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 2 1⁄2–3 years and usually breed only once.
- Helmut Schmalfuss (2003). "World catalog of terrestrial isopods (Isopoda: Oniscidea) — revised and updated version" (PDF). Stuttgarter Beiträge zur Naturkunde, Serie A. 654: 341 pp.
- "The Care of Woodlice (Crustacea, Isopoda, Oniscidae)". November 11, 2005.
- Susie Ballerstedt (2005). "Common sea slater - Ligia oceanica". Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme. Retrieved October 27, 2015.
- Juan Luis Menéndez (July 3, 2005). "Ligia oceanica (Linnaeus, 1767)" (in Spanish). Naturaleza Cantábrica.
- Richard Fox (2001). "Invertebrate zoology laboratory exercises". Lander University.
- "Sea slater (Ligia oceanica)". ARKive.org. Retrieved December 9, 2006.
- Data related to Ligia oceanica at Wikispecies