Ligia oceanica

CRUSTACEA > MALACOSTRACA > ISOPODA > Oniscidea > Diplocheta > Ligiidae

This distinctive, semi-terrestrial coastal species is Britain's largest woodlouse with fully mature adults attaining a length of 25-30mm. They're typically a uniform, dull-grey colour, sometimes with greenish or brownish hues, but also variable to some degree as they have the ability to alter their colour in order to blend in with the environment. The individual featured below that was photographed in a shallow rockpool where the bottom was covered in bleached encrusted white algae is a good example as the specimen's pale mottled appearance made it quite difficult to spot, especially when there was wind ripple on the surface; all in all providing pretty good camouflage given the starkness of the pool.

The habitat alone makes Ligia oceanica almost impossible to mistake, although in the field immatures could possibly be confused with the uncommon and nationally scarce Halophiloscia couchii, which is associated with unvegetated rocky cliffs or stone beaches, but not rock pools. Both species have a tendency to hide away in rock crevices during the day, emerging at night to feed, and both can run very fast if disturbed.

Ligia oceanica has large eyes with numerous ocelli and antennal flagella composed of multiple bead-like segments as can be clearly seen in these photos. Halophiloscia couchii on the other hand is in the Crinocheta section having three flagella segments as well as a stepped body outline, so although it's somewhat reminiscent of an immature Ligia oceanica they can easily be identified from photos.

The species is regarded as common around the entire coastline of the country wherever there are rocky cliffs, or hard beaches with rock pools, boulders or shingle. They may also be found on harbour walls, jetties or other hard surfaces in close proximity to the sea. Halophiloscia couchii has a far more restricted range with most records being from around southwest and southern coastlines.

I've only had the opportunity to photograph Ligia oceanica on a couple of occasions; the first during a coastal macro photography workshop in South Pembrokeshire when I spotted an individual in a rockpool as noted above, and the other at Chesil Beach where I found a few hiding under stones just above the high-tide line.

Updated, Sept.26 (v.2)