What is a Sea Slug ?

Sea slugs are marine gastropod molluscs, mostly heterobranchs, characterised by having a soft body and a shell that is absent, greatly reduced or internalised in the adult stage. The term “sea slug” is a common name based on morphology and used for general science communication, rather than a formal taxon, and encompasses several evolutionary lineages that exhibit a great diversity of forms, anatomy and lifestyles.

Several groups are commonly known as sea slugs. The best-known are nudibranchs, which include both dorid nudibranchs, often with a broad, flattened body, and aeolid nudibranchs, many of which have numerous finger-like projections along their backs. Sacoglossans are another fascinating group, with some species resembling tiny green leaves. Sea hares are generally larger animals with soft, broad bodies. Cephalaspideans include head-shield slugs and bubble snails, while pleurobranchs, or side-gilled slugs, form another distinctive group. Smaller groups, such as runcinids, also contribute to this diversity. Although these animals share a general slug-like appearance, they differ considerably in their anatomy, ecology and evolutionary history.

In many sea slug lineages, development begins with a tiny larva that bears a small shell. During metamorphosis, as the larva transforms into a juvenile, this shell is lost or reduced, depending on the group. In other lineages, a shell may persist as an internal or greatly reduced structure. These differences reflect the diverse evolutionary pathways through which marine gastropods have adapted to life with a reduced or absent external shell.

One of the most striking features of sea slugs is their extraordinary variety of colours and shapes. Some display intense hues, surprising colour combinations or silhouettes reminiscent of small branches, leaves or feathers. Others, by contrast, go almost unnoticed because they blend in with the algae, sponges or other organisms on which they live. Some species are so well camouflaged that finding them requires a keen eye, even when they are right in front of us. This diversity makes observing sea slugs particularly rewarding, but it also means that many species are difficult to find despite their size.

Their diet is equally varied. Depending on the species, sea slugs may feed on algae or other marine organisms, including sponges, hydroids, bryozoans, sea squirts and anemones. Many have highly specialised diets and depend on particular organisms for food. For example, many dorid nudibranchs feed on sponges, numerous aeolid nudibranchs prey on cnidarians such as hydroids, sacoglossans feed on algae, and sea hares generally consume algae. Knowing which organisms inhabit a particular area can therefore help us discover which sea slugs might live there.

Sea slugs can be found in seas around the world, from polar regions to tropical waters. They occupy a wide range of habitats, including coastal and intertidal zones, rocky seabeds, algal communities, seagrass meadows, and sandy or muddy bottoms. Some species live among sponges or other invertebrates, while others are associated with particular algae or marine plants. There are also species that inhabit deep-sea environments, far beyond the depths normally explored by recreational divers. In addition, some related gastropods live in the open ocean, drifting in the water column rather than crawling along the seabed.

Their size varies considerably. Some species measure only a few millimetres and can easily go unnoticed, whereas others, such as sea hares, can grow much larger. Observing the smallest species often requires patience, careful searching and the help of macro photography. Even a small patch of algae or a single sponge can harbour several different species.

Their biology also includes some surprising phenomena. For example, some sacoglossans can retain chloroplasts obtained from the algae they feed on within their own cells. This phenomenon, known as kleptoplasty, allows these chloroplasts to remain functional for a period of time and, in some species, contribute to the animal’s metabolism. It is one of the most remarkable examples of the unusual relationships that sea slugs can establish with their food and environment.

Studying sea slugs allows us to discover a fascinating part of marine biodiversity that is often unknown even to people who regularly visit the coast. Observing them carefully helps us better understand the richness of marine ecosystems and appreciate the extraordinary diversity of organisms hidden among algae, rocks and other inhabitants of the seabed.

References

• Ballesteros, M., Hopkins, A., Salicrú, M., & Nimbs, M. J. (2025). Heterobranch sea slugs s.l. (Mollusca, Gastropoda) from the Southern Ocean: Biodiversity and taxonomy. Diversity, 17(5), Article 330. https://doi.org/10.3390/d17050330
• Cruz, S., & Cartaxana, P. (2022). Kleptoplasty: Getting away with stolen chloroplasts. PLOS Biology, 20(11), e3001857. https://doi.org/10.1371/journal.pbio.3001857
• Krug, P. J., Vendetti, J. E., Ellingson, R. A., Trowbridge, C. D., Hirano, Y. M., Trathen, D. Y., Rodriguez, A. K., Swennen, C., Wilson, N. G., & Valdés, Á. A. (2015). Species selection favors dispersive life histories in sea slugs, but higher per-offspring investment drives shifts to short-lived larvae. Systematic Biology, 64(6), 983–999. https://doi.org/10.1093/sysbio/syv046
• Wägele, H., & Klussmann-Kolb, A. (2005). Opisthobranchia (Mollusca, Gastropoda)—more than just slimy slugs. Shell reduction and its implications on defence and foraging. Frontiers in Zoology, 2, Article 3. https://doi.org/10.1186/1742-9994-2-3
• Wägele, H., Klussmann-Kolb, A., Verbeek, E., & Schrödl, M. (2014). Flashback and foreshadowing—a review of the taxon Opisthobranchia. Organisms Diversity & Evolution, 14, 133–149. https://doi.org/10.1007/s13127-013-0151-5

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