Sea slugs,
Order of nudibranchs (Nudibranchia)
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These marine gastropod molluscs belong to the opisthobranch family. They are benthic organisms (they live on the substrate at the bottom of the oceans). Please note that they are not the aquatic equivalent of land slugs. Nudibranchs are sea slugs, but not all sea slugs are nudibranchs: there are other orders whose common name is “sea slugs”. They leave behind a trail of mucus with a very distinctive chemical and optical signature1,2.
Their gills are naked, hence their name: from the Latin nudus, meaning “naked”, and the ancient Greek βράγχια, brankhia, meaning “gills”.
There are now more than 4,700 known species of nudibranchs3-5, but there are certainly many more yet to be discovered! They are highly diverse and often colourful. The structure of their colours is complex6 and sometimes reflects their chemical defences7. some have effective camouflage to blend in with the substrate on which they live, many are fluorescent8 and some are even bioluminescent9! They measure from a few centimetres to 60 centimetres for the largest.
They are found in all the world's seas, from the Arctic to the Antarctic, including temperate and tropical waters and the Mediterranean, from coastal areas to the depths of the ocean10-12. They most often live in the euphotic zone (the zone where there is sufficient light for photosynthesis, usually up to 200 m in the open sea) but can live at depths of up to 700 m. One species has even been discovered living at depths of up to 4,000 m: Bathydevius caudactylus, described in 2024, which is also one of the few bioluminescent species9.
Nudibranchs’ Anatomy
Nudibranchs have poor eyesight and can only distinguish between areas of light and dark, but they have rhinophores5,13, two extensions on their heads, a sensory organ with chemical receptors that enable them to detect other nudibranchs or food14. They can retract them to protect themselves in case of danger!
Some species living close to the surface live in symbiosis with photosynthetic algae: zooxanthellae5,15-17, which are also found in corals. These algae perform photosynthesis, providing energy to their hosts in exchange for support, constituting an evolutionary advantage for the survival of individuals5,18. Other nudibranchs prefer to associate with bacteria19.
Schema. anatomic description of nudibranchs (after Dean and Prinsep, 2017).
4 big morphotypes groups
· Doridina : a flattened body and the presence of a gill plume at the rear of the mantle
Ex : Chromodoris willani, Goniobranchus geminus
· Dendronotina : The gill appendages are located at the periphery of the mantle.
Ex : tritonia festiva
· Aeolidiina : a varying number of papillae (‘cerata’) on the back, which serve as gills, extensions of the digestive gland and poison glands for defence (presence of cnidocytes that store stinging cells).
Ex : Flabellina iodinea
· Arminina : the surface of the mantle is wrinkled (with or without dorsal appendages), the retractable rhinophores and gills are located under the mantle in its anterior part.
Ex : Armina occulta
They live between a few weeks and a year depending on the species, environmental conditions and predation pressure. They are hermaphroditic organisms (they have both male and female reproductive organs). They lay large quantities of eggs20, in spirals or ribbons on the marine substrate. These eggs are often coloured. In the absence of fertilisation, they are capable of parthenogenesis, whereby the eggs develop into clones. After the eggs hatch, the larval stage (veliger) begins. At this stage, the larva is microscopic and planktonic (it floats in the water column) and moves using small cilia, feeding on tiny organisms21. It then attaches itself to a substrate and becomes a juvenile nudibranch, undergoing a metamorphosis that gives it the characteristics of a miniature nudibranch and causes it to lose its larval characteristics. It loses its shell after the larval stage. The juvenile then continues to grow, feeding on prey suited to its size and acquiring the adult characteristics of its species (size, colour, etc.). Once adult, it reproduces and the cycle repeats itself.
Most nudibranchs are carnivorous. Although it was long believed that all species were carnivorous, recent studies show that some species are herbivorous22. Their radula (the cartilaginous lamellae on which ‘teeth’ are found, varying greatly in number and shape depending on the species)23 is very powerful and allows them to feed by scraping the food below them. Each species has specific food preferences, and their diet consists of sponges, bryozoans, ascidians, hydroid polyps and sometimes even other nudibranchs. Many of these animals are often toxic, and numerous species are able to recycle the toxins in specialised cells to protect themselves from their own predators24-27, particularly species in the Aeolidiina group, which store them in their cerata. These stinging cells are an effective means of defence against potential predators. Their main predators are fish, crabs, anemones, starfish and even other nudibranchs.
Bibliography
1. Ryogo Takano & Euichi Hirose. Optical Properties of Body Mucus Secreted from Coral Reef Sea Slugs: Measurement of Refractive Indices and Relative Absorption Spectra. Zoological Studies 無, (2024).
2. Stuij, T. et al. Exploring Prokaryotic Communities in the Guts and Mucus of Nudibranchs, and Their Similarity to Sediment and Seawater Microbiomes. Curr Microbiol 80, 294 (2023).
3. Bouchet, P. & Gofas, S. MolluscaBase eds. (2025). MolluscaBase. Nudibranchia. World Register of Marine Species (WORMS) (2018).
