Nudibranch

Sea slugs are very pretty.

They live in the salt sea. Some live in warm water. Some live in cold water.

These animals do not have shells. They lose their shells when they grow up. They have many bright colors. Some look like dragons or rabbits.

They use these colors to stay safe. Many kinds live in the ocean.

Nudibranchs are soft sea animals.

They belong to a group called molluscs. Most molluscs have hard shells. But nudibranchs shed their shells when they grow up. Their name means "naked gills." This is because they breathe through gills on their backs.

These animals come in many shapes and colors. People give them fun nicknames. Some are called "sea rabbits" or "dragons."

They use colors to stay safe. Some colors help them hide. Other bright colors warn predators that they taste bad. Some even steal stinging cells from their food. They keep these cells in parts on their backs called cerata. This helps them fight back.

Nudibranchs live in salt water all over the world. They live in warm reefs and cold oceans. Most crawl on the sea floor. Some float near the surface. One special kind lives very deep in the dark sea. It can even glow in the dark.

They lay eggs in jelly ribbons. Some lay only two eggs. Others lay 25 million eggs!

Nudibranchs are amazing soft-bodied sea animals. They belong to a group called molluscs. Most molluscs have hard shells to stay safe. However, nudibranchs shed their shells after they are larvae. Their name comes from Latin and Greek words meaning "naked gills."

This is because they breathe through gills on their backs. They are often called sea slugs by many people. There are about 3,000 known species of these creatures.

These animals have many ways to stay safe. Some use camouflage to look like sponges or corals. This helps them hide from predators. Other nudibranchs use bright colors to warn others. These bright patterns show that they might taste bad. Some species even steal stinging cells from their food. They move these cells into parts on their back called cerata. This process helps them defend themselves from being eaten.

Nudibranchs live in salt water all over the world. You can find them in the Arctic and near Antarctica. Most live in warm, shallow coral reefs. Some crawl on the sea floor. Others float upside down just under the surface. A special kind called Bathydevius lives very deep in the dark.

This species was described in 2024. It is one of the few that can glow in the dark. This is called being bioluminescent.

Every nudibranch has its own unique body parts. They have tentacles on their heads to smell and feel. They also have club-shaped rhinophores to detect odors. Their eyes are very simple and small. These eyes can only see light and dark.

Some species can even produce sounds. In 1884, Philip Henry Gosse reported hearing sounds from them. He said they sounded like a steel wire hitting a jar. These sounds might help them talk to each other.

Life begins with eggs laid in jelly ribbons. Nudibranchs are hermaphrodites, which means they have both male and female parts. They must find a partner to mate.

The number of eggs can change a lot. Some lay only two eggs at a time. Other species can lay as many as 25 million eggs! After they hatch, the babies look like tiny adults. They are just much smaller than the parents.

Nudibranchs are a diverse group of soft-bodied marine gastropod molluscs. They belong to the order Nudibranchia within the phylum Mollusca. Most gastropods, like snails, rely on hard shells for protection. However, nudibranchs undergo a major change during their life cycle. They begin as swimming larvae with coiled shells. When they undergo metamorphosis into adults, they shed these shells entirely. This loss of a shell is why their name is so descriptive. The word comes from the Latin *nudus*, meaning naked, and the Ancient Greek *brankhia*, meaning gills.

Because they lack shells, nudibranchs have developed complex biological mechanisms to survive. Many species use aposematism, which is the use of bright colors to warn predators of danger. Some nudibranchs use camouflage to mimic the textures and colors of sponges or soft corals. Others use chemical defenses by concentrating toxins from the sponges they eat. This process, known as sequestering, allows them to become distasteful to predators. Some species can even produce their ‘de novo’, meaning they create their own chemical defenses from scratch without needing food sources.

One of the most fascinating mechanisms is how some species steal defenses from their prey. Certain nudibranchs feed on hydrozoans, which are animals with stinging cells called nematocysts. Instead of being harmed, the nudibranch moves these cells through its digestive tract. These stolen cells, called kleptocnidae, are then moved to specific placements in the animal's dorsal body wall. These protrusions are called cerata. By storing these stinging cells in their cerata, the nudibranch becomes armed against anything that tries to eat it.

Nudibranch anatomy is highly specialized for life without a shell. They possess cephalic tentacles on their heads to sense touch, taste, and smell. They also have club-shaped rhinophores, which are organs used specifically to detect odors in the water. Their eyes are quite simple and measure about a quarter of a millimeter in diameter. These eyes consist of a lens and five photoreceptors, allowing them to discern light and dark. While their external bodies appear bilaterally symmetrical, their internal organs remain asymmetrical. This is due to a process called secondary detorsion.

Nudibranchs inhabit almost every ocean on Earth. They can be found in the Arctic, tropical regions, and the Southern Ocean near Antarctica. Most species are benthic, meaning they crawl along the sea floor. However, there are unique exceptions to this lifestyle. The genus *Glaucus* is neustonic, meaning they float upside down just below the ocean surface. The species *Cephalopyge trematoides* is pelagic, meaning it swims within the water column. In 2024, scientists described *Bathydevius*, a unique bathypelagic nudibranch. It lives at extreme depths and is one of the few species that is bioluminescent.

Reproduction in nudibranchs is a complex biological process. All nudibranchs are hermaphrodites, meaning they possess both male and female reproductive organs. However, they cannot fertilize themselves and must find a partner for mutual impregnation. Mating often involves a courtship period that looks like a dance. After mating, they deposit eggs in a gelatinous, spiral-shaped mass that resembles a ribbon. The scale of reproduction varies wildly between species. For example, *Vayssierea felis* may lay only one or two eggs, while *Aplysia fasciata* can produce an estimated 25 million eggs.

History shows that humans have long been curious about these creatures. In 1884, Philip Henry Gosse reported that certain species might produce audible sounds. He noted observations from a "Professor Grant" regarding the species *Tritonia arborescens*. The sound was described as resembling a steel wire clinking against a glass jar. The sounds occurred when the animals were active and seemed to come from the mouth. While scientists have not confirmed if these sounds are for communication or defense, they remain a notable historical observation. Today, scientists recognize approximately 3,000 different species of nudibranchs worldwide.