What is a Nudibranch?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 12 October 2019
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A nudibranch is a marine gastropod in the suborder Nudibranchia. There are over 3,000 unique nudibranch species, with numerous more being identified each year, making nudibranchs among the most diverse of marine organisms. They are also extremely widespread; nudibranchs can be found in shallow coastal waters all over the world, except in extremely cold regions.

These animals are sometimes referred to as “sea slugs,” but this is not a very precise term. Like other slugs, nudibranchs are shell-less mollusks, but there are numerous other slugs in the sea, so to speak. In fact, a wide range of animals could be considered sea slugs, and most of the scientific community avoids the use of this term, since it has become so generic that it is essentially meaningless. “Nudibranch” is much more descriptive.

Nudibranchs have a number of interesting traits. The first is their coloration, which is often very ornate, and also very bright. Nudibranchs come in a wide range of colors, from hot pink and yellow to zebra-striped, and they are eye-catching inhabitants of shallows and tide pools as a result. Nudibranchs also develop outgrowths from their bodies which are sort of like soft spines, and they are capable of storing venom, stinging cells, and toxins from their prey in these outgrowths so that when they are bitten or attacked, they can fight back.


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.

In Latin, “nudibranch” translates to “naked gills,” referencing the fact that these gastropods breathe through their skin, rather than through specialized gills. They also have very simple nervous systems and digestive tracts, like other mollusks, and they are simultaneous hermaphrodites, 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.

Some aquaria which focus on marine animals have nudibranchs on display, because many people are intrigued by these brightly-colored sea creatures. They can also be seen in situ along many coastlines of the world, and thanks to the fact that they inhabit the shallows, no special equipment is required. When tidepooling to look for nudibranchs and other interesting creatures, people should remember to keep an eye on the ocean, as sneaker waves can pose a significant danger to the unwary.


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