Reproductive ecology of the scallop Pecten novaezelandiae

Abstract

Key factors influencing the reproductive success of the scallop Pecten novaezelandiae were examined in the Hauraki Gulf, North Island, New Zealand using a combination of field and laboratory studies. Size at maturity was objectively estimated to be 65 mm shell height, although small differences in size at maturity among sites may exist owing to differences in environmental conditions that affect growth rates. Reproductive activity was monitored at four sites from 2000 to 2003 using multiple techniques. Gonad indices appropriate for measuring reproduction were assessed and a method that directly scaled gonad mass to shell height was applied. A visual grading index based on gonad appearance was significantly correlated with quantitative histological and gonad mass data (r² = 0.74-0.87). Visual grading facilitated rapid assessment of reproductive condition and could be conducted non-destructively. Repeated observations of visual grade in tagged scallops tethered to the seabed were made in situ, the first time that longterm sequential reproductive activity of individuals has been described for a mobile freespawning animal. Scallops exhibited episodic spawning from spring through to early winter. Temporal variability in gonad size and condition was the result of multiple (serial) spawnings per year. Four to 75% of individuals spawned synchronously between sampling dates. Partial spawnings were common and gonads redeveloped between spawnings. Partial spawning may be part of a bet-hedging strategy to manage the risk of fertilisation failure in asynchronously spawning populations. The timing of spawning varied among sites and years, but often coincided with a sharp drop in sea surface temperature. Scallop spawning behaviour reflected their fertilisation requirements, releasing millions of gametes in a series of contractions over a short period of time. In vitro fertilisation success was primarily dependent on sperm concentration, but was also influenced by gamete age and sperm-egg contact time. Maximum fertilisation rates (up to 92%) occurred only at high sperm concentrations (106-107 sperm ml'1). A polyspermy-adjusted fertilisation kinetics model explained 87% of the variation in fertilisation success. Pecten novaezelandiae gamete traits appear to be adapted to fertilisation at high sperm concentration. Consequently, low-density populations may be susceptible to Allee effects and reproductive failure.