The Reproductive Biology, Population Genetics and Origin of Invasion of the Asian paddle crab, Charybdis japonica, in north-eastern New Zealand

Abstract

The introduction and spread of invasive species is a serious threat to global biodiversity. Our understanding of successful invaders and invasions depends on understanding the fundamental biology of these invasive species, which in many cases is severely lacking. This thesis describes aspects of the Asian Paddle crab, Charybdis japonica’s, reproductive biology and maternal investment and investigates its population genetic structure, spread and origin of invasion in New Zealand. Reproductive seasonality largely depends on female condition, since males contain spermatozoa yearround. Female gametogenesis begins during late autumn with oocyte development occurring throughout winter and early spring. Based on the appearance of gravid females in early November, mating begins during early spring with spawning first occurring in mid-spring as indicated by a drop in the gonad index. Based on morphological measurements and histological observation sexual maturity is achieved between 44 and 52 mm carapace width. Females produce at least two broods per year of upwards of 413,000 eggs. Female C. japonica invest similar amounts of maternal reserves to their eggs when compared to other brachyurans of a similar egg size, although lipid class percent composition showed slightly higher average quantities of phospholipids and proteins. Females invested an estimated 10.2 kcal per brood. Population genetics indicated a significant bottleneck on colonization. The molecular data (mtcr and ITS- 1) indicates widespread genetic homogeneity among the invasive New Zealand populations which according to F-statistics and mixed-stock analysis indicated the most genetic similarity with populations from Nagashima, Japan. There is also preliminary evidence of the occurrence of a separate introduction from Japan into Whangarei, New Zealand. While mitochondrial diversity was greatly reduced, comparatively higher levels of ITS-1 diversity were maintained despite the bottleneck, possibly due to the ability of sperm storage from multiple males and egg brooding. The successful invasion and establishment of C. japonica in New Zealand is likely due in part to the phenotypically plastic reproductive attributes including a long reproductive season, high fecundity, fast growth and early sexual maturity, in addition to higher propagule pressure to counter the founder effects afforded by multiple invasions, sperm storage from multiple males and egg brooding.