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Work Package 5 - Crustacean Diseases

 Diseases of Crustaceans

  • Prof. Andrew Rowley (Swansea University)
  • Dr. Emma Wootton (Swansea University)

The crustacean work package examines two diseases of lobsters and crabs, namely bitter crab disease caused by the dinoflagellate parasite, Hematodinium, and shell disease syndrome.

 Bitter crab disease (Pink crab disease)

This disease is considered by some authorities to be an ‘emerging disease’ because of its apparent increase in host and geographical range, and prevalence over the last few decades (Morado et al. 2011). It is caused by the dinoflagellate parasite of the genus Hematodinium. A large number of crab and lobster species have been reported to be hosts for this parasite (Morado, 2011) and it has caused significant losses in crustaceans of economic significance including snow crabs (Chionoecetes bairdi), velvet swimmers (Necora puber) and Norway lobster (Nephrops norvegicus) in various locations in the northern hemisphere (Shields, 2011).

A great deal is known about the pathology of this disease in a variety of hosts. Hosts appear to be most vulnerable to infection immediately following moulting and although there is some experimental evidence that it can be transferred via cannibalism of infected animals (e.g. Walker et al., 2009) recent reports suggest that this is not of major importance at least in the blue crab, Callinectes sapidus (Li et al., 2011). Overall, our understanding of how this disease is transmitted in the wild is unclear.

Once inside its host, Hematodinium quickly replicates such that it invades a variety of tissues (see Figures 1 & 2). Animals with significant infections have ‘milky’ blood filled with the parasite and reduced numbers of blood cells. The host response to Hematodinium is largely absent (Stentiford, 2008) suggesting it either suppresses the immune system or more likely uses molecular mimicry to outwit its host’s defences.

Figure 1. Histological section of the gills from a Hematodinium infected edible crab. Note the characteristic nuclei of the parasites (arrows) that facilitate their easy identification. H&E stained.

   

Figure 2. Phase contrast micrograph of the blood from a Hematodinium infected edible crab. The Hematodinium are phase bright and variable shape (white arrows) while the blood cells (haemocytes) are of variable morphology but often phase dark (red arrows). Cells fixed in sea water formalin.