Nudibranchs: Beautiful but Dangerous Marine Creatures

by | Nov 15, 2007 | 0 comments

In 1943 Jacques-Yves Cousteau used aqualung on French south coast for the fist time in the history. From that very moment, both scientists and divers were able to admire and discover the wonders of the sea, unachievable for them until then. Among the fascinating creatures that live under the sea, nudibranchs are the most stunning of all of them due to their incredible colours, their beauty and their exotic forms. Also are known as sea slugs and butterflies of the sea due to their undulating way of swimming. They are considered the most beautiful creatures of the sea and as they move very slowly, they are very popular among submarine photographers. Exactly, thanks to these submarine photographers, nudibranchs are so famous and admired by divers and marine naturalists.


Berghia coerulescens (right), Chromodoris luteorosea (left)

Nudibranchs are invertebrates from the Phylum Mollusca, which major groups are clams (Class Bivalvia), squid and octopus (Class Cephalopoda) and snail and slugs (Class Gasteropoda). There are more than 7500 molluscs alive today and they arouse on Earth more than 600 millions years ago. During evolution gastropods went through a torsion process that twists the animal head and foot counterclockwise by 180°. So a new anatomical configuration was reached and then the mantle cavity, anus, and gill were positioned over the head, on the unique shell opening. Class Gastropoda is divided on three subclasses and Opistobranchia (marine slugs) exhibits a major evolutionary trend, the reduction or loss of the shell and a new distortion, aas in Order Nudibranchia. There are more than 3000 different species of nudibranchs all over the world. Although the shell is present during their larvae stage, disappears during metamorphosis, so when adults shell is not present. Nudibranchs have gills totally “naked” and secondary respiratory structures called cerata. In fact, the word “nudibranch” comes from Latin nudus meaning “naked”, and Greek brankhia meaning “gills“. Due to their shell lack, they have not been very known through the history, and actually, Linné in his 12th edition of “Sistema Naturae” on 1767 just described 17 species. On 1817 Cuvier described nine new genera and set up the group with that name, Nudibranch. The Englishmen Alder and Hancock created systematic basis of the group at the end of XIX century. But only last years, and thanks to the submarine technology development, study this fascinating animal group has been possible.


Chromodoris krohni (left, right)

Anatomy and more

Nudibranchs are little marine animals between 3 mm to 15 cm long, though some species can reach 30 cm. They can be very different in shape, but generally are elongated and symmetrical. They are really ephemeral; normally the life span for a nudibranch is one year or less, though some more evolved species can live 3 years. They are benthonic species, living in nearly every marine bottom and can be found all over the world, from Antarctic to tropics, even in our coast. This group can be divided into two main groups, Dorids, with just two structures on its back, a pair of rhinophores in the top of the head and a tuff of feathery gills on the back, and Aeolids, also with rhinophores but with the back covered with numerous finger-like appendages called cerata. Their small eyes are located below the tissue of the animal on the head near the brain, and they cannot see actual images but only take measures of light intensity. As every molluscs, nudibranchs have a file like organ on the month called radula, used to scrap off their food. This organ is covered on top with rows of chitinous teeth, which are produced by the radula sac as they are warned down. But the most distinctive characteristic is the pair of tentacles on the head, the rhinophores, chemical sensors used to smell and taste that can be retracted for protection. Dorids have a branchial plume surrounding the anus. These “naked” gills are very irrigated so gaseous exchange occurs. Aeolids have not gills, but cerata function as gills. Cerata are branching extensions of the digestive system with three functions, breath, digestion and defence. The cerata epidermis is so thin that gaseous exchange can occur.


