I have a childhood seaside memory of my first sea slater – a monster of a thing scuttling over a crumbling sea wall.
It resembled the woodlice found under the stones in my parents’ rockery, but this one was so big I could see its twinkling compound eyes with ease. Was it a freak of nature? Gigantism in a crustacean? Nope, this animal, at up to 3cm long (without the antennae!), was a sea slater (Ligia oceanica), the UK’s largest woodlouse.
Where do sea slaters live?
The sea slater doesn’t turn up in the average garden compost heap or under a woodland log, because it’s a specialist of rocky coastal locations. Still, it is relatively widespread in the UK, living under boulders, cliffs and rip-rap (human-placed rocks that prevent erosion). If you’re planning a trip to the beach, it’s well worth looking out for them.
What does a sea slater look like?
Comparisons with the woodlouse are obvious because, in effect, that’s what a sea slater is. However, the sea slater’s broad body is a changeable canvas – yellow, green, brown or grey, and often speckled with a variety of black freckles – helping it to blend into the craggy surfaces it hugs.
It can actively adjust the pigments in its cuticle thanks to cells called melanophores. This colour change is mainly a response to light levels, possibly detected by its large, well-developed eyes, but it can react to humidity and temperature, too.
As a group, isopod crustaceans (which include the familiar woodlouse and Ligia) have just left the sea, evolutionarily speaking at least, where most of their crustacean brethren evolved and can still be found.
Surprisingly, terrestrial isopods still rely on gills to breathe, which is why they are denizens of the dark and damp. A look underneath any woodlouse reveals that the last legless segments consist of a series of closely fitting, thin flaps called pleopods. The membranes of these flaps must be coated in a film of water – the definition of a gill – for oxygen to pass across from the air to the woodlouse’s blood.
Why can't sea slaters live on land?
The only significant difference – and the reason that sea slaters are not roaming the land – is that Ligia have a particularly permeable skin and gills that are less adapted to land than other terrestrial isopods, meaning they are restricted to the thin, humid line betwixt land and sea. This exposed lithic zone is a tough place for any life to get a grip. Regularly misted with salt spray or, worse, total inundated by waves, there is little in the way of plant growth. There is also the constant risk of dehydration from both sun and wind.
Due to these conditions, sea slaters are only seen during daylight hours if their hiding place has been disturbed. The best time to find them is on a dark, moonless night, although a bright torch beam will send them quickly scuttling for cover on their seven pairs of long legs. Just like their garden cousins, they are tempted out of their crevices at night by the lack of predators and higher humidity.
What do sea slaters eat?
Sea slaters are scavengers and part of the beach clean-up crew, chewing up anything from their favourite brown seaweeds to mosses, lichens and even dead animal material. We have tended to ignore Ligia in favour of its terrestrial cousins, which is a disservice, as not only is it larger and more eye-catching, but it is also, arguably, more interesting.
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