Biological and ecological attributes of a population of the invasive Asian paddle crab, Charybdis japonica, in northeastern New Zealand

Abstract

This thesis describes biological and ecological characteristics of the invasive Asian paddle crab, Charybdis japonica and is the first such study of the species in its invaded habitat. Based on the first appearance of gravid females in November, mating begins at the end of winter and continues throughout the summer. After attaining a reproductive size of between 40 and 50mm carapace width after one year, females hatch at least one brood of up to 400,000 eggs between austral spring and autumn (November to April) with a peak in austral summer. Young crabs that hatch early in the reproductive season attain recruitment size in autumn and settle into estuaries. Comparisons of zoeal survival rates to historical sea surface temperatures and salinities show that C. japonica Stage 1 zoeae tolerate broad environmental ranges and could survive natural conditions and continue to spread throughout New Zealand. Throughout its life, C. japonica is an opportunistic omnivore with a preference for sessile and slow moving macroinvertebrate prey items, based on diet analysis. Starved C. japonica preferentially prey upon smaller bivalves and can consume up to 42 juveniles of recreationally and ecologically important bivalve species in 24h. In one-on-one competition for food, male C. japonica behaviourally dominated both male and female native New Zealand paddle crabs, which were not successful in defending or acquiring the prey items from C. japonica. Surprisingly, despite its predatory and competitive abilities, field experiments and comparisons with historical surveys did not detect an impact of C. japonica on native fauna. With a long larval life that facilitates dispersal, rapid growth to maturity, high fecundity, a broad voracious diet, high competitive ability, and broad environmental tolerances, it appears that C. japonica has biological and ecological attributes that have aided their integration into a novel New Zealand habitat.