4. Cheney, K. L. & Wilson, N. G. Nudibranchs. Current Biology 28, R4–R5 (2018).
5. Dean, L. J. & Prinsep, M. R. The chemistry and chemical ecology of nudibranchs. Nat. Prod. Rep. 34, 1359–1390 (2017).
6. Humphrey, S. et al. Nudibranch color diversity shares a common origin in guanine photonic structures. preprint https://doi.org/10.1101/2025.09.06.674434 (2025) doi:https://doi.org/10.1101/2025.09.06.674434.
7. Van Den Berg, C. P. et al. Chemical defences indicate distinct colour patterns with reduced variability in aposematic nudibranchs. Preprint at https://doi.org/10.1101/2023.01.30.525844 (2023).
8. Betti, F., Bavestrello, G. & Cattaneo-Vietti, R. Preliminary evidence of fluorescence in Mediterranean heterobranchs. Journal of Molluscan Studies 87, eyaa040 (2021).
9. Robison, B. H. & Haddock, S. H. D. Discovery and description of a remarkable bathypelagic nudibranch, Bathydevius caudactylus, gen. et. sp. nov. Deep Sea Research Part I: Oceanographic Research Papers 214, 104414 (2024).
10. Chavanich, S., Viyakarn, V., Sanpanich, K. & Harris, L. G. Diversity and occurrence of nudibranchs in Thailand. Mar Biodiv 43, 31–36 (2013).
11. Moles, J. et al. Giant embryos and hatchlings of Antarctic nudibranchs (Mollusca: Gastropoda: Heterobranchia). Mar Biol 164, 114 (2017).
12. Valdés, Á., Lundsten, L. & Wilson, N. G. Five new deep-sea species of nudibranchs (Gastropoda: Heterobranchia: Cladobranchia) from the Northeast Pacific. Zootaxa 4526, (2018).
13. Murphy, B. F. & Hadfield, M. G. Chemoreception in the nudibranch gastropod Phestilla sibogae. Comparative Biochemistry and Physiology Part A: Physiology 118, 727–735 (1997).
14. McCullagh, G. B., Bishop, C. D. & Wyeth, R. C. One rhinophore likely provides sufficient sensory input for odour-based navigation by the nudibranch mollusc, Tritonia diomedea. Journal of Experimental Biology jeb.111153 (2014) doi:10.1242/jeb.111153.
15. Burghardt, I. Symbiosis between Symbiodinium (Dinophyceae) and various taxa of Nudibranchia (Mollusca: Gastropoda), with analyses of long-term retention. Organisms Diversity & Evolution 8, 66–76 (2008).
16. Mizobata, H. et al. The highly developed symbiotic system between the solar-powered nudibranch Pteraeolidia semperi and Symbiodiniacean algae. iScience 26, 108464 (2023).
17. Wagele, M. Observations on the histology and photosynthetic performance of ‘solar-powered’ opisthobranchs (Mollusca, Gastropoda, Opisthobranchia) containing symbiotic chloroplasts or zooxanthellae. Organisms Diversity & Evolution 1, 193–210 (2001).
18. Silva, R. X. G., Cartaxana, P. & Calado, R. Prevalence and Photobiology of Photosynthetic Dinoflagellate Endosymbionts in the Nudibranch Berghia stephanieae. Animals 11, 2200 (2021).
19. Zhukova, N. V., Eliseikina, M. G., Balakirev, E. S. & Ayala, F. J. Multiple bacterial partners in symbiosis with the nudibranch mollusk Rostanga alisae. Sci Rep 12, 169 (2022).
20. Harrigan, J. F. & Alkon, D. L. LARVAL REARING, METAMORPHOSIS, GROWTH AND REPRODUCTION OF THE EOLID NUDIBRANCH HERMISSENDA CRASSICORNIS (ESCHSCHOLTZ, 1831) (GASTROPODA: OPISTHOBRANCHIA). The Biological Bulletin 154, 430–439 (1978).
21. Thompson, T. E. Feeding in nudibranch larvae. J. Mar. Biol. Ass. 38, 239–248 (1959).
22. Camps-Castellà, J., Ballesteros, M., Trobajo, R., Pontes, M. & Prado, P. Not all nudibranchs are carnivorous: trophic ecology of Polycerella emertoni in the Ebro Delta. Mar. Ecol. Prog. Ser. 645, 67–82 (2020).
23. Nybakken, J. & McDonald, G. RESPECT TO THE RADULA. Malacologia 20, 439–449 (1981).
24. Cheney, K. L. et al. Choose Your Weaponry: Selective Storage of a Single Toxic Compound, Latrunculin A, by Closely Related Nudibranch Molluscs. PLoS ONE 11, e0145134 (2016).
25. Faulkner, D. & Ghiselin, M. Chemical defense and evolutionary ecology of dorid nudibranchs and some other opisthobranch gastropods. Mar. Ecol. Prog. Ser. 13, 295–301 (1983).
26. Greenwood, P. G. Acquisition and use of nematocysts by cnidarian predators. Toxicon 54, 1065–1070 (2009).
27. Haber, M. et al. Coloration and Defense in the Nudibranch Gastropod Hypselodoris fontandraui. The Biological Bulletin 218, 181–188 (2010).
January 2026
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