In July 2015, while torching some rock pools at night east of Lizard Point, I noticed a crab that was unfamiliar, but also familiar. I secured a record shot with the help of Luke Marsh, which was essential for the next steps, and released the crab.

As I started to research the identity its familiarity dawned on me as a rock crab species, a family that I, and anybody who has visited the area, will be familiar with as being found on the Atlantic Islands.  The Red Rock Crab is a large and obvious species there, along with the smaller Marbled Rock Crab. I checked over the features and sent in the record for verification.

The Marbled Rock Crab Pachygrapsus marmoratus is usually a Mediterranean and warm Atlantic Island species, but there has been a handful of records over the last 10 years in the Teign Estuary, Southampton Water, Fal Estuary and South Wales.  The species has been considered a non-native / introduced species but, with increasing numbers up the Atlantic coastline, and even just to the south on the French Coast (2012–2014 sampling, 22 individuals of P. marmoratus at Saint-Germain-sur-Ay), it appears that, like the Little Egret, it could well be another colonising species. If so, this is no doubt linked to global warming / climate change.

The species has a shell (carapace) length of up to 3.6 cm. It is brown to almost black in colour with a marbled pattern of yellowish brown. The carapace is almost square, which is a useful identification feature, and is slightly broader than long, with distinct diagonal ridges visible. The front edge between the eyes is particularly straight and wide with three pointed teeth on each side, which is another useful identification feature. The chelipeds (pincers) are large and slightly unequal in size.

The Marbled Rock Crab occurs on rocks and reefs in the upper to middle shore, in the crevices of breakwaters, pier piles or similar habitats. It is semi-terrestrial, leaving the water to pick over algae and weed surfaces. It even appears to sunbathe although, when warm and disturbed, they are very agile and dive into the water and slide between rock cracks and crevices.   On The Lizard, it was found in a very much upper shore pool with dense fine green algae mat growth.

I have found limited information on the species in the UK and, given its status, it could become out of date very quickly. I also think that this is a species that is being overlooked and could very easily have some local populations. Keep a look out and do send in your records.

Published: January 2016
Author: Tony Blunden, Consultant Ecologist

References

Edwards, R.V., 2005. Pachygrapsus marmoratus a marbled rock crab. In Tyler-Walters, H. and Hiscock, K. (eds.), Marine Life Information Network: Biology and Sensitivity Key Information Reviews [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from http://www.marlin.ac.uk/species/detail/1978.

Ingle, R.W. and Clark, P.F., 2007. First reported occurrences of the marbled crab, Pachygrapsus marmoratus (Crustacea: Brachyura: Grapsoidea) in southern coastal waters of the British Isles. Marine Biodiversity Records, 12/2007; 1. DOI: 10.1017/S1755267206002454.

Pezy, J.-P. and Dauvin, J.-C., 2015. First record of the marbled crab Pachygrapsus marmoratus (Fabricius, 1787) on the coast of Calvados (Bay of Seine, English Channel). Cahiers de Biologie Marine, 56: 151–154.