Nudibranch Colors: Awesome Defenses

nudibranch colors
Unidentified species, probably in the Dorid genus Nembrotha. Photographed in the Philppines.

WITH FLAMBOYANT SWAGGER, NUDIBRANCH COLORS CALL ATTENTION TO THEIR SLUGY TORSOS – and that’s the point. As beautiful as they are, nudibranchs’ bright markings are statements telling potential predators to leave them alone.

In brief, nudibranch tissues tend to be foul-tasting, if not toxic. Nudibranch’s colors are warning signs to potential predators that they’ll regret attempts to dine out on their nudibranchy flesh.

There are other nudibranch defenses in the mix, as well. Some have a knack for consuming the stinging nematocysts of sea anemones and hydroids and utilizing them as part of their personal arsenal against their enemies.

They do this without inflicting harm on themselves but it’s not known how they manage it.


Add fantastical body architecture to their vivid personas and you can see why  this articular category of sea sligs has its devoted enthusiasts.

Biologists estimate that there are more than 3,000 species in Order Nudibranchia, exhibiting a remarkable spectrum of colors and designs. With so many, it’s no surprise that Nudibranch World features a wide range of color and anatomical designs, with bodies that are flattened or thick, long or short, fantastically or demurely gilled.

nudibranch defenses
This little guy, Jorunna funebris, clearly depticts nudibranchs’ characteristics of exernal “naked” anal gills, as well as chemically sensitve rhinophores at the head.


To begin with, the term nudibranch comes from the Greek words for “naked gills.” That’s because nudibranchs don’t wear their hearts on their sleeves but they do wear their lungs on their backs. Well, they’re marine animals, so wear their gills on their backs.

Pretty much all nudibranchs sport external gills of some sort, often resembling feather-like structures surrounding their anuses. If this sound like a questionable design, it appears to work. These anal gills may be enhanced by skin ruffles or other appendages that maximize their oxygen-absorbing surfaces.

nudibranch colors
Although most abundant in tropical waters, nudibranchs are found pretty much worldwide. This Dendronotus robustus, making its living on hydroids, was photographed at Cape Ann, Mass.


Flamboyant nudibranch colors are designed to call attention to the little guys – and the hazards of dining on them. Some manufacture their noxious compounds themselves but many s have a real talent for absorbing the compounds given off by prey – like some sponges – that use toxins for their own defense.

In some species the compounds absorbed are literally toxic to predators. In others, they merely render the nudibranch in question thoroughly foul-tasting. Surprisingly, the nudibranchs themselves are immune to the .

Rather, after ingesting the toxins, they store them in their mantles, awaiting anyone who tries to ingest them.  Some species are able to emit a toxic mucus when disturbed.

nudibranch defenses
The cerata on nudibrnachs like this Indo-Pacific specimen, Pteraeolida ianthina, serve multiple functions, including respiration, digestion – and storage for nematocysts to discourage attackers.


Some nudibranch species take non-chemical  approach, ingesting provocative things and storing them for defensive uses. Rather than toxins, their weapons of choice are nematocysts, the harpoon-like stingers utilized by cniderians like sea anemones and stinging hydroids.

Somehow, without harming themselves, intrepid  nudibranchs transfer ingested nematocysts into their mantle, fully intact and ready to greet attackers.


One other detail: Not all nudibranchs depend on chemical or ballistic defenses. In some cases, nudibranch colors serve as camouflage, aiding them in hiding out on the ocean bottom.

And some nudibranchs simply rely on mimicry. They’re not distasteful or toxic at all. They just make a point of looking like their nudibranch fellow travelers who are.

PRINCIPAL SOURCES: “Nudibranchs of the Northeast,” Jerry Shine;  Reef Creature Identification, Florida,   Caribbean, Bahamas, Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; Marine Life of the North Atlantic, Andrew Martinez; “Nudibranchia,” Oxford Dictionary of Zoology; “About Nudibranchs,” National Geographic; “Nudibranchs: Armed & Fabulous,” Hakai Magazine; “Nudibranchia,” Encyclopedia of Life; “Nudibranch,” Wikipedia.