December 12, 2025
Archidoris pseudoargus, commonly known as the Sea Lemon, was first described by Rapp in 1827. They are a common sea slug found around the UK, with over 1,000 observations on iNaturalist, and over 6,000 observations made worldwide from GBIF.
The Sea Lemons are primarily yellow or orange, with some being lighter colours, and have various blotchy markings on the body. The colours of the blochy marking vary in colour, and each colour varies depending sponges they feed on.
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These slugs come in a variety of colours, from orange, yellow, green some pale yellow. Each slug can come in variations with blotchy colours on the body, which those blotchs vary in colours.
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The sea lemon closely resembles Geitodoris planata, and the two species can easily be confused.
Size is a helpful identifying feature. Adult sea lemons can reach up to 12 cm in length, whereas Geitodoris planata typically grows to around half that size. This makes it an easy and uninvasive way to identify between the species reliably.
On the other hand, to be more certain of the identification, especially if the species is smaller, examining under the mantle is helpful. The sea lemon’s mantle is typically smooth in colour and does not usually have spots, although individuals with small purple spots have occasionally been recorded. In contrast, Geitodoris planata has numerous brown spots covering its mantle.
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The life of the sea lemon begins as a tiny egg, laid alongside hundreds to thousands of others in a distinctive ribbon-like mass.
After hatching, the larvae become free-swimming plankton. During this stage, they drift in the water column before eventually settling on the seafloor, usually near or directly on sponges — their main food source.
Juveniles are typically pale grey or yellow. As they grow, their colour deepens into the richer yellow most commonly seen in adults.
Unlike many sea slug species, which live for around one year, the sea lemon can live for up to two years.
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These slugs are carnivorous, with their main prey being the breadcrumb sponge (Halichondria panicea) in the low intertidal zone, and Halichondria bowerbanki (type of sponge) and Suberites ficus (another type of sponge) in the sublittoral zone.
The sea lemon is native to Europe and is particularly abundant around the coasts of the United Kingdom.
In recent years, the species has also been recorded in North America, with sightings reported in the United States in 2013 and Canada in 2023. In these regions, it is considered a non-native species.
This range expansion is thought to be linked to human activity. During its planktonic larval stage, the sea lemon can be transported in ships’ ballast water. It may also be introduced via sponges (its primary food source) that attach to boat hulls and are carried between regions.
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
Ager, O.E.D. 2008. Archidoris pseudoargus Sealemon. In Tyler-Walters H. and Hiscock K. Marine Life InformationNetwork: Biology and Sensitivity Key Information Reviews, [on-line].Plymouth: Marine Biological Association of the United Kingdom. [cited20-02-2026]. Available from: https://www.marlin.ac.uk/species/detail/1856
Davies, J.(1993) Aspects of the life history and physiological ecology of long-livednudibranch molluscs, Thesis. University of St Andrews.
Picton, B.E. & Morrow, C.C. (2016). Archidorispseudoargus (Rapp, 1827). [In] Encyclopedia of Marine Life ofBritain and Ireland.
https://www.habitas.org.uk/marinelife/species.asp?item=W14030 Accessed on2026-02-20
Raviraj,R., Scheetz, C., Carlton, J., Burns, K., Crosby, S., Cuzco, K., Donato, M.,Fajardo, M., Morrison, I., Olavarria, M., Romanello, D., Schwarzenbach, H.,Spiller, N., and Susarchick, J. (2025) Sea slug invasions: the non-nativeArchidoris pseudoargus (Nudibranchia: Dorididae) moves south and the nativeDoriopsilla pharpa (Nudibranchia: Dendrodorididae) moves north on the U.S.Atlantic coast. BioInvasions Records 14(3): 687–699.
Yang, Y.,Vannier, J., Lhéritier, M., and Ou, Q. (2026) Rethinking artiopod evolution:insights from the anatomy of Acanthomeridion from the early Cambrian ofChina. Royal Society Open Science 13(2): 250956.
