A novel disease of the shore crab Carcinus maenas

For F. Happy Birthday!

Aquaculture is often seen as an important way to address the growing food needs of human populations, without the environmental costs of harvesting from wild populations (such as habitat destruction and overfishing). Unfortunately, ‘farming’ of marine species for food often has significant environmental impacts of it’s own, for example the destruction of large swaths of mangroves for the construction of shrimp farming ponds, and the organic enrichment of marine habitats by wastes from fish farms.

Aquaculture has other problems to contend with, aside from the potential environmental effects. Rearing of large numbers of individuals of a single species, in close quarters, inevitably raises the risk of disease – which can lead to significant economic costs: lost production, administration of antibiotics, treatment of infected animals, etc.

One species which is under consideration for aquaculture is the European shore crab, Carcinus maenas, which is used as bait by anglers and (apparently) as a food in continental Europe (those crazy continentals….. I can’t say that I find myself tempted to eat this particular species). However, there seem to be some problems with culturing this particular species – one of which is a previously unknown disease which is seen to infect a significant proportion of crabs under aquaculture conditions (present in less than 1% of wild individuals). Scientists at Swansea University (Eddy et al., 2007) investigated this previously undescribed disease, which they call ‘milky disease’ due to the fact that the haemolymph (crab blood, to the uninitiated) of infected crabs takes on a milky appearance, and to find out what the disease agent is, what effect it has on infected crabs, and how it is transmitted.

What effects does the disease have? Crabs with milky disease show no-external symptoms, the only obvious sign being the milky appearance of the haemolymph. Infected crabs have higher levels of ammonia and glucose in the blood, but reduced numbers of circulating blood cells and protein. Certain cells of the hepatopancreas and gills of infected crabs appear to be physically affected by the disease, but other tissues appear normal. Nonetheless, the infection is lethal; usually within seven days of when the crabs are first observed to have milky blood.

What’s causing the disease? Haemolymph samples from infected crabs appear to contain large numbers of tiny, bacteria-like particles which are not present in healthy crabs. Detailed electron microscopy work also shows the presence of these bacteria in hepatopancreas cells, as well as bacteria apparently escaping from ruptured cells.

The causative agent turns out to be, as suspected, a bacterium. While it obstinately refused to be cultured, DNA techniques show that the organism responsible for milky disease is a previously unknown member of the alpha-proteobacteria.

In situ hybridisation is a molecular technique which is used to identify the physical presence of a specific DNA sequence. It works by using short DNA sequences (called ‘primers’) which will only attach to the DNA sequence that you’re looking for. These primers are labelled with chemical compounds which allow (after further processing) you to actually see where the DNA is. In this case, a sequence unique to the milky disease bacteria was used, so that once developed, slides of crab tissues could be inspected to see exactly where in the organism’s body the bacteria were most concentrated. This technique confirmed the presence of the bacteria in the hepatopancreas, and also in some blood vessels and connective tissue, while showing that it was, indeed, absent from most other cells and tissues.

The researchers were not able to induce infections in healthy individuals, so the route of infection is, as yet, unknown. Crabs injected with haemolymph from infected animals do die within 24 hours (even if the injection is free from bacterial cells), despite the absence of typical milky disease symptoms. This suggests there might be some toxic effect killing the crabs, perhaps due to bacterial waste products. However, direct injection of infected haemolymph is not a natural situation, and crabs fed infected food did not show any sign of infection, so further work will be needed reveal the natural route of infection.

So, while there are obviously still questions, this study goes some way to helping us understand this ‘new’ disease, and it’s effect on one particular species. Presumably, work will continue.

While the disease is rare in the wild (less than 1% of crabs infected), under aquaculture conditions the disease is much more prevalent (up to about 25% of crabs infected) – though only during the warmer months. This has several implications. Firstly, it shows that even diseases which are not a serious problem for wild organisms are still a potential threat to aquaculture. Perhaps aquaculture operations could thus also act as reservoirs for disease agents, which could then spread into wild populations. Design of aquaculture operations should attempt to take this into account, for example by treating effluent and preventing escapes.

The fact that infection rates are highest in warmer months is also of concern. It means that climate change may have negative impacts on all aquaculture operations where water temperatures are rising, but also that in general rising water temperatures could lead to higher disease incidences even in wild populations.

Eddy, F., Powell, A., Gregory, S., Nunan, L.M., Lightner, D.V., Dyson, P.J., Rowley, A.F., Shields, R.J. (2007). A novel bacterial disease of the European shore crab, Carcinus maenas molecular pathology and epidemiology. Microbiology, 153(9), 2839-2849. DOI: 10.1099/mic.0.2007/008391-0