A beautiful crab shell

September 24th 2018

Crab shells are so commonplace on shorelines that we may not give them a second look.  They are often empty moulted shells, lightweight and not smelly, unlike dead whole crabs.  The underparts and legs frequently become detached, leaving just the fragile carapaces.  Commonest are the shells of the shore crab Carcinus maenas, less so the oval brown ‘piecrust’ ones of the edible crab Cancer pagurus.  Just occasionally, the triangular encrusted shells of the common spider crab Hyas araneusturn turn up.

On 2ndAugust 2018, I took my nephew and his family to hunt along the shoreline at Clashnessie (NC0531).  Amongst our finds was a small crab carapace, about 30mm wide, which wasn’t any of the above.  It resembled that of the shore crab in shape, but had a very different pattern, an elegant symmetrical set of reddish-brown markings on a pale background.

I could not remember seeing one like it, so back at home reference books were consulted.  The illustrations in James Merryweather’s User-friendly Seashore Guide (2014) suggested one of the swimming crabs.  Further progress required the use of the Handbook of the Marine Fauna of North-West Europe (Hayward and Ryland eds., 2012) a massive tome, at 800 pages, but with excellent diagnostic keys and line drawings.

Deciding characters were the smooth (not wrinkled) carapace, and three rounded teeth of equal length in the middle of the front edge. These led us to the marbled swimming crab Liocarcinus marmoreus.  Marbled is a good description of the pattern, at least to anyone familiar with the intricate patterns on the end-papers of old books.

This crab is said to occur on ‘fine sand and gravel, from LWST [low water spring tides] to 200m’ and to be ‘common’.  However, local beachcombers of my acquaintance have not previously come across it and the NBN Atlas maps just three records from the north-west, none of them on the coasts of Assynt.   Having said which, I found further specimens on the beach at Traigh Allt Chailgeag, east of Durness (NC4465), just three days later.  Either there was some seasonality to the finds, or I had got my eye in, as is often the case.

Have a look for yourself and let us know what you come across.

Ian M. Evans

 

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