These marine animals are considered as one of the ocean’s most beautiful creatures as they are flamboyantly decorated with extremely striking and brilliant color. They are grouped to the Class Gastropoda and belong to the Order Nudibranchia, in the Phylum Mollusca (molluscs or soft-bodied animals which include squids, octopi, abalones, shells, scallops, oysters, mussels, Chitons, and etc.). They are also known as Nudibranchs which means “naked gill” in Latin as they carry their exposed plume-like external gills on their backs. Unlike the snails, they do not have shell to cover their soft bodies from injuries and scratches. Despite the absence of the hard shells, they use a defend strategy called cryptic coloration or in a simple word “camouflage” to protect themselves.

Scientists believed that Nudibranchs are naked gill molluscs whose ancestors shrugged off their shells millions of years ago and thus Nudibranchs crawl through their lives as slick and naked creature when they are born. Approximately 3,000 species of these marine molluscs are found living particularly from shallow reefs to two miles beneath the sea floors all over the world. The species from the Family Chromodorididae alone are found to be more than 360 species distributed around the oceans of Caribbean, Mediterranean and Indo-Pacific. There are about 350 species of them being identified from the Great Barrier Reef.

Divers may come across these marine animals with their skins, muscles and organs sliding on trails of slime onto the sandy shallows and reefs to the ocean beds and coral heads. Some of them may be seen thriving in warm and cold waters but some even be seen billowing around deep-sea vents.

Though these naked sea slugs have a relatively large arsenal, their sizes are roughly of a human’s index finger. Their average sizes can reach to about 50mm. That is why they always become a quick snack and an easy digested food for turtles, sea stars, humans, and even marine organisms. No matter how beautiful and dazzling coloration of their bodies, they live no longer than a year.

In contrast to their beauty, they are born blind as their tiny eyes discerning little more than light and dark. Despite of the blindness, they use their head-mounted sensory appendages called rhinophores and oral tentacles to smell, taste, and feel their world. Their chemical signals assist them to detect food sources like barnacles, small fish, eggs, and other tiny marine organisms. The adaptation of their brightly colored bodies is signals to warn their predators that they are tasteless or even deadly for them to feed on. Some may even have toxic glands just below the surface of their mantles to defend themselves from enemies. However, some species are able to swim swiftly and vigorously when disturbed.

As they are hermaphrodities in nature, and thus they have both male and female reproductive organs, they can fertilize one another. That means during mating, the partners fertilize each other that both become pregnant and lay eggs. They may lay eggs in tangled clumps, ribbons or even in coils depending on their specific species. They will lay up to two millions eggs at a time to double their reproductive success. Some of their young are born with mollusk shell to protect their tender bodies but their shells will quickly shed off thereafter.

Many types of these boneless species use the camouflage by blending their bodies to the colorful surrounding of the reefs while attracting their prey as their food source. This adaptation helps them from being spotted by other predators dwelling deep beneath the sea bed. As they are empowered with skin glands that produce potent poisons, some of them can even make sulphuric acid, and others may produce non-acidic substances. The species like orange peel Nudibranchs for example, secreting toxic chemicals to defend themselves. They will sting any marine creatures that they curious or suspicious by simply giving them a bite. They have no problems to eat toxic sponges or any other harmful marine animals without being suffered from toxicological consequences as they can alter the toxins by turning them into secretions that they will use against their potential predators. Specifically, when they dine on harmful marine organisms, they will reprocess and store the harmful substances from eating stinging-celled animals into their bodies which will be secreted from their skin cells or glands when attacked or disturbed. When threatened or in danger, some Nudibranchs will shed their cerata off to be attached to their predators firing nematocysts to enable them to escape from being captured.

They are carnivores and thus they feed on sessile invertebrates like sea fans, hydroids, sea anemones, bryozoans, barnacles, soft corals, ascidians, fish eggs, sponges and other species of marine organisms. Most of them are adapted for specific diet that may be restricted themselves to particular species of animals or plants. They have a ribbon of teeth called radula that are adapted to match their food choices. Nudibranchs which feed on sponge for example, have broad radulae together with several teeth to help them scraping while feeding on their prey. In contrary, Nudibranchs which feed on hydroids and bryozoans have narrow radulae but firm jaws to enable them crunching while eating. Some of the Nudibranchs will just rely on enzymes instead of teeth to break down their prey. It was reported that some Nudibranchs will change their colors in accordance to what they feed on.

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