Multicoloured Harbour Crabs

When I was teaching at Millport a couple of weeks ago, one of the exercises we did involved conducting a short trawl in the Clyde so the students could see the technique and then get some experience in identifying and analysing the species present. What was interesting about these catches though is that one of them contained a rather unusual harbour crab (Liocarcinus depurator).

Trawling for animals in the Clyde Sea Area, from the Millport Marine Biological Station, Cumbrae.
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Harbour crabs are very common around the coasts of the UK, and are usually a fairly light orange-brown colour with purple patches on their swimmerets, a bit like this one:

A harbour crab (Liocarcinus depurator) showing the "normal" colour pattern.
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Unusually, the one we trawled up two weeks ago had a much darker body and completely white legs: 

An unusually-coloured harbour crab, caught by trawl in the Clyde Sea Area, just opposite the Millport Marine Biological Station, Cumbrae. I unfortunately lost my photos of the specimen, but Martina has kindly allowed me to use hers instead!

We keyed this guy out, and it appears to be Liocarcinus depurator, but a different colour morph. The REALLY cool thing though, is that this isn't the first time I've seen a crab like this. In 2010, a similarly-coloured crab was trawled up from the Clyde and brought to Glasgow University. I happened to have my camera with me, so was asked to photograph the specimen because it was cool and unusual, but I don't think we ever really followed it up. Anyway, the images were still sitting in my archives, and it turns out that they are rather similar (AND showed off its pink belly!): 

A similarly-strange harbour crab (Liocarcinus depurator) caught in the Clyde Sea Area in 2010.
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I've had a bit of a look online for any reports of different colour-morphs in the Harbour Crab, but I haven't been able to find any previous reports of albinism or partial albinism in this species. A quick search pulls up plenty of examples of temperate crustaceans that have well-known colour variants, such as the red and green colour morphs of the common Shore Crab (Carcinus maenas), the Pacific shore crab (Hemigrapsus oregonensis), juveniles of the red rock crab (Cancer productus) and American (Homarus americanus) and Atlantic lobsters (Homarus gammarus) to name just a few.

Colouration patterns in crustaceans are driven largely by genetics (though environmental drivers can also be important in changing the brightness or colouration over shorter time-scales). It may be then, that the unusual harbour crabs we have caught from the Clyde have some mutation that gives rise to the different patterns. It would be interesting to know whether there are more crabs like this elsewhere in the UK, and what causes the change (unfortunately there's only so much you can tell from a photograph!). So, if you ever spot a harbour crab looking a bit like these ones, please let me know!