What is a nudibranch?

Nudibranchs are mollusks, as the snails, sea slugs or clams.  The Nudibranchia, is a group with probably more than 3000 described species.

Distribution and Habitat

Nudibranchs are distributed worldwide in all oceans from the intertidal down to the deep sea, but reach their greatest size and variation in warm, shallow waters. Seventeen species of nudibranchs have to date been recorded in the Colombian Caribbean.

Description

The word "nudibranch" comes from the latin nudus, naked, and the greek brankhia, gills, “naked gills”, referencing the fact that these gastropods breathe through their skin, rather than through specialized gills. Nudibranchs are often casually called "sea slugs", a non-scientific term. This has led some people to assume that every sea slug must be a nudibranch.

The body forms of nudibranchs vary enormously, they are bilaterally symmetrical. Some species have venomous appendages on their sides. These are used to deter predators. Many also have a simple gut and a mouth with a radula. Their eyes are simple and able to discern little more than light and dark, Nudibranchs vary in adult size from 20 to 600 millimetres (0.79 to 24 in). They also have very simple nervous systems and digestive tracts, like other mollusk.  

But the most amazing characteristic is their beautiful appearance, which has been vividly described by Helmut Debelius, an avid oceanic photographer and author: “They look like the creation of talented painters, like exercises in imagination. They come mostly in bright colors, decorated with such profusions of undulating flaps, sensory organs and waving forests of ‘fingers’ that it is difficult to tell which end is which. Delicate, seemingly unprotected, they are beautiful to look upon, the underwater analogues of butterflies. Yet, they too are animals surviving in an eat-or-beeaten world, sometimes predator and sometimes prey. Moreover, they have taken an evolutionary gamble by giving up the protective shell that has always sheltered their kind, and appear to have won handsomely”.  

Feeding

Most nudibranchs are carnivorous. Some feed on sponges, others on hydroids, others on bryozoans and some eat other sea slugs or, on some occasions, are cannibals and prey on members of their own species. Other groups feed on tunicates, barnacles or anemones.

They can also take in plants' chloroplasts (plant cell organelles used for photosynthesis) and use them to make food for themselves.

Reproduction

They are simultaneous hermafrodites possessing sex characteristics of both genders at the same time. Most nudibranchs prefer to seek out partners rather than self fertilizing, laying clutches of fertilized eggs in areas where the young will be dispersed after hatching.

Defense Mechanisms

In the course of evolution, sea slugs have lost their shell because they have developed other defense mechanisms. Their anatomy may resemble the texture and color of the surrounding plants, allowing them to camouflage (cryptic behavior). Others, as seen especially well on chromodorids, have an intense and bright coloring, which warns that they are distasteful or poisonous (aposematic coloration)

The ability to sequester and re-use potentially harmful components of their prey makes the nudibranch rather unique. Most animals would die or become extremely sick if they consumed a wide variety of venomous creatures, yet nudibranchs have evolved to not only roll with their punches, but to actively reuse them.

Nudibranchs that feed on hydroids can store the hydroids' nematocysts (stinging cells) in the dorsal body wall. The nematocysts wander through the alimentary tract without harming the nudibranch.

Some sponge-eating nudibranches concentrate the toxins from their prey sponge in their bodies, rendering themselves toxic to predators. Another method of protection is the release of an acid from the skin. Once the specimen is physically irritated or touched by another creature, it will release the slime automatically.

Because of their world wide distribution and astonishing array of color patterns many countries have issued postage stamps to show off their native nudibranchs. For example, at least 84 stamps have been issued depicting these unique animals. One of the best known nudibranchs, the Spanish Dancer, Hexabranchus sanguineus, Family Hexabranchidae, has appeared on three stamps – Fiji Islands #697, Tuvalu #465 and the United States #3831g.

So, next time you go diving in the Tayrona Park, try to spot this beautiful creature, it can be an unforgettable experience and a wonderful picture

Meanwhile you can enjoy a collection of beautiful pictures in National Geographic Gallery.