Chromodoris purpurea (right), Discodoris atromaculata (left)

Opisthobranchs are carnivorous feeding on sponges, cnidarians (anemones, soft corals, hydroids, etc.), tunicates, ascidia, bryozoans, and sometimes on algae and even on others nudibranches. Each specie tends to be very selective feeders and may only eat one certain prey species. Thanks to their capacity to digest preys that often are toxic, as sponges or anemones, and even to be able to use these toxic metabolites for their own defence, nudibranchs have been able to feed on those dangerous preys avoiding competence of other predators that will dye due to preys’ defence systems. But nudibranchs also have enemies, like spider crabs, some other kind of crabs and fishes. They also can suffer from parasitic copepods.


Crimora papillata

Sea slugs are simultaneous hermaphrodites, which means that they possess both male and female sex organs at the same time, though auto-fecundation is rare. This strategy increases the probability of finding a mate, since nudibranchs are solitaries and they use to be scattered. Copulation is usually reciprocal, so both function simultaneously as male and female, giving and receiving sperm and both partners will lay eggs. The genitalia are located at the right side of the animal, so for copulating they position themselves opposite side-by-side, subsequently the right sides of their bodies are next to each other. After mating sea slugs lay their spiral egg masses on or near the prey they feed on. Normally eggs are white, though the colour could change and be pink, orange or red depending on diet. Egg development can take between 5 and 50 days, depending on temperature. Usually from eggs arise little planktonic larvae, called veliger, with shell, which will lose during metamorphosis, when they begin their benthonic life as adult.

Sophisticate defensive systems

In most molluscs the shell is the primary defence in which the animal can retreat from danger. But shells are heavy and make difficult to move and slow down growth. So the loss of the shell gave them freedom to evolve spectacularly shaped bodies, they could growth more rapidly and move faster. But sea slugs had to evolve a series of strategies to protect themselves from predation. Those amazing, effective and sophisticated defence systems not only reduce death in direct predators’ attacks but also have dissuasive effect, which means that enemies that had attack a nudibranch before will not do it again because they remember that bad experience. These defence systems, real chemical and biological weapons, could have been either a post-adaptation to shell loss, either a pre-adaptation that made the shell loss possible.


Discodoris rosi (right), Doriopsilla areolata (left)

One of the easiest defence strategies is escape. Some nudibranchs have evolved muscular foot that can secrete mucus to be able to escape, sometimes even swimming, though never very quickly. But they also present some other defences like tubercles that vary in size, shape and number and calcareous spicules in the mantle tissue that can heart the enemy. There are a number of species that have evolved the ability to drop parts of their body, which can be rapidly regenerated, to confuse predators and escape. This behaviour is called autotomy.

Other species have developed colour patterns entirely for defence against predators, mainly fishes. The first line of defence is to try to become invisible. In some cases both the texture and the colour of the body of the nudibranch match the colour and the texture of the prey or the substrate they live on. It is called cryptic coloration. Nudibranchs can obtain the pigments from their diets, though sometimes they can produce pigments themselves. Their translucent body permit the digestive system to be seen and also nutrients that travel through it. In many cases, slugs can vary their colour as they change the food source.



Some other nudibranchs have a bright and spectacular colour patterns. These colours are used to warm potential predators that these animals are full of distasteful chemicals and not worth to attempting to eat. This is known as aposematic colouration. Even have the ability to change their colours when danger. Normally these slugs contain distasteful or even toxics compounds and one a fish has attacked one nudibranch it is not likely to forget this odd experience, which will correlate with this colour pattern, and will not attack it again. Nudibranchs can obtain the pigments from their diets, sponge or cnidarians, or even they can produce de novo themselves. Sometimes different species, both toxics, have evolved similar colour patterns to “help” predators to remember their danger. This is known as Müllerian mimicry. But in other cases, a non-toxic specie evolved the toxic specie’s colour pattern to confuse and to avoid been attacked by predator. This is known as Batesian mimicry.


