Sea slug

Lore & Background

Sea slugs are a paraphyletic set of marine gastropods that have either entirely lost their shells or have significantly reduced or internal shells. This loss has evolved many times independently within the class Gastropoda. The most familiar are nudibranchs, which have no shell at all and are often brightly colored. Other groups called sea slugs include sacoglossans (sap-sucking slugs), headshield slugs, sea hares, and pteropods (sea butterflies and sea angels). Some sea slugs, like the pulmonate family Onchidiidae, are air-breathing and shell-less. Many sea slugs have striking colors that warn predators of toxic stinging cells (nematocysts) or foul taste. For example, the genus Glaucus stores nematocysts from its prey. The lettuce sea slug (Elysia crispata) uses kleptoplasty, absorbing chloroplasts from algae to photosynthesize. Headshield slugs use shovel-shaped heads to burrow in sand. Sea hares can eject ink or secrete slime as defense. The largest sea hare, Aplysia vaccaria, reaches 75 cm and 14 kg. They possess small, razor-sharp teeth called radulas and sensory tentacles (rhinophores) for smell. Most have feathery cerata that act as gills. All genuine sea slugs depend on a selected prey animal, including jellyfish, bryozoans, sea anemones, plankton, and other sea slugs.

Reader's Guide

Sea slugs represent a remarkable evolutionary convergence across multiple gastropod lineages toward shell loss or reduction. Their significance lies in their extreme morphological and ecological diversity, from reef-dwelling nudibranchs with warning coloration to pelagic pteropods and air-breathing pulmonates. The term 'sea slug' is not a taxonomic group but a common name applied to numerous families within Opisthobranchia and Pulmonata, reflecting independent evolutionary paths. Their legacy includes serving as model organisms in neuroscience—Aplysia californica's simple brain has been crucial for studying learning and memory. Ecologically, sea slugs are specialized predators, often coevolving with toxic prey like jellyfish and sea anemones, which they repurpose for defense. The lettuce sea slug's kleptoplasty blurs the line between animal and plant metabolism. Some acochlidian species have transitioned to freshwater, and at least one lineage moved to land, illustrating gastropod adaptability. Sea slugs also highlight the complexity of common naming: the same term covers shell-less forms, those with internal shells, and even shelled pteropods. Their bright colors and bizarre forms make them popular in marine biology and public aquariums, though their dependence on specific prey makes them challenging to study in captivity.

Did You Know?

• Most sea slugs have a pair of rhinophores on their head used primarily for the sense of smell, with a small eye at the base of each.
• The lettuce sea slug (Elysia crispata) uses kleptoplasty, absorbing chloroplasts from algae to photosynthesize sugars.
• The largest sea hare, Aplysia vaccaria, can reach 75 cm in length and weigh 14 kg.
• Some species of acochlidian sea slugs have made evolutionary transitions to living in freshwater streams.