CRUSTACEA > MALACOSTRACA > ISOPODA > Oniscidea > Diplocheta > Ligiidae

This distinctive, semi-terrestrial coastal species is the largest British woodlouse, achieving a length of 25-30mm when fully mature. It's typically a uniform dull grey colour, sometimes with greenish hues, but also variable to a degree as it has the ability to alter its colour to blend in with its environment. I've only encountered the species once, whilst on a coastal macro photography group trip to South Pembrokeshire, so I haven't got any comparison photos but, from those that I have seen, this individual does look paler and certainly more mottled than most. I guess that's to do with the almost white background within this particular rock 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 also 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 numerous 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.