic gametes and early life stages act to delimit the geographical distribution of these species around Antarctica. Narrow tolerance limits and showed gametogenic and developmental processes make free-spawners extremely vulnerable to environmental change (as predicted by global warming models). The development rates of Antarctic molluscs can be 15-18 times greater than temperate and tropical molluscs. Reproductive processes, fertilization and embryonic development in Antarctic bivalves and possibly in other Antarctic invertebrates shows no temperature compensation.

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