Protandry in the New Zealand geoduck, Panopea zelandica (Mollusca, Bivalvia)

Unequal sex ratios favouring males, especially in younger or smaller animals, is a feature of many infaunal bivalve species, including geoducks. Size and age at sexual maturity, sex ratios, and growth rates of the New Zealand geoduck, Panopea zelandica (Quoy and Gaimard, 1835), were investigated in specimens collected from Kennedy Bay, Coromandel Peninsula, and Shelly Bay, Wellington, from June 1999 to March 2001. Analyses of internal growth bands of polished shell sections were used to determine age at sexual maturity, and to construct von Bertalanffy growth curves for males and females in both populations. Patterns of sexual maturity were equivalent for both populations with 50% maturity calculated to be at ∼55 mm and ∼57 mm at Shelly Bay and Kennedy Bay, respectively. Length-frequency analysis of geoducks in Kennedy Bay indicated that females became more prevalent as the size increased, with significantly more females occurring in the largest size classes for both populations. However, sex ratios were equal with respect to age after about age 5 years. Similar patterns were observed in Shelly Bay, although the population was made up of larger, older individuals. Protandric development of P. zelandica for a significant proportion of individuals is considered likely for this species as all immature geoducks matured into males in their third year of life. Differences in growth rates could not be used as an explanation for the observed sex-ratio patterns as they were very similar for both sexes in each population. This paper suggests that functional protandric dioecy exists in P. zelandica. This finding may help to explain apparent sex ratio disparities reported in other geoduck species.