Where to dive with nudibranchs

What they are

Nudibranchs are soft-bodied marine molluscs that lack an external shell. They range from a few millimetres to several centimetres long, and most sea slugs are small, though Hexabranchus has been reported at 520 mm or more off Djibouti in the Red Sea.^1,2,3

They are carnivores, and each species tends to feed on a particular prey such as a hydroid, sponge, anemone, bryozoan or tunicate. Many are toxic: rather than making their own toxins they reuse those of their prey, and some take up the stinging cells of the animals they eat to protect themselves.^1,2,4

Where and when divers see them

Each species tends to feed on a particular prey, and divers most often spot them crawling around looking for food. Depth ranges vary by species; in Hawaii, for example, the gold lace nudibranch is found around caves at roughly 5 to 30 metres.^2,4

Even temperate seas hold many species: the Wildlife Trusts count more than 100 in UK waters and say they can be found all year round.¹

Conservation status

Nudibranchs are a large group and we found no single IUCN Red List status for them. Hawaii's State Wildlife Action Plan lists its featured nudibranch species as not considered by the IUCN Red List, and notes that some are targeted by the aquarium trade.⁴

How to behave around them

Reef conservation guidelines ask divers to stay neutrally buoyant, not to touch or handle marine life and never to chase or harass it. Photography guides add that you should never touch or move a nudibranch for a better picture, and should keep your silt off it.^2,5

• Stay neutrally buoyant at all times.⁵
• Do not touch, move or reposition the animal.^2,5,6
• Watch that your presence and stirred-up silt are not bothering it.²