Onchidella celtica, commonly known as the Celtic Sea Slug was first described by Cuvier in 1817. They are most abundant in the South-West of the UK, with fewer records in the North of the UK. Currently, there are around 400 research-grade observations from iNaturalist and over 25,000 observations from Global Biodiversity Information Facility (GBIF).

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Key identifying features

The Celtic sea slug is very small, reaching a maximum length of around 13 mm and a width of 6 mm.

One of its main identifying features is the pair of bumpy tentacles on the upper surface of its body. However, these tentacles are not always clearly visible and may appear as small, rounded blobs when the animal is out of water.

Habitat can also help with identification. Unlike most sea slugs, the Celtic sea slug is typically found in the high intertidal zone (high tide zone). It often hides within barnacles, among honeycomb worms, or in small crevices in rocky shores.

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Colour variations

They are primarily black in colour, but occasionally spotted to be grey, brown and even dark grey. They always have the same key identifying features, so make sure to look for their bumpy tubercles!

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Prey

Once the tide is low, they forage for various algae among barnacles.

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Occurance Data

For the most up-to-date distribution records, see:

National Biodiversity Network (NBN) - Europe (mostly UK) database of species occurrence

iNaturalist - most up to data records of the species occurrence

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Fun facts

Rather than fitting in with the true sea slugs, this mobile gherkin is a pulmonate slug, grouping it with all of the terrestrial slugs of Britain. This means that the Celtic Sea Slug is effectively a land slug that has evolved to survive in the intertidal, not the other way round. While most sea slugs feed when the tide is high, the Celtic sea slug spends this period surviving off an air bubble it traps under its body.

Lastly, there is much mystery as to the location the Celtic sea slug is found during winter, but the common theory is that they retreat and hibernate in old barnacle shells. Nonetheless, their prime time to find them is between April and November.

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References

Rowley, S.J. 2005. Onchidella celtica Celtic sea slug. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 20-02-2026]. Available from: https://www.marlin.ac.uk/species/detail/2076