Nudibranchs: The Rainbow of the Reef

What Is a Nudibranch?

A nudibranch is a soft-bodied marine mollusc — a sea slug — that has evolved to display its defence system on the outside of its body. The name comes from the Latin nudus (naked) and the Greek brankhia (gills): literally, 'naked gills.' The gill-like cerata or plumes visible on the dorsal surface of most nudibranchs are exposed respiratory organs, and in many species they serve additional functions: storing stinging cells from prey for secondary defence, housing photosynthetic zooxanthellae, or providing additional camouflage.

Over 3,000 species of nudibranchs have been described, with new ones documented regularly from understudied regions. They range from 4mm to 60cm (the giant Spanish dancer, Hexabranchus sanguineus). They occur in every ocean, including the Arctic and Antarctic, though the greatest diversity is in tropical Indo-Pacific waters.

Defence Chemistry

The extraordinary colouration of nudibranchs is primarily aposematic — warning colouration signalling toxicity or unpalatability to potential predators. The system is elaborated differently across the major groups:

Dorid nudibranchs (the ones with gills on their backs) typically produce defensive chemicals from their own tissues — often sequestering compounds from the sponges they eat. Many sponges produce powerful cytotoxic chemicals to deter predators; dorids can consume these sponges and reuse the chemicals for their own defence.

Aeolid nudibranchs (the ones with finger-like cerata) feed on cnidarians — hydroids, soft corals, even anemones — and sequester the unfired stinging cells (nematocysts) from their prey. The nematocysts are transported through the cerata into special cells (cnidosacs) where they can be discharged if the nudibranch is attacked. A sea slug carrying another animal's weapons.

The Diversity Problem

Identifying nudibranchs to species level is a skill that takes years to develop. The variation within species (colour, pattern, and size can all vary significantly) and the similarity between species (in some genera, distinguishing species requires microscopic examination of reproductive anatomy) makes even expert identification difficult in the field.

For non-specialist divers, the key groups to learn:

• Chromodorids (Chromodoris genus): Typically blue, white, and black or purple; the most commonly photographed group; includes the famous Chromodoris willani and Chromodoris annulata
• Phyllidiids: Warty, rounded dorsal surface; typically white with black markings and coloured tubercles; contain some of the most toxic chemical defence systems in the group
• Flabellinids: Slender aeolids with clustered cerata in rows; brightly coloured; common throughout the Indo-Pacific
• Glaucus (Glaucus atlanticus): Pelagic; silver-blue; floats at the ocean surface, frequently washed ashore; feeds on Portuguese man-of-war and sequesters its nematocysts

The Best Nudibranch Destinations

Nudibranch diversity tracks the Coral Triangle generally, with Lembeh Strait (Indonesia), Anilao (Philippines), and Milne Bay (Papua New Guinea) consistently producing the greatest number of species per dive. Dedicated nudibranch dives at these sites with an expert guide can produce 20–30 species per 60-minute dive.

In cooler waters, Monterey Bay (California) and the Cape Peninsula (South Africa) offer different species assemblages — including some of the world's largest aeolids and spectacular cold-water dorid species rarely seen elsewhere.