Spatial variation in reproduction in southern populations of the New Zealand bivalve Paphies ventricosa (Veneroida: Mesodesmatidae)

Paphies ventricosa is a large surf clam endemic to New Zealand, and whose populations have substantially declined during the past century owing to overfishing and habitat degradation. Poor recruitment is now evident, and therefore, understanding the reproductive patterns of P. ventricosa is a key to developing and implementing conservation strategies for the species. This study examines the reproductive cycle of P. ventricosa over one year in a population at Oreti Beach, Southland, the southernmost known extent of the species. At the same beach, we quantify spatial variation in reproduction among four sites using quarterly surveys. Reproductive status is quantified from body indices and histological examination of gonads. Based on changes in oocyte sizes, gametogenic stages and condition index, we observed a species with a primary spawning in spring and a second spawning event in autumn, with no resting phase but minimal reproductive activity over winter. Seasonal reproduction corresponded with warmer sea surface temperature and a peak in chlorophyll-a concentrations in the region. Small-scale (<15 km) variation in the timing of spawning was also evident along Oreti Beach, and these patterns maybe an important consideration when identifying areas that may be considered for conserving source populations.