Ligia oceanica, the Sea Slater is a marine crustacean which is closely related to woodlice.
L. oceanica is oval, twice as long as broad and may reach up to 30 millimetres (1.2 in) in length, making it the largest oniscid isopod. Its colour may vary from grey to olive-green, and it has large compound eyes and long antennae, ⅔ as long as its body.
L. oceanica is found in temperate waters from Norway to the Mediterranean Sea, and from Cape Cod north to Maine. It is a common species, occurring wherever the substrate of the littoral zone is rocky, and is especially common in crevices and rock pools and under stones. It is a nocturnal omnivore, eating many kinds of seaweed, diatoms and debris, with a particular fondness for bladder wrack (Fucus vesiculosus).
L. oceanica individuals live for 2½–3 years and usually breed only once.