sacoglossans

Sacoglossans are an order of small to medium-sized marine opisthobranch gastropods, commonly known as sacoglossan sea slugs. They are primarily herbivores that feed on algae, using a specialized radular apparatus to pierce algal cells and extract the contents. Many species specialize on particular algal hosts, and some are found on shallow coastal shores worldwide.

A distinctive feature of many sacoglossans is kleptoplasty, the sequestration of algal chloroplasts into their own tissues after feeding. The retained plastids can remain functional for weeks to months, enabling the slugs to perform photosynthesis and supplement their diet. This phenomenon has made sacoglossans a key focus in studies of endosymbiosis and cellular interactions between animals and plant organelles. Not all sacoglossans exhibit kleptoplasty, and the extent and duration of plastid function vary by species and algal source. Elysia chlorotica is one of the best-known examples of long-lasting kleptoplasty.

In terms of morphology, adults generally lack a large external shell, though a small internal shell is present in some lineages. Body forms range from slender to leaf-like, with a broad foot and often a mantle skirt or dorsal projections depending on the species. Reproduction is hermaphroditic, and individuals commonly exchange sperm before laying gelatinous egg masses that hatch into free-swimming larvae or direct-developing juveniles, subject to the species’ life cycle.

Sacoglossans inhabit a wide range of temperate and tropical marine environments, from intertidal to subtidal zones, where their algal prey are abundant. They are studied for their ecological roles as herbivores and for the intriguing symbiotic relationships enabled by kleptoplasty.