Lagoon sea slug

Tenellia adspersa


This tiny, rare sea slug is, at its largest, 8mm long, and often reaches only half that size.  Much more attractive than their name suggests (and than land slugs), lagoon sea slugs have mobile, finger-like structures around the edges of their bodies.  They are pale brown, and marked with small dark spots.

Lagoon sea slugs live in shallow water, and can be found on the shore between the tides.  They like the sheltered conditions of bays and harbours.  Lagoon sea slugs prefer landscapes of pebbles, larger stones and small rocks and can also be found in seaweed and seagrass beds.  They do not eat plants, however, and are actually predators that feed on hydroids (anemone or jellyfish-like creatures that live on the seabed).

Lagoon sea slugs can be found in different areas, between which the saltiness of the water varies considerably.  They have been found upstream in estuaries in virtually freshwater conditions and, at the other extreme, in highly salty coastal lagoons.

The sea slugs’ common name comes from their association with these lagoons, and it is the threats to the lagoons from pollution, in-filling and coastal defences that present the most serious known threat to the populations of the sea slugs.


European distribution

The few British records of the lagoon sea slug have been on the southern and western coasts of England, but the full European distribution extends to the eastern and western North Atlantic, and the Baltic and Mediterranean Seas.


Conservation status/need

  • Lagoon Sea Slug FactThis is a UK BAP Priority Species (BAP species are now Species of Principal Importance/Priority Species).
  • Species of principal importance for the purpose of conservation of biodiversity under the Natural Environment and Rural Communities Act 2006
  • Protected under Schedule 5 of the Wildlife and Countryside Act 1981
  • Nationally rare marine animal
  • Listed as Insufficiently known but at least Rare inthe British Red Data Book
  • The coastal lagoons in which the sea slugs can be found are a UKBAP Priority Habitat and are listed on Annex I of the Habitats Directive


Further information

Marine Life Information Network


JNCC - UK BAP Priority Species and Habitats