The Elysia sea slug, a sacoglossan mollusk, defies the rigid boundary between the animal and plant kingdoms. Nicknamed the “solar-powered sea slug,” this unique marine gastropod harnesses energy from sunlight. Its biology involves a complex interplay of theft and cellular integration, allowing it to mimic the function of a plant.
Identification and Habitat
The most well-studied species, Elysia chlorotica, commonly known as the eastern emerald elysia, is a small, leaf-shaped mollusk that typically grows to a length of 1 to 6 centimeters. Its bright green coloration is not innate but is acquired from its diet, providing natural camouflage against the algae in its environment. This species is found along the Atlantic coast of North America, inhabiting salt marshes, tidal pools, and shallow creeks from Nova Scotia down to Florida.
The slug’s physical form aids in its unconventional energy strategy, featuring a body with large, ruffled lateral folds called parapodia, which can be unfurled to maximize surface area exposure to sunlight. The specialized diet of Elysia chlorotica consists almost exclusively of the filamentous, yellow-green alga Vaucheria litorea.
Kleptoplasty: The Stolen Power Source
The term for the slug’s unique ability is kleptoplasty, which literally means “to steal” (klepto) and “plastids” (plasty), the group of organelles that includes chloroplasts. The slug feeds on the algal filaments by puncturing the cell wall with a specialized, single-row tooth called a radula, then sucking out the cellular contents. During this process, the slug’s digestive system selectively destroys the algal cell nucleus and other components, but isolates and retains the photosynthetically active chloroplasts.
Once sequestered, these stolen chloroplasts, or kleptoplasts, are incorporated into the cells lining the slug’s highly branched digestive diverticula, which spread throughout its body. This integration is not merely storage, as the kleptoplasts remain structurally intact and photosynthetically active for an extended period, sometimes for up to ten months in laboratory conditions. The resulting photosynthesis provides the slug with fixed carbon products, essentially turning it into a photoautotroph that can survive long periods of starvation by harnessing light energy.
The Mechanism of Chloroplast Retention
The long-term function of the kleptoplasts is particularly puzzling because chloroplasts require hundreds of proteins for maintenance, most of which are encoded in the original algal cell’s nucleus. When the slug consumes the algae, the algal nucleus is digested, leaving the chloroplasts without their genetic support system. Early research suggested that the slug solved this problem through Horizontal Gene Transfer (HGT), incorporating an algal nuclear gene, psbO, into its own genome to produce a protein necessary for Photosystem II repair.
However, this simple explanation has been complicated by subsequent genomic studies, which failed to find widespread algal gene transfer into the slug’s germline DNA. The current understanding suggests that the exceptional stability of the Vaucheria litorea chloroplasts themselves may be a major factor, as they appear to be inherently robust. The slug’s digestive cells are thought to provide necessary proteins and a favorable cellular environment, helping to prevent the breakdown of the fragile photosynthetic machinery. This host-driven support is now believed to be the primary mechanism, providing the slug with a crucial nutritional supplement during times of food scarcity.
Scientific Implications
The study of the Elysia slug and its kleptoplasty capability offers profound insights that challenge conventional biological classifications. The organism acts as a natural experiment, demonstrating the potential for long-term, functional organelle transfer between kingdoms (animal and algae). This unique symbiosis is of particular interest to bioengineering and synthetic biology, specifically in the field of creating “planimal” cells.
Successfully identifying these mechanisms could be a foundation for developing photosynthetic cells in mammals. This may have applications in tissue engineering, such as generating oxygen within lab-grown tissues to prevent cell death.