Nudibranchs are shell-less mollusks that constitute the order Nudibranchia within the class Gastropoda. They exhibit extraordinarily vibrant body colors and patterns, which function as both camouflage and warning coloration. Owing to their striking appearance, nudibranchs are also referred to by other common names, such as clown, marigold, splendid, dancer, dragon, and sea rabbit.
These mollusks possess the unusual ability to acquire functional stinging cells from their cnidarian prey and employ them for their own defense.
Approximately 3,000 species are found across all oceans of the world.
Depending on the species, nudibranchs vary in size from 0.16 to 23.62 in (4 to 600 mm). They can weigh up to about 3.3 lb (2 kg).[1]
These animals have a soft, bilaterally symmetrical body (only externally), lacking a mantle cavity. They do not possess a shell (operculum) in adulthood, even though most species begin their life as larvae with a coiled shell that is lost during metamorphosis.
They possess sensory tentacles on their head for touch, taste, and smell, as well as specialized odor-detecting organs, the club-shaped rhinophores.
Their eyes are simple and embedded in the body, capable only of distinguishing light from dark.
Depending on the species, nudibranchs have a diverse range of body colors, ranging from vibrant blues and yellows to brilliant reds and purples. The skin is often patterned with stripes, spots, swirls, or gradients. In fact, certain body patterns help nudibranchs mimic sessile invertebrates, such as sponges or soft corals, helping them avoid predators.
The name ‘nudibranch’ derives from the Latin word nudus (meaning naked) and the ancient Greek word bránkhia (meaning gills).
Traditionally, the order Nudibranchia was classified under the subclass Opisthobranchia, which is no longer a formal group. Since 2005, this order has been placed along with pluerobranchs (family Pleurobranchida) within the superorder Nudipleura.
According to the traditional classification (based on the work of Johannes Thiele, 1931), nudibranchs are classified into 2 broad groups: Anthobranchia (dorids) and Cladobranchia (aeolids).
While dorids possess a cluster of dorsal gill plumes (naked gills, as the name of the order refers) arranged like a rosette, aeolids respire through finger-like projections or cerata spread across their backs.
In a 2024 study, a new family, Bathydeviidae, containing a single genus, Bathydevius, was revealed. The revised classification is as follows:
These animals are found worldwide, ranging from the Arctic to the Southern Ocean, with most species found around Southeast Asia. Although they are typically marine, a few, such as Bohuslania matsmichaeli, are found in brackishwater environments.[2]
Their highest diversity is observed in warm, shallow reefs, though one specimen, belonging to the species Bathydevius caudactylus, was collected at a depth of about 8,200 ft (2,500 m). This species is the only known nudibranch to be found in the bathypelagic zone.[3][4]
Most nudibranchs are benthic, found crawling on a substrate, though members of the genus Glaucus float upside down just under the ocean’s surface (neustonic). Similarly, the species Cephalopyge trematoides is pelagic and swims in the water column.
Although all nudibranchs are carnivores, their diets vary widely. Many species feed on sea sponges, while others, such as those in the genus Cuthona, specialize in eating hydrozoans. Nudibranchs belonging to the genera Tambja, Limacia, Plocamopherus, and Triopha commonly feed on bryozoans. Some species, like Favorinus tsuruganus, even consume the egg masses of other nudibranchs, especially those laid by species of Hexabranchus.[5]
The blue glaucus (Glaucus atlanticus) specializes in consuming siphonophores, such as the Portuguese man o’ war (Physalia physalis). Other nudibranchs feed on jellyfish and sea anemones.
Depending on the species, their lifespan typically ranges from a few weeks to about a year.[6]
These mollusks are simultaneous hermaphrodites, meaning each individual has both male and female reproductive organs at once. After a brief period of courtship dance, the pair mates by aligning their bodies side by side. The sperm fertilize the eggs internally, after which the eggs are laid as a ribbon-shaped gelatinous spiral embedded within a soft matrix.
In 2 to 5 weeks, the eggs of most species hatch as planktonic, shelled veliger larvae. The larvae drift in the water column and finally settle on a sponge or a coral. They then shed their shell and transform into a benthic juvenile. In some species, such as Cadlina laevis, the eggs develop directly into juveniles, bypassing the larval stage.
The juvenile grows in size, attains adult body coloration, and reaches sexual maturity in 2 to 8 weeks.
Very few predators of nudibranchs have been documented, primarily because predation events are rarely observed in the wild. However, they have occasionally been recorded to be eaten by sea spiders, crabs, fish, and sea stars.[7]
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