Sea slugs are not a single species but a diverse collection of shell-less snails

Nature · Published 3 October 2026 Swipe more like this →

In short: Often mistaken for simple creatures, sea slugs are a fascinating, varied group of marine gastropods. From solar-powered slugs that photosynthesize to those that weaponize the stinging cells of their prey, these shell-less mollusks have evolved extraordinary survival strategies across the world's oceans, proving that losing a shell is a brilliant evolutionary move.

The term sea slug is a common label for various marine gastropods that have either lost their shells entirely or possess significantly reduced, internal ones. This trait has evolved independently multiple times across different lineages, including nudibranchs, sea hares, and sacoglossans. While they share a common ancestry with sea snails, these creatures have adapted to life without the protection of a traditional shell, often developing vibrant colors to warn predators of their toxicity or foul taste.

Their biology is as diverse as their appearance. Many species, such as the nudibranchs, utilize cerata—feathery structures on their backs—for both gas exchange and defense. Some, like Glaucus atlanticus, ingest stinging cells from prey like the Portuguese man o' war and store them in their own bodies to deter attackers. Others, such as the lettuce sea slug, engage in kleptoplasty, absorbing chloroplasts from algae to photosynthesize sugars, effectively becoming solar-powered. These animals also possess radulas, which are small, razor-sharp teeth used for feeding on specific prey like jellyfish, bryozoans, and sea anemones.

Sea slugs are not just passive drifters; they exhibit complex behaviors and physical adaptations. Species like the California black sea hare can grow up to 75 centimeters in length and weigh 14 kilograms, while others, like the headshield slugs, use shovel-shaped heads to burrow into the sand. Some have even transitioned into freshwater or onto land. Despite their soft bodies, they are neurologically capable, with some species possessing brains containing thousands of nerve cells, such as the 20,000 cells found in Aplysia californica.

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