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Introduced shore crabs escape from parasites

European shore crab, Carcinus maenas

The European shore crab, Carcinus maenas, in a sheltered habitat on the coast of the Cape Peninsula, South Africa [Photo: Charles Griffiths].

The European shore crab Carcinus maenas is one of the most successful of all marine invasive species and has established populations on both Atlantic and Pacific coasts of North America, Australia, Argentina, Japan and South Africa. Despite centuries of trade between Europe and South Africa the species was first recorded in Table Bay Docks only in 1983 and remains restricted to sheltered habitats on the Cape Peninsula – mainly in Cape Town and Hout Bay harbours, with small outlying populations in sheltered embayments on the adjacent open coast. Experimental studies by Hampton and Griffiths (2007; Afr. J. Mar. Sci. 9: 123-126) have shown that the species has a poor ability to grip the substratum in the face of strong wave action and is thus unlikely to become established on South Africa’s open wave-swept coastline, although there is a strong risk that it will soon reach Saldanha Bay, where it is likely to have devastating impacts on aquaculture industry there (Griffiths et al. 2011; In: Galil, B., Clark, P.E. & Carlton, J.T. eds In the Wrong Place: Alien Marine Crustacea – Distribution, Biology and Impacts, pp. 269-282, Springer, Berlin).

Another reason to its invasion success has been in part attributed to the presence and prevalence of parasites and its impact on the fitness of male crabs. A recent paper co-authored by C·I·B core team member, Charles Griffiths, compared the parasite loads of male crabs within the indigenous range with those introduced populations from South Africa and Australia (Zetlmeisl et al. 2011). South African crabs were completely devoid of any European parasites, while those in Australia had low parasite loads (under 5%) and were infected only by local rather than introduced parasites (the main reason for this is probably that the invaded sites lack intermediate hosts necessary for the parasites to complete their life cycles).

The paper also used testes weight of the sampled crabs as an indicator for reproductive fitness and a possible resource trade-off in response to parasite infestation. Unexpectedly the parasite-free populations do not appear to show ‘parasite release’ at least when measured as enhanced gonad weight. However, South African populations do appear to grow to an exceptional carapace width, and might exhibit enhanced growth rates relative to the European relatives.

Overall, the results did not hint at a relevant effect of helminth parasites on the reproduction fitness of the European shore crab, either at population level, or for individual crabs. Parasite prevalence was found to be highly diverse between different locations. Local growth rates have not been measured to date and are a promising topic for later study.

Read the paper:

Zetlmeisl C , J. Hermann , T. Petney , H. Glenner , C. Griffiths, H. Taraschewski (2011) Parasites in the shore crab Carcinus maenas (L.): implications for reproductive potential and invasion success. Parasitology 138: 394-401

For further details, e-mail Prof. Charles Griffiths