Monday 31 August 2015

Crabs, parasites and other wonderful afflictions

So it's been a while since my last blog post... I know! Since finishing my PhD in January it's been a hectic 6 months. I have been busy writing up some bits and bobs from my thesis which weren't quite published.. and you will all be excited to hear that my research has moved a little towards the crabby side... (groan!).

For one of my PhD chapters, I looked at a disease called Hematodinium. Well, more of a parasite than a disease, this dinoflagellate infects over 40 species of decapod crustaceans worldwide. But not lobsters of the clawed kind, apparently.... I set out to test this theory.

So a bit of background. What is a parasite? According to the dictionary; "noun an organism which lives in or on another organism (its host) and benefits by deriving nutrients at the other's expense." There are different types of parasites, endo (those that live within an organism) and ecto (those which live outside of one). An example of an ectoparasite, is the 'lobster louse'; endoparasitic copepod Nicothoe astaci, another critter I have worked extensively on and may have mentioned in the past. In my lab, we have worked on it's histological morphology, revealing the point of attachment to the lobster, surface morphology revealing the attachment mechanism and the effects of the parasite upon the host.

Anyway, back to the parasite at hand. As an endoparasite, Hematodinium live inside the host, specifically in the haemolymph (blood)... pretty grim I know. A couple of French scientists Chatton and Poisson first reported the disease in France in both harbour Liocarcinus depurator and shore crabs Carcinus maenas in the 1930s. It has since been found to infect over 40 species of decapod crustaceans worldwide, and because infected animals become unmarketable due to poor muscle quality, Hematodinium spp. infections have had huge economic impacts on commercial fisheries. For example, in France, the velvet swimming crab Necora puber fishery suffered a catastrophic collapse (>96 %) due to Hematodinium spp. in 1985. In the US, outbreaks of Hematodinium spp. have infected up to a third of the Tanner crab Chionoecetes bairdi and snow crab Chionoecetes opilio stocks in southeast Alaska and Newfoundland respectively and in Virginia, loss to the blue crab Callinectes sapidus fishery is estimated to be between 0.5 and 1 million USD per year. In the UK, the Scottish Nephrops fishery also loses approximately £2-4 GBP million annually due to Hematodinium spp. infection.

There are only two species of Hematodinium that have been described so far. This is due to their lack of distinct characteristics and poorly understood life cycles. The type species, Hematodinium perezi, was first described from the crabs on the Normandy and Mediterranean coast of France by our friends Chatton & Poisson in 1931H. perezi, or a closely related species, has since been reported in epidemics from edible/brown crabs Cancer pagurus and velvet swimming crabs off Brittany, France, and from the English Channel. A second species, H. australis, was described from Australia and was separated from H. perezi on the basis of size of the vegetative stage (called a trophont), the presence of rounded plasmodial stages and the austral location.

My experiment, in a nutshell

So, why do I want to see if my beloved European lobsters are susceptible to infection? Judging by the above effects upon fisheries worldwide, it's an important critter to keep an eye on, and since it infects our native Cancer pagurus (edible, or brown crab), for me, that's a little too close for comfort! Edible crabs share habitats with European lobsters and are often found together in parlour pots (fishing traps) - often injured from some aggressive run ins. We know from my past research that injury can lead to disease and although Hematodinium infections have been found more in juvenile crabs, it is still an important issue. We don't know where the parasite resides before it enters the host, and so it is interesting to investigate the susceptibility of different species in order to further understand the infectivity.

In order to do this, we did two experiments, or 'exposure studies'.  First, we collected some edible crabs from the South Wales coast, from spots known to harbour Hematodinium infected crabs in the past, and inspected the blood for the parasite. Just to be sure, we kept them for a few weeks, checking every week for infective stage parasites. Once we were happy we had some crabs sufficiently 'infested' enough, we took live samples of Hematodinium by drawing the blood (haemolymph) and separating out the parasites into a clean saline solution. This solution was to be injected into our disease-free, juvenile European lobsters.

We first did a preliminary, or pilot, study, which was run side-by-side with a similar study artificially infecting edible crabs Cancer pagurus (just to be sure that the Hematodinium species we were injecting was viable). In the pilot, the crabs injected became infected after a matter of weeks, but the lobsters did not... However, the number of lobsters we used was small and we wanted to run a longer study with more sampling points, so we decided to try again. On the second attempt we took blood samples from the experimental (and control!) lobsters before injection, just after, 24h after, 1 week and then every month thereafter. The results were as expected... all negative (even the 24h post injection one!). To look for the parasites, we used microscopy (blood smears), polymerase chain reaction (PCR) with primers specific for Hematodinium spp. (yes, that's species, just in case!) and also histology from the final time point.

What is it that a lobster has and a crab doesn't? There have been some pretty cool molecular studies of late at a collaborators lab in Canada, looking at gene expression (i.e. what genes are expressed in disease animals vs. those which aren't diseased...) I think it would be really interesting to find out exactly what it is in the lobster immune response which renders it unable to maintain this infection.

Although we weren't surprised at our results, it is still an interesting study. It does seem that the EU lobster has something that other decapods don't. Another example is my earlier disease work where we looked at transmission of epizootic shell disease (ESD) from American lobsters into European ones... to no avail. It seems EU lobsters are the strong men of the lobster kingdom?

To read the full study, see the citation below (if you click the DOI, it will take you to a download page). If you can't access the papers, feel free to comment or email me and I can send you a copy.

Davies, C.E. and Rowley, A.F. (2015) Are European lobsters (Homarus gammarus) susceptible to infection by a temperate Hematodinium sp.?. Journal of Invertebrate Pathology 127, 6-10 doi: 10.1016/j.jip.2015.02.004

For further reading, my supervisor recently wrote a mini review on this interesting parasite...

Rowley, A.F., Smith, A.L. and Davies, C.E. (2015) How does the dinoflagellate parasite, Hematodinium outsmart the immune system of its crustacean hosts? PLOS Pathogens 11 (5), e1004724 doi: 10.1371/journal.ppat.1004724

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