Nudibranch

Quick Take

• Some nudibranchs steal a weapon from their prey and turn it against predators, and the mechanism behind this is stranger than it sounds. See what nudibranchs eat →
• One group of these sea slugs has essentially figured out how to become part plant, which gives them a surprising energy advantage. Discover the chloroplast trick →
• Despite being hermaphrodites, nudibranchs face a reproductive constraint that forces a surprisingly elaborate solution. Explore their mating biology →
• The blue dragon nudibranch hunts prey that most ocean predators avoid entirely, and it gets away with it. Meet the blue dragon →

The nudibranch is a soft-bodied marine gastropod mollusk, also called a sea slug, that sheds its shell after the larval stage. Its name means “naked gill.” Some species steal the nematocysts, or stinging cell organelles, from their prey in order to release the toxins as a defensive mucus against predators. The creature lives at all depths in mostly saltwater oceans around the world, while a few species live in brackish water.

5 Nudibranch Facts

• Nudibranchs are carnivorous.
• They are hermaphrodites.
• Some absorb chloroplasts from algae to benefit from photosynthesis, which gives them energy.
• There is a pair of tentacles (rhinophores) with scent receptors on their heads, which they can hide from predators in a flesh pocket.
• Nudibranch means “naked gill.”

Classification and Scientific Name

The nudibranch belongs to the order Nudibranchia. One notable scientific fact about this creature is that it is one type of sea slug, or opisthobranch, in the phylum Mollusca (mollusks) and the order Nudibranchia. The phylum Mollusca includes snails, slugs, limpets, and sea hares. Nudibranch means “naked gill” and comes from the Latin word nudus (naked) and the Greek word brankhia (gills). The creatures are divided into two types: dorid and aeolid, which are suborders Doridacea and Aeolidida. Other types of sea slugs are not closely related to nudibranchs.

Nudibranch Species

There are over 3,000 known species of nudibranchs, with new species still being discovered regularly. Some examples are:

• Anemone sea slug (Berghia coerulescens): lives in the waters of the Mediterranean and Atlantic Ocean.
• Blue dragon, blue angel, or sea swallow (Glaucus atlanticus): lives in temperate and tropical waters around the world.
• Berghia nudibranch (Berghia stephanieae): lives in the warm and temperate waters of the Atlantic Ocean, Caribbean, and Mediterranean.
• Opalescent sea slug (Hermissenda opalescens): lives in both sides of the Pacific Ocean.
• Hooded nudibranch, lion nudibranch, or lion’s mane nudibranch (Melibe leonina): lives in the eastern Pacific Ocean of North America, ranging from Alaska to Baja, California.
• Sea bunny (Jorunna parva): a dorid, lives in the waters of Japan, the Philippines, Oceania, and the Indian Ocean.
• Phyllodesmium poindimiei: an aeolid that lives in tropical waters around the world.

Appearance

Nudibranchs vary in size, form, and color depending on the species, often displaying stunning, vivid hues. They are often confused with other types of sea slugs, such as Sacoglossa and Aglajidae, which are not closely related to them. The size of the nudibranch ranges from a few millimeters to over 60 cm (0.25 to 25 inches), with a weight of up to 3.3 lbs. Dorids are smooth with feather-like gill plumes on their backs, while aeolids have cerata over their backs to breathe with.

• Anemone sea slug (Berghia coerulescens): an aeolid, has a size of 1.6 to 2.8 inches (40 to 70 mm).
• Blue dragon, blue angel, or sea swallow (Glaucus atlanticus): an aeolid, is 1.2 in long, has dark blue head stripes, a tapering, flat body, and six appendages displaying cerata, pale blue sides, and a silvery grey belly.
• Berghia nudibranch (Berghia stephanieae): an aeolid, is up to 0.8 inches (20mm) long.
• Opalescent sea slug (Hermissenda opalescens): an aeolid, has bright colors.
• Hooded nudibranch, lion nudibranch, or lion’s mane nudibranch (Melibe leonina): an aeolid, has a pale, translucent yellow or green body, 4 inches (102mm) in length and 1 inch (25mm) in width, a large oral hood usually over 2 inches (51 mm) wide with 2 rows of cirri, a single pair of rhinophores, and two rows of three to six pairs of cerata.
• Sea bunny (Jorunna parva): a dorid, has an average size of 0.4 inches (1cm) long, black-and-white rhinophores similar to rabbit ears, a body that is yellow, white, or green, covered in black papillae as well as papillae matching its main body color.

Distribution, Population, and Habitat

Nudibranchs are found in all the world’s oceans, both cold and warm water. They can be found on the seafloor and at all depth levels from 30 to 6,500 ft below the surface.

Predators and Prey

Nudibranchs are carnivores that eat other animals. Many species have a primary prey item. The Favorinus genus eats other sea slugs and their eggs, and Robastra eats other nudibranchs. Onchidoris bilamellata eats barnacles, Cladobranchia (including Aeolididae) eat anemones, and others eat sea sponges or other creatures.

• Anemone sea slug (Berghia coerulescens): eats aiptasia, a coral or anemone species.
• Blue dragon, blue angel, or sea swallow (Glaucus atlanticus): eats hydrozoans, such as the Portuguese Man O’ War.
• Berghia nudibranch (Berghia stephanieae): eats aiptasia.
• Opalescent sea slug (Hermissenda opalescens): eats sea anemones, bryozoans (moss animals), sea pens, sea squirts, corals, sponges, small crustaceans, and others of its kind.
• Hooded nudibranch, lion nudibranch, or lion’s mane nudibranch (Melibe leonina): eats planktonic invertebrates.
• Sea bunny (Jorunna parva): a dorid, eats sea butterflies.
• Phyllodesmium poindimiei: an aeolid that eats Alcyonacea, including Carijoa riisei, a soft coral species.

Reproduction and Lifespan

All nudibranch species are hermaphroditic, meaning they have both male and female sexual organs. However, they cannot fertilize themselves, so they have to meet another nudibranch to reproduce with it. They begin with a courtship ritual involving dancing. During the few minutes of mating, one deposits its eggs in a gelatinous spiral, while the other donates sperm. They can lay as few as 1 to 2 or as many as 25 million eggs, which all contain toxins from sea sponges for defense against predators. The baby nudibranchs look like adults except for their smaller size and fewer cerata. Nudibranch lifespan ranges from a few weeks to up to 4 years, depending on the species.

Fishing and Cooking

Nudibranchs are caught and sold as exotic pets.

Population

No nudibranch species are listed as vulnerable, threatened, or endangered. The hydrothermal vent nudibranch (Dendronotus comteti) has an unknown population and is listed as Least Concern.