Entire identification books are dedicated to the rainbow-colored nudibranch. Here are some quick facts about these diver favorites.
What is a nudibranch?
Nudibranchs are a family of sea slugs within the phylum Mollusca, but not all sea slugs are nudibranchs. Their name means “naked gills,” which is one of their identifying characteristics compared to sea slugs. Some species have exposed gills on their back while others, such as aeolid nudibranchs, have outgrowth all over their body which helps them breathe.
They vary in size from very small (a few millimeters) up to almost 23 inches (60 cm) for the biggest species. They also vary in body shape and color, with more than 3,000 known species and new ones discovered regularly. Most of them are very bright and colorful, and despite their lack of shell they have a good protection as some of them are toxic to predators. Nudis may also sting predators or use camouflage techniques to protect themselves.
Nudibranchs are hermaphroditic, which means they have both male and female organs. However, they cannot fertilize themselves. They produce thousands of eggs, which they lay on the reef, usually in a characteristic spiral form. Most of them crawl on the sea floor, although a few species can swim in the water column such as the well-known Spanish dancer.
They are carnivorous and, depending on the species, they feed on sponges, hydroids, anemones or even on members of their own species.
Where can you find nudibranchs?
You can find them in any ocean in the world and up to a depth of over 6,500 feet (2,000 m), although they live most commonly in shallow, tropical waters. Some of the species are limited to a specific area while some others are more commonly distributed around the world.
It is very likely that you have seen them before while diving, although you may not have known what you were looking at. You may have seen some divers with their noses close to the ocean floor, looking for species they have never seen before, or a dive guide pointing at something very small. Once you discover these tiny jewel boxes on your next dive, you too will be scouring the coral and sea floor for them. They are of course very popular among macro photographers.
Are they hard to spot?
Their camouflage abilities can make it quite difficult to spot them, but they are common on many reefs around the world. By taking a closer look and slowing down on your dives, you’ll no doubt become a nudibranch enthusiast as well.