Charybdis japonica (lady crab)

C. japonica is a large portunid crab native to coastal marine environments of the western Pacific. Introduced to New Zealand, it was first discovered in Auckland’s Waitemata Harbour in 2000 (Webber, 2001). By April 2002 the invader had spread widely throughout the harbour (Gust and Inglis, 2006). It appears to have become established there and was still found to be widespread in the harbour in April 2008 (Willis and Morrisey, 2008). C. japonica may impact native estuarine faunal assemblages as an opportunistic predator of benthic invertebrates, particularly small, native shellfish. In its native range it is a host or carrier of the white spot syndrome virus, a serious fisheries threat to a broad spectrum of crustaceans (Lightner, 1996; Maeda et al., 1998). Listed as one of ten most likely invaders into Australian marine waters C. japonica is categorized as a 'medium high priority' species based on its invasion potential/impact (Hayes et al., 2005).

C. japonica is a large (up to 110 mm carapace width) portunid (swimming) crab, with a typical portunid body-form. The carapace and limbs are pilose (hairy), though the extent of cover varies considerably within populations. In some New Zealand individuals it is confined to recesses in the carapace, between anterolateral spines on the carapace, and in recesses on the chelae. Maximum size (carapace width) in Korean population is 109 mm (males) and 96 mm (females). Corresponding values for New Zealand population are 110 mm and 90 mm (Miller et al., 2006). Detailed descriptions of external morphology are given by Wee and Ng (1995) and Smith et al., (2003). The colour of the New Zealand population is very variable, ranging from pale green and off-white, thorough olive green to deep chestnut with purplish markings on the carapace and upper surfaces of the appendages (Smith et al., 2003). Most specimens from New Zealand also have yellow-orange markings, particularly on the chelae. There seem to be few published descriptions of colour in individuals from its native range, but Wee and Ng (1995) describe the colour of crabs from Japan as “dorsal surface mottled cream and purple”.

First recorded in September 2000 in the Rangitoto Channel near the entrance to Waitemata Harbour, Auckland, New Zealand and the following (austral) summer inside the harbour. It has since been recorded during all trapping programmes in the harbour (in 2002, 2003, 2004, 2006 and 2008). It has also been found on several occasions in two other, nearby harbours approximately 36 km to the north (Mahurangi Harbour) and approximately 10 km southeast (Tamaki Estuary) of Waitemata Harbour (Gust and Inglis, 2006; M Morrison, NIWA, New Zealand, personal communication, 2008). In September 2003 a single specimen (a mature female) was collected in Whangarei Harbour, on the northeast coast of the North Island, but none has been captured since in this location despite regular trapping programmes (in 2004, 2006 and 2008).

Given the volume of shipping between ports in the species’ native range and those in Australia and New Zealand, further introductions are not unlikely. Spread within its introduced range is very likely by natural dispersion of adults and larvae (gravid females have been caught in New Zealand) or by shipping-related vectors (ballast water or hull fouling (Smith et al., 2003)). Since the species is eaten by humans in its native range, and is the target of a commercial fishery, deliberate introduction or translocation is possible (Smith et al., 2003; Vazquez Archdale and Kuwahara, 2005).

Very similar to other portunids in general and to other members of the same genus in particular (Webber, 2001; Smith et al., 2003).

In its native range it occurs in intertidal and subtidal habitats to depths of about 15 m, including sandy and muddy or stony bottoms with seaweed (Smith et al., 2003; Vazquez Archdale and Kuwahara, 2005). In Korea, juveniles (less than 25 mm carapace width) occur in seagrass (Zostera marina) beds (Hu and An, 1998, cited in Smith et al., 2003). In the Waitemata Harbour, New Zealand, it occurs in a range of substrata from fine, silty muds to coarse, shelly sands, sandstone reefs, and beds of the introduced bivalve Musculista senhousia, and in water depths from 1-13 m (Gust and Inglis, 2006).

Genetics

See Smith et al. (2003) for a study on the DNA and morphological identification of C. japonica in New Zealand waters.

Reproductive Biology 

In Japan, C. japonica releases larvae in late summer (Oishi and Saigusa, 1997). Wang et al. (1996), cited in Smith et al. (2003), described a bimodal reproductive season in China, with spawning in spring and autumn when sea temperatures are between 20 and 28°C. Females may produce multiple broods each year. Some species of Charybdis are able to store sperm and produce several broods from a single mating (Dineen et al., 2001).  

In New Zealand the similarly-sized native portunid Ovalipes catharus is preyed on by various species of fish (McLay, 1988) and it is likely that the same species would also consume C. japonica, though the latter is reputed to be more aggressive than the native species (Webber, 2001). In Korea, C. japonica is host to the parasitic rhizocephalan barnacle Heterosaccus papillosus but reported infection rates were low (3.8% of females and 1% of males) (Kim, 2001) and it is not known what effect they may have on the population dynamics of their host. This parasite was not recorded during a study of parasites of C. japonica in New Zealand, even though the sample size had a high estimated probability (79%) of detecting at least one individual infected with a rhizocephalan if it was present in the New Zealand population (Miller et al., 2006). Very few other parasites were found in the sample of C. japonica or in sympatric and allopatric populations of the native Ovalipes catharus. An unidentified juvenile ascaridoid nematode was found in the hindgut of 5-9% of the C. japonica examined. Melanised lesions were noted in the muscle tissue of 46.6% of the C. japonica examined, some of which were spherical bodies resembling melanised trematode metacercariae. Low levels of infection with parasites are consistent with arrival of the species in New Zealand as larvae via ships’ ballast water.

Possible adverse effects on populations of commercially important, estuarine bivalves in introduced range (Gust and Inglis, 2006). Potential distribution of C. japonica in New Zealand may not overlap substantially with that of the only commercially important native portunid crab (Ovalipes catharus), reducing the likelihood of direct competition between them (Gust and Inglis, 2006). C. japonica is also a known host of the white-spot syndrome virus, a serious fisheries threat to a broad range of crustaceans (Lightner, 1996; Maeda et al., 1998), though the New Zealand population has not so far been found to carry it.

See comments in section on Economic Impact for potential effects of predation on native bivalve populations on disease on native crustaceans.

Lack of basic information on biology and ecology, including environmental tolerances and requirements, reproductive biology, natural and human-related dispersal, and ecological interactions (predation and competition) in native and introduced range.