Nudibranchs: Fantastic Mollusks Explained

Nudibranchs are hermaphroditic – each one produces both eggs and sperm. Two of them will line up side by side and fertilize each others’ eggs in an exchange of sperm. Here, a pair of Nembrotha lineolata.

THERE ARE TWO KINDS OF DIVERS: THOSE WHO ARE BONKERS ABOUT NUDIBRANCHS and those who ought to be. For those in the second category, here are some nudibranch facts.

First, as the name “nudibranch” suggests – it’s from the Greek for “naked gills” – nudibranchs absorb their oxygen from the water through external breathing structures located on their backs. There’s great variation in gill architecture among the many species of nudibranchs, from flamboyant to demure.

And they’re  not all the feathery gills that most predominate nudibranch photo albums. Some sport cerata, structures that run the full length of their backs, supporting not just respiration but digestion and a secret arsenal of stinging nematocysts.


Secondly, nudibranchs are generally small and would be hard to spot if it wasn’t for the vibrant colors many of them sport – and their penchant for hanging out in the open. The two are related. Their colors are warning labels to potential predators that they taste bad, may be toxic to anything that eats them, or may harbor stinging namatocysts within their tissues.

Unless they’re don’t and are just faking it. See “Nudibranch Colors Shout Out Nudibranch Defenses.”


1   Nudibranchs are one order of sea slugs – basically, snails that lack shells – in the Gastropod Subclass Opisthobranchia. Nudibranchs are found in Order Nudibranchia.

2   There are more than 3,000 species of nudibranchs found worldwide and in all environments – Arctic waters, deep ocean depths and shallow, tropical reefs.  Actually, they’re most abundant in shallow tropical waters.

3   With so many species and environments, it’s to be expected that there are great variations in coloration and body architecture. And, there are.

4   Typically, nudibranchs are oblong in shape, but their bodies can be flattened or thick, as small as a quarter inch or as long as a foot. Their mantles, the surface layer covering their backs, often feature ornate designs – and those colors.


5  Nudibranchs have eyes, but they’re mostly able to only differentiate light from dark. Instead, they search for prey using two chemically sensitive rhinophores, or tentacles on their heads that constantly test the water for cues to prey. They also sport tentacles near their mouths, although these oral tentacles may not be immediately obvious.

nudibranch facts
Bright yellow and four inches or more in length, Notodoris minor nudibranchs are easy to spot. This one was on the Great Barrier Reef, feeding on a sponge.

6  Almost all nudibranchs are benthic, or bottom-dwellers, crawling along the seafloor sniffing out things to eat. But the Oxford Dictionary of Zoology points out that many can swim well.


7   Nudibranchs are divided into four subgroups: Dorids, Aeolids, Dendronotids and Arminids.

nudibranch facts
The crown of external, feathery anal gills on this Jorunna funebris specimen marks it as a member of the dorid group of nudibranchs. /em>

8  Most species are found in the dorid group. Generally broad and flattened, they’re the ones with clusters of feathery gills surrounding their anal cavities.

nudibranch facts
Aeolid nudibranchs like this Pteraeolida ianthina are generally characterized by clusters of cerata along their bodies. Cerata serve multiple functions, including respiration, digestion – and storage for nematocysts to discourage attackers.

9   Aeolids are remarkable for their multitudes of elongated cerata – their version of gills that also serve digestive and defensive functions. They use them to store toxins filched from prey. They also store the nematocysts they acquire from cnidarian prey in the tips of their cerata, ready to surprise any predator that decides to sample nudibranch flesh.

10   Dendronotids tend to exhibit feathery gill structures in pairs along the length of their dorsal surfaces. Arminids tend to hide their gills underneath their mantles, which often display stripe-like lines along their lengths.


11  They’re all slugs, albeit slugs with nudibranch colors and features. Like all snails and slugs, they attack food with radulae, tooth-like rasping structures for tearing or scraping food particles.

12   Unlike algae-eating sea slugs in Order Sacoglassa, nudibranchs are carnivorous, preying on sea anemones, hydroids, sponges, corals and fish eggs.

13   Typically, each species specializes in one particular type of prey. In line with this, their radulae are adapted to their preferred victims, whether sponges, sea anemones, hydroids or others.

14   One free-swimming nudibranch, which swims upside down near the ocean surface, enjoys feasting on Portuguese Men of War.

15   Some nudibranchs are cannibalistic, known to eat other members of their own species. This may occur more in desperation than as a routine.

nudibranch facts
This twisting little guy resembles but doesn’t  exactly match a half-dozen species in genus Chromodoris, in the dorid group. It shows off its anal gills very well.


16  Lifespans among the 3,000 species vary significantly, from perhaps a month to a year. To some extent, the availability of food plays a role in this.

17   Nudibranchs are hermaphroditic – each one produces both eggs and sperm. Two of them will line up side by side and fertilize each others’ eggs in an exchange of sperm. They then lay their eggs in long ribbons, in patterns that vary by species. Although they possess both male and female sex organs, they can’t self-fertilize.

18   There is a National Sea Slug Day, observed each October 29. To celebrate it, the estimable California Academy of Sciences announced the discovery of 17 new species identified throughout the Indo-Pacific basin.

19   Although they have studied the genus for several decades, the CAS scientists were able to use advanced genetic tools to re-evaluate the Hypselodoris family tree.

nudibranch facts
A collage of the 17 newly identified nudibranch species announced by the California Academy of Science in October (one specimen is pictured twice). All are members of Genus Hypselodoris in the dorid group. Courtesy, California Academy of Sciences.

20  For the scientists, nudibranchs are a long-time thing. Of the 3,000 known species, more than 1,000 have been discovered by CAS senior curator Terry Gosliner.

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; “Scientists from the California Academy of Sciences describe 17 new species of vibrantly patterned sea slugs from the Indo-Pacific,” California Academy of Sciences; “Nudibranch,” Wikipedia.