Comparative aspects of gonad development in field and laboratory conditioned surf clams from Foxton Beach, New Zealand

Abstract

The surf clams Mactra murchisoni Deshayes and Spisula aequilatera (Deshayes) were sampled between August 1995 and November 1996 from Foxton Beach, New Zealand. Patterns of gametogenesis and spawning were determined from histological preparations of the gonads. The reproductive cycles of M.murchisoni and S.aequilatera are similar to those of other members of the Mactridae in the northern hemisphere. For both species, gonad maturation commenced around mid to late winter, followed by a period of rapid maturation leading to a major spawning event during spring. This major spawning event occurred over a period of about two weeks, and was subsequently followed by a secondary minor spawning by S.aequilatera, and a trickle spawning by M.murchisoni. Rapid gonad development and spawning occurred during periods of moderate sea water temperatures (around 16°C) and frequent phytoplankton 'blooms'. No evidence of hermaphroditism was observed. The sex ratios of both M.murchisoni and S.aequilatera did not differ significantly from 1:1. Sexual dimorphism was not observed for either M.murchisoni or S.aequilatera. For M.murchisoni, the first sign of gametogenic activity was observed in clams (both male and female) two years of age. However, since smaller M.murchisoni (and therefore younger) were not sampled, this result is provisional. Sex cells were encountered in one female S.aequilatera not greater than one year old. By two years of age both male and female S.aequilatera contained sex cells. Between January to October 1996, the condition index of M.murchisoni and S.aequilatera declined from approximately 17 to 13 and 19 to 10 respectively. For both species, the greatest inter-sample decline was observed between September and October 1996. Only three Mactra larvae were found in water samples collected between December 1995 to October 1996. Four further bivalve larvae were unidentifiable. M.murchisoni and S.aequilatera could not be successfully conditioned in the laboratory under independent temperature (2 regimes) and food delivery (3 delivery rates) regimes. Future directions are discussed.