Biodiversity & Conservation

Celtic sea slug - Onchidella celtica

Onchidella celtica

Image Peter Barfield - The celtic sea slug Onchidella celtica. Image width ca XX cm.
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Distribution map

Onchidella celtica recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

Why do the maps differ?

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Onchidella celtica is not listed under any importance categories.

Taxonomy icon Taxonomy Taxon English term
Phylum Mollusca Snails, slugs, mussels, cockles, clams & squid
Class Gastropoda Snails, slugs & sea butterflies
Authority Cuvier, 1817
Recent synonyms None
Map icon Recorded Distribution in Britain and Ireland The Celtic sea slug is abundant on the north and south coasts of Devon and Cornwall. Also recorded from Croyde Bay, Bristol Channel; Upper Loch Fyne, Scotland, the Farne Islands and Jersey, the Channel Isles.
Habitat information icon Habitat information The Celtic sea slug can be found on exposed rock, usually on sandy shores amongst mussels, barnacles and hiding in crevices. It forages down the shore as the tide goes out, crawling on small algae and returning up the shore as the tide rises. Mainly observed between April and November.
Text page icon Description The Celtic sea slug has a solid, oval and fleshy body up to 13 mm in length and 6 mm in width. It is a dark green-black inconspicuous slug with a mantle that is covered in large, coarse, evenly spaced tubercles, to which sand grains often adhere. When Onchidella celtica is motionless, the mantle completely obscures the head and pale grey foot. The head bears two short, thick, cylindrical tentacles with eyes at the tips. The head is only visible when the Celtic sea slug is mobile.
Identifying features
  • Up to 13 mm long and 6 mm wide.
  • Dark green-black in colour.
  • Mantle covered with large, coarse, evenly spaced tubercles.
  • Head and grey foot obscured by tuberculate mantle when stationary.
Additional information icon Additional information This species is not an opisthobranch sea slug. It is a pulmonate slug more adapted to terrestrial and freshwater habitats, and related to most land snails. Being a pulmonate gastropod means that it has no gills. Therefore the mantle cavity acts as a lung and is closed by a sphinctered opening called the pneumostome, situated close to the anus. Pulmonate sea slugs also have no shell. It has a veliger larval stage which is completed within the egg capsule.

This review can be cited as follows:

Sonia Rowley 2005. Onchidella celtica. Celtic sea slug. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 27/11/2015]. Available from: <>