Lagoon sea slug

‘Sea slugs are to the molluscs what the butterflies are to the arthropods or the orchids to other flowering plants’ T. E. Thompson, author of various books on molluscs

Fast Facts

Latin name: Tenellia adspersa

Rarity in the UK: Rare / Common

Sea slugs are molluscs, in the order nudibranchia, some lack shells and others have half shells hidden in body folds. They are known to crawl along the sea bed or swim through the water with wing-like flaps called ‘cerata’. Sea slugs can be colorful and beautiful and have been referred to as ‘sea butterflies’ or ‘sea angels’. Although most sea slugs are marine some species can be found elsewhere.

The lagoon sea slug (Tenellia adspersa) is a rare species found in only a handful of sites in the UK. This sea slug is particularly attractive, varying in colour from pale yellow to brown with black speckles across its body and wing-like flaps called cerata. The animal uses it cerata to move in the water but they are also used for breathing and in some circumstances as a form of defense. It is a tiny animal about the length of your little finger, reaching a maximum of 10mm in length.

Feeding on Hydrozoa and living in lagoons

The lagoon sea slug is a predator feeding mainly on hydrozoa – plant like animals related to jellyfish, sea anemones and corals. The hydrozoa preferred by the lagoon sea slug are those in a static and colonial plant-like form. The lagoon sea slug can be found in a range of habitats from the salty waters of estuaries and lagoons (where it gets its name) to almost freshwater habitats. It requires sheltered, shallow water and can be found on pebble beds or on vegetation.

Lagoons under threat

As the species name suggests the Lagoon sea slug lives in the habitat of saline lagoons. This habitat is threatened by pollution, in-filling and sea defenses. The Lagoon sea slug is currently protected under Schedule 5 of the Wildlife and Countryside Act 1981. The species was listed as Insufficiently known but at least Rare in the British Red Data Book and classified as Nationally rare in a recent review of benthic marine species.

Current action is being taken to maintain, enhance and restore populations of the Lagoon sea slug. Some of the habitats known to host the sea slug are designated as areas of conservation importance such as the Fleet is a Site of Special Scientific Interest (SSSI) and Special Area of Conservation. The Essex site lies within a SSSI and Portishead Beach forms part of a SSSI. For more information on this species visit the The Marine Life Information Network website.

Where has it been found?

The most recent records for the species are from Portishead (Bristol Channel), the Fleet (Dorset) and St. Osyth (Essex). The species has been recorded from four other localities in Britain this century – Snettisham Pits lagoon and a creek near Dersingham (both Norfolk), New England Creek (Essex), and saltmarsh pools in the Firth of Forth. Records from before 1900 include Rotherhithe (London docklands), where the species was reported to be common.

Outside of the UK the species is widespread but in north-western Europe it is sporadically distributed and apparently not common at any locations.

Visit the Sea slug forum and other links

To find out more about the Lagoon sea slug visit Natural EnglandMarine Life and Information Network and the Sea slug forum.