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   Charybdis japonica (crustacean)
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    Taxonomic name: Charybdis japonica (A. Milne-Edwards, 1861)
    Common names: Asian crab, Asian paddle crab, blue crab, paddle crab, swimming crab
    Organism type: crustacean
    The Asian paddle crab Charybdis japonica is a portunid (swimming) crab native to marine environments of Central and South East Asia. It may impact native esturine communities by competing for space and resources with native crabs. As it transmits disease and preys on native shellfish it is a potential threat to fisheries and traditional shell-fishing.
    Charybdis japonica have a carapace width of up to 12cm (Gust et al. 2003). They have a pilose (hairy) carapace (although amount of hair varies to little or none). The carapace has ridges with six frontal teeth, triangular and sharp. The inner supraorbital lobe is broadly triangular (Smith et al. 2003). Wee and Ng (1995) record the colour of C. japonica in Japan as mottled cream and purple. In the Waitemata harbour (New Zealand) specimens varied from pale green and off-white, through olive green to a deep chestnut with purplish markings on the carapace and upper surfaces of the appendages (Smith et al. 2003). In addition, most Waitemata specimens have yellow-orange markings, some with only a hint of yellow-orange and some with very noticeable brown-orange on parts of the carapace and the legs, especially on the chelae where the upper colouration grades into the white to off-white ventral surfaces (Smith et al. 2003).
    Occurs in:
    estuarine habitats, marine habitats
    Habitat description
    In Korea, juvenile C. japonica (<25mm carapace width W) are abundant in eelgrass (Zostera marina) meadows (Huh and An 1998, in Smith et al. 2003), but there are few other accounts of their distribution and habits in wild populations. In Auckland (New Zealand) it is found in estuarine habitats comprised of ‘firm sand-muddy fine sand flats’ or ‘muddy-shelly fine sand’ habitats (Gust and Inglis 2006).
    General impacts
    Disease transmission is one of the key potential impacts of the paddle crab in introduced environments. C. japonica is known to be a host or carrier of the White Spot Syndrome Virus (WSSV) (Maeda et al. 1998, in Potential next pests 2003). WSSV is a serious fisheries threat as it infects a broad spectrum of crustaceans, and can cause cumulative mortalities of up to 100% within 3 to 10 days from the first sign of disease (Lightner 1996, in Potential next pests 2003).
    This crab is found and exploited in Asia where it is trapped using pots and gill nets (Archdale et al. 2006).
    "Charybdis" is Greek for whirlpool.
    Geographical range
    Native range: The native range of Charybdis japonica includes coastal regions of China, Japan, Korea, Taiwan and Malaysia (Smith et al. 2003).
    Known Introduced range: New Zealand (Gust and Inglis 2006).
    Introduction pathways to new locations
    Seafreight (container/bulk): Ships’ sea chests are another likely mechanism of spread for this species. A preliminary investigation carried out on the sea chests of a group of large fishing vessels, small freighters and small tankers operating around New Zealand and in adjacent areas of the South Pacific revealed that crabs are an importent component of the suite of organisms found alive in sea chests (Tim Dodgshun., pers.comm., April 2007).
    Ship ballast water: The paddle crab may have been introduced from ship ballast water (Gust et al. 2003). This is known to be a potential route of spread of the Asian paddle crab.
    Management information
    Preventative measures: A two year study was undertaken for the Department of Environment and Heritage (Australia) by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) to identify and rank introduced marine species found within Australian waters and those not found within Australian waters, categorised C. japonica as one of ten most likely invaders using the most influential measure of environmental similarity and one of ten potentially most damaging species. C. japonica is catergorised as a 'Medium High Priority' species based on its invasion potential/impact. These rankings will be used by the National Introduced Marine Pest Coordinating Group in Australia to assist in the development of national control plans. See Hayes et al. 2005 for full details.

    Physical: For eradication and exploitation purposes Archdale et al. (2006) examined the performance of two pots commonly used in Japan to catch the crab : a box-shaped pot with slit entrances and a dome-shaped pot with open funnel entrances. The latter was found to be 100% efficient while the box-shaped box was only 66% efficient.
    Swimming crabs are known to be highly sensitive to saccharides, in response to which they show strong food searching behaviour (Kawamura et al. 1995). Kawamura and collegues conducted a study to examine the possibility of using sugarcane as an attractant in basket traps for Charybdis japonica during their reproductive season and found that a bait combination of sugarcane and fish was found more effective than fish bait alone, whereas sugarcane alone was ineffective.

    This large crab is known to be an opportunistic predator of bivalves, fish, cephalopods and other benthic invertebrates (Jiang et al. 1998, in Smith et al. 2003).
    Wang et al. (1996) report a bimodal reproductive season in China, with spawning in spring and autumn when sea temperatures are between 20°C and 28°C. Females lay an average of c. 85 000 eggs per brood (Wang et al. 1996, Smith et al. 2003) and may produce multiple broods in a single year.
    Reviewed by: Dr. Nick Gust, Marine Ecologist, National Institute of Water and Atmospheric Research Ltd Christchurch, New Zealand
    Compiled by: IUCN/SSC Invasive Species Specialist Group (ISSG) with support from La Fondation d'entreprise Total
    Last Modified: Monday, 14 May 2007

ISSG Landcare Research NBII IUCN University of Auckland