Ligia oceanica, the sea slater, common sea slater, or sea roach, is a species of isopod in the family Ligiidae. This isopod lives in the littoral zone and is abundant on the rocky shores of the North Sea, the western Baltic Sea, and the Atlantic coasts of Europe. This isopod is the largest species in the suborder Oniscidea, reaching 35 mm in length. Phylogenetic analysis based on molecular data, however, casts doubt on the placement of this species in that suborder, suggesting that the genus Ligia is more closely related to the marine isopods in the suborders Valvifera and Sphaeromatidea than to the terrestrial woodlice in the suborder Oniscidea.

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