Crabs, camouflage, and the eye of the beholder

A juvenile shore crab (Carcinus maenas) explores my palm.
A juvenile shore crab (Carcinus maenas) explores my palm.

(Originally published March 21, 2014)

Along coastlines throughout the UK, the shore crab (Carcinus maenas) is a common sight, either scrambling for cover in a rock-pool, peering out of a bucket, or pugnaciously clinging onto a baited line held by a giggling child. Yet, despite their abundance, they still provide a fascinating insight into evolution, with their camouflage being a classic example of selection and adaptation.

As the days start to lengthen and spring bounds towards us, I’m beginning my MSc project, working with the Sensory Ecology group of the University of Exeter, and supervised by Dr Martin Stevens. The study will add to the growing body of knowledge on camouflage in real species in their natural environment, with shore crabs providing an excellent opportunity to do so. An important addition to the study is the modeling of bird vision to analyse the camouflage; crabs are a popular prey choice, and many bird species will scour the rock-pools to feast on them. Bird vision differs from humans, however, as they can see ultraviolet light. We’d like to know how the crab’s camouflage looks to its common predators. By modelling bird vision and using a UV filter with digital photography we aim to find out!

Juvenile shore crabs in particular exhibit a wide array of striking colours and markings, especially when compared to the adults of the species, which develop murky, cryptic greens and browns as they mature. By studying shore crabs across different coastal habitats, including rock-pools, mud-flats, and mussel-beds, we hope to discover if different colours and markings of the crabs are closely matched to the environment in which they are found.

A few early periods of scouting and methodology practice has already turned up an amazing variation in colour morphs, with the two images above brilliant examples. Over the course of the study I expect to uncover a few hundred more crabs from their hiding places, and in doing so I’ll hopefully be able to add to knowledge on the evolutionary dynamics of camouflage in the natural world. In addition though, I obviously won’t be forgetting to enjoy the simple pleasure of sploshing around in rock-pools again, turning over rocks and wowing at wildlife like a curious child!

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2 thoughts on “Crabs, camouflage, and the eye of the beholder

    1. Brilliant, thanks! Great markings there. We turned up so much variety in colour and marking morphs it was fascinating. Definite recurring themes, but then you’ll turn up a pattern and colour combination you’ve not seen before and be amazed all over again. I’m sure you’ve seen a huge amount of interesting morphs in your Cornish rockpool adventures.

      Keep an eye out for the follow up to this piece. We finished the project last month, so just in the process of tweaking a blog about it (and getting other photo blogs transferred from my old site!)

      Liked by 1 person

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