Flabellina babai (right), Janolus cristatus (left)

But usually nudibranchs have a chemistry or biological active defence. Sometimes slugs possess skin glands that secret toxic products, like sulphuric acid. In some cases these compounds are known to be toxics and can kill animals near them, but in other cases they are not so toxics but still enough repellents to confuse predators. But the sophisticatest defence system is known cleptodefence. Slugs are evolved to be immune against other invertebrate defences, swallow them and transport them along tracts to the cerata, where they are used for defence, being replaced every few days. Some species are able to use really toxic compounds from sponges and accumulate them in some glands ready to be used if is in danger. Some others, like aeolids, fed on cnidarians (hydrozoans, corals, anemones, jellyfish, etc.) that content cnidocytes, sting and toxic cellules with nematocysts used to defence themselves from predators. To avoid them, some aeolids are able to secrete mucus, which change with cnidarian species, and that inhibits nematocyst discharge. Other species acquire the mucus from the cnidarian and trick their prey. Once the aeolids eat them, cnidocystes are transport and stored in cnidosacs in the tip of their dorsal cerata ready to be used.

Defence compounds of nudibranchs present biological activity, and are being investigated to be used against some really dangers disease like cancer.

Nudibranchs in our aquariums?

These colourful and exotic creatures are categorically inappropriate for casual keeping. It is more advisable to admire these animals from the photographs that divers take directly under the sea.

Most sea slugs are either very difficult or impossible to keep in small private aquariums. They are very delicate to ship and sensitive to acclimation. Their diets are very specific, and, as much species are toxics, after dying, there is a very real danger that, due to their decomposition, some toxic compounds could be released into the aquarium and disturb or even kill other organisms.


Eubranchus farrani (right), Flabellina pedata (left)

If these advices do not frightened the future keeper, a long quarantine and then to limit their number and mass in the system are recommended. Normally, nudibranchs are peaceful and, because sea slugs are toxic animals, usually they are very active by day. They are not gregarious and, as they are hermaphrodites, there will not be any problem to find the right mate. But before buying our specimen it is of paramount importance to know all its needs, especially diet, and buy it on or with the food it eats, that can be algae, sponge, anemone, coral, worms, etc. It is needed too to be able to find its food or to culture it. Also it is essential to select for a healthy and active specimen, because a sick one will die and dissolve quickly and release its toxic compounds in our aquarium.


Hypselodoris cantabrica (left, right)

Once the healthy specimen has been chosen, the animal must be quarantined at least for one month. In isolation, information about the animal, specially its diets, must be verified. Nudibranchs are able to live long time, weeks even months, without eating, so if you cannot confirm that the animal is eating during quarantine, do not send the animal to the aquarium. Many aquarists on seacoasts may be able to collect nudibranchs, which is further better than ship and stress the animals. Above all, you will have the opportunity to verify the specific food they eat and collect together, and also pick up more in the future. But do not forget to investigate local regulations first.


Hypselodoris tricolor

Little is known about nudibranchs and even less about their diets. Some of them are fed on sponge, others on anemones, even corals, and few on algae. A lot of sea slugs are very specific in their diets and just eat one sponge or cnidarian’s specie. So before buying a nudibranch, their diet must be known, and you must be able to give it to the animal, buying the prey or culturing it, or do not buy nudibranchs at all. In recent years, Scientifics have discovered that some nudibranch species are fed on nuisance organisms in the aquarium, like Berghia verrucicornis that eats pest anemone Aiptasia, Chelinodura sp. that eats flatworms, or Lettuce slugs Elysia spp. that eats algae.


Polycera quadrilineata

Due to its extraordinary diversity and distribution in habits of the world’s sea, there are not standardized water parameters for the sea slugs and they are very different for each specie. For tropical species, for example, stable and high Redox, high-saturated oxygen levels, and very stable salinity are required. Medications in the form of metals, organics dyes and other products like some kind of antibiotics are not safe to use.

Beyond any issues of stress from feeding, compatibility or acclimation, we will find that sea slugs’ life expectancy is really short and they will die within months naturally. Normally they are collected as adult and few live more than a year. After their sudden death, their toxic compounds will be released on our aquarium. To reduce the danger, good chemical filtration like carbon changed weekly, adequate water flow, skimmer and ozonation, and huge water changes will be needed.

