Early stages of a New Zealand invasion by Charybdis japonica (A. Milne-Edwards, 1861) (Brachyura: Portunidae) from Asia: population demography

Charybdis japonica (A. Milne-Edwards, 1861) is an Asian swimming crab from the northern hemisphere whose native range includes China, Japan, Russia, and Malaysia but has recently been introduced to the eastern coast of northern New Zealand. We have undertaken the first study of its population dynamics and reproductive biology in the southern hemisphere during the early stages of its invasion and colonization. Trappings indicate that the Weiti Estuary population (northeastern New Zealand) represents an established population due to the presence of ovigerous females and size distributions demonstrating continued recruitment. Ovigerous females (43.7-79 mm carapace width) hatch at least one brood of up to 415 000 eggs between the austral spring and autumn (November to April) with a peak in summer. Crabs that hatch early in the reproductive season (November, December) attain recruitment size in the following autumn and settle into estuaries. Individuals hatched later in the reproductive season continue to recruit into the population during the winter and early spring. In New Zealand, C. japonica seems to have indeterminate growth as evident by multiple post-maturity instars. When comparing native and introduced populations, it appears that C. japonica does not gain any reproductive advantage in its new environment, e.g., larger brood sizes, increased number of broods, smaller size at maturity. In fact, in Russia, C. japonica females exhibit larger average brood size (1.3×), larger overall female size, and increased fecundity with female size when compared with New Zealand populations. Charybdis japonica has life history characteristics that positively influence invasion success, i.e., early maturation, relatively long life at large body sizes, high fecundity, and long spawning seasons. As New Zealand offers further habitats within its environmental envelope, C. japonica has the potential to be a formidable invader within New Zealand and provide a stepping-stone to other temperate environments via transfer in ship ballast water.