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Ligia oceanica, the sea slater, common sea slater or sea roach, is a littoral woodlouse. L. oceanica is oval, twice as long as broad and may reach up to 30 millimetres (1.2 in) in length, making it one of the largest oniscid isopods. 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 21⁄2–3 years and usually breed only once.
Under a rock at low-tide in an urban harbour.