So, it is understandable that there are many reasons for leaving these beauties in the sea.


  1. Barnes, R.D. 1983. Zoología de los invertebrados. Ed. Interamericana.
  2. Bertsch, H. Nudibranchs: marine slugs with verve.
  3. Benkendorff, K. 2000. Molluscan medicines. Nature Australia. Winter 200, pp. 50-57.
  4. Calfo, A.; Fenner, R. 2003. Reef invertebrates. An essential guide to selection, care and compativility. Reading trees & Wet Web Media Publications.
  5. Clavin, J.C. Nudibranquios. La belleza del ataque.
  6. Cousteau, J.Y.; Dumas, F. 1959. Un mundo silencioso. Ed. Éxito, SA.
  7. Cimino, G.; De Rosa, S.; Se Stefano, S.; Sodano, G.; Villani, G. 1983. Dorid nudibranch elaborates its own chemical defense. Science, vol. 129, pp. 1237-1238.
  8. Debelius, H. 1996. Nudibranchs and sea snails. Indo-Pacific field guide. Ed. IKAN.
  9. Doe, J. 2005. Adaptations for defense by the nudibranch Aeolidia papillosa. Biol., vol 105.
  10. Faulkner, D.J.; Ghiselin, M.T. 1983. Chemical defense and evolutionary ecology of dorid nudibranchs and some other opistobranch gastropods. Marine Ecology, vol. 13, pp. 295-301.
  11. Ghazali, S. 2006. Displays of defense: behavioural differences in antagonist avoidance four opisthobranch molluscs. Water Resource Archives, Biology and Geomorphology of Tropical Islands (ESPM 107/IB 158).
  12. Grassé, P.P.; Poisson, R.A.; Tuzet, O. 1985. Zoología. I-Invertebrados. Ed. Masson.
  13. Greenwood, P.G.; Garry, K.; Hunter, A.; Jennings, M. 2004. Adaptable defense: a nudibranch mucus inhibits nematocyst discharge and changes with prey type. Biol. Bull., vol 206, pp. 113-120.
  14. Hall, S.; Todd, C.D. 1986. Growth and reproduction in the Aeolidia papillosa (L.). J. Moll. Stud., vol. 52, pp. 193-205.
  15. Harrigan, J.F.; Alkon, D.L. 1978. Larval rearing, metamorphosis, growth and reproduction of the eolid nudibranch Hermissenda crassicornis (Eschscholtz, 1831) (Gasteropoda: Opisthobranchia). Biol. Bull., vol. 154, pp. 430-439.
  16. Karisson, A.; Haase M. 2002. The enigmatic mating behaviour and reproduction of a simultaneous hermaphrodite, the nudibranch Aeolidiella glauca (Gastropoda, Opisthobranchia). Can. J. Zool., vol. 80 (2), pp. 260-270.
  17. Mayer, A.M.S.; Gustafson, K.R. 2003. Marine pharmacology in 2000: antitumor and cytotoxic compounds. International Journal of Cancer, vol. 105, issue 3, pp. 291-299.
  18. Mebs, D. 1985. Chemical defense of a dorid nudibranch, Glossodoris quadricolor, from the red sea. Journal of Chemical Ecology, vol. 11 (6), pp. 713-716.
  19. Menendez, J.L.
  20. Nakumara, K. 2007. Especies bentónicas de Opisthobranchia (Mollusca: Gastropoda) presentes en el litoral del norte peruano. Reu. Peru. Biol.., vil. 13, pp. 255-257.
  21. Sisson, C.G. 2005. Life history dynamics and biogeography of a nudibranch with contrasting development models: A hypothesis for the evolution of larval types. Journal of Natural History, vol. 39 (20), pp. 1719-1733.
  22. Todd,C.D.; Doyle, R.W. 1981. Reproductive strategies of marine benthonic invertebrates: A settlement-timing hypothesis. Marine Ecology, vol. 4, pp. 75-83.


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