Sea slugs are diverse marine mollusks, often referred to as Opisthobranchs, a subclass that includes the vividly colored Nudibranchs. Unlike their shelled relatives, these soft-bodied gastropods fill specialized ecological niches. Their feeding habits reflect this specialization, ranging from selective herbivory to complex, active predation. While the diet of an individual sea slug is typically narrow, often restricted to a single species or genus of prey, the group as a whole exploits nearly every major invertebrate phylum.
Primary Food Sources of Sea Slugs
The majority of sea slugs are carnivores that center their diets on sessile, or stationary, invertebrate prey. Many Dorid nudibranchs specialize in consuming Sponges (Porifera), which are chemically defended and avoided by most other marine life. These slugs utilize a wide, abrasive radula—a ribbon of microscopic teeth—to rasp away and swallow pieces of the sponge tissue. Specific species, such as Hypselodoris zephyr, feed almost exclusively on particular types of sponges, reflecting high dietary fidelity.
Another major food group consists of Cnidarians, such as Hydroids, which resemble tiny, branching plants but are actually colonies of stinging animals. Aeolid and Dendronotid nudibranchs consume these organisms, often stripping polyps from the hydroid colony. They use a specialized radula to ingest the hydroids without triggering their defensive stinging cells. Bryozoans, also known as moss animals, are preyed upon by other nudibranch groups, including Phanerobranchs like Tambja.
A significant portion of the sea slug population consists of herbivores, notably Sacoglossan sea slugs and some members of the Anaspidea (sea hares). These slugs graze on Algae and Diatoms, which are scraped from surfaces or sucked out using specialized methods. Sacoglossans, often called “sap-sucking slugs,” possess a single, sharp tooth on their radula used to pierce the algal cell wall. This allows them to suck out the cellular contents. This specialized feeding apparatus limits their diet to specific types of filamentous or siphonous algae.
Highly Specialized Predation
Beyond consuming sessile organisms, some sea slugs exhibit extraordinary predatory behaviors, actively hunting mobile or complex prey. Certain nudibranchs are “slugivores,” preying on other sea slug species or engaging in cannibalism. Others, such as Favorinus tsuruganus, specialize in oophagy (egg-eating), piercing and devouring the egg masses laid by other dorid nudibranchs.
Feeding on Cnidarians like Sea Anemones and Corals requires specialized tactics, as these prey are equipped with powerful stinging cells (nematocysts). The nudibranch Berghia stephanieae, for instance, is a dedicated specialist that feeds only on the sea anemone Exaiptasia diaphana. To overcome dangerous prey, these slugs sometimes engage in social predation, forming temporary groups to feed simultaneously, which may reduce the risk of injury.
Jellyfish and their relatives, the siphonophores, are also part of this specialized diet, hunted by pelagic species like the blue sea dragon, Glaucus atlanticus. Glaucus will nibble on the fishing tentacles of its prey, which are densely packed with potent stinging cells. These predatory acts are often the means by which the sea slug acquires its own defense mechanisms, linking diet and survival.
Biological Adaptations Linked to Diet
The unique diets of sea slugs have led to remarkable biological adaptations that allow them to utilize their food source for defense and metabolism. One such adaptation is kleptoplasty, a phenomenon seen in Sacoglossan sea slugs. After piercing an algal cell and ingesting its contents, these slugs digest everything except the chloroplasts—the organelles responsible for photosynthesis.
The stolen chloroplasts, known as kleptoplasts, are integrated into the slug’s digestive cells, where they can continue to function, sometimes for months. This allows the slug to produce energy from sunlight, functioning like a solar-powered animal and providing a nutritional boost during periods of food scarcity. Species like Elysia chlorotica are famous for this capability, which depends on consuming specific siphonous green algae.
Another sophisticated adaptation involves the sequestration of toxins and stinging cells, a process known as kleptocnidae. Nudibranchs that feed on sponges absorb and concentrate the noxious chemical compounds produced by their prey, storing them in specialized mantle glands to deter predators. Similarly, species that consume hydroids, anemones, or jellyfish do not digest the nematocysts. Instead, they transport the undischarged stinging cells through their digestive system and store them in sacs at the tips of their cerata (finger-like projections on their back), effectively stealing their prey’s weapon for their own defense.