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Big less than a fingernail or as a foot, with a flattened or sausage shape, but, above all, very colorful: the nudibranchs (Nudibranchia), molluscs with a soft and flexible body belonging to the family of sea snails, are many and varied. .

It is estimated there are over 3, 000 known species, but new ones are identified almost every day. They are known as the "harlequins" of the abyss: their bright colors, often combined with a certain toxicity, serve to keep predators away, warning them of the possible danger.

In the photo, a nudibranch of the Indo-Pacific species Chromodoris kuniei.

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Their curious name derives from the Latin nudus (nude) and from the Greek word brankhia (gills), and means with bare gills: most of these creatures have in fact tufts of gills and antennas positioned outside, on the back. Practically blind - their eyes barely perceive darkness and light - the nudibranchs explore the surrounding world through a set of receptors positioned on the head, called rhinophores.

In the photo, a Spanish dancer (Hexabranchus sanguineus): if disturbed, this nudibranch moves away with sinuous movements given by the contraction of the ventral back muscles, moving the reddish-colored coat like a flamenco skirt. Up to 60 centimeters long, it is one of the largest nudibranchs in the world and one of the few that can swim: the others are generally benthic, ie they live in close symbiosis with the seabed.

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Most nudibranchs measure from 3 millimeters to about 30 centimeters, and prefer the low and warm tropical bottoms. These creatures are carnivorous and feed on small molluscs, crustaceans, dead fish, sponges, corals and even other nudibranchs. Some of them are able to assimilate the poisonous substances and the stinging tentacles present in the preys and use them in turn when attacked by predators (fish, turtles, starfish, crabs).

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The life of these creatures is short-lived, almost always less than a year and sometimes even less than a month. This is why it is particularly difficult to study them, even though scientists are interested in some chemical mechanisms that these animals set in motion in self-defense situations and during mating.

The nudibranchs are simultaneous hermaphrodites: that is, they have both male and female reproductive organs and can give or receive sperm as appropriate. Moreover, some of them are able to lose their penis immediately after mating and have it regrow within 24 hours (read here to understand how they do): a unique feature among all living things known so far. In the photo, the convulsive coupling between two nudibranches of the genus Thuridilla.

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But the penis is not the only part of the body that these creatures can "lose". Some species of nudibranch are able to leave part of the mantle to distract the predators with bright colors and run away undisturbed (a defensive strategy called autotomy). A bit like lizards with tails, or some crabs with claws.

In the photo, a nudibranch of the genus Chromodoris photographed while feeding on a sponge.

During mating these animals fertilize each other, and then lay down millions of eggs grouped in strips or skeins. In the photo, a nudibranch intent on depositing the brood on an hydroid, an animal organism similar to a plant actually formed by polyps with filiform tentacles.

A nudibranch of the genus Flabellina, common also in the Mediterranean Sea up to 50 meters deep. If some nudibranchs, like this one, scare off the attackers with the bright colors of their coat, others opt for a more discreet strategy and camouflage themselves with the seabed in which they live.

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It is estimated there are over 3, 000 known species, but new ones are identified almost every day. They are known as the "harlequins" of the abyss: their bright colors, often combined with a certain toxicity, serve to keep predators away, warning them of the possible danger.
In the photo, a nudibranch of the Indo-Pacific species Chromodoris kuniei.
Look also at the funniest creatures of the abyss