Elysia viridis, commonly known as the sap sucking slug, is a gastropod belonging to the family Plakobranchidae. This fascinating animal is an example of sacoglossans, a group of sea slugs capable of performing photosynthesis by acquiring chloroplasts from the algae they consume. The name Elysia refers to the ‘Elysian Fields,’ the paradise of heroes in Greek mythology, while viridis means ‘green’ in Latin, alluding to its characteristic color. This species is found in the eastern Atlantic, from the British coasts to the Mediterranean, commonly inhabiting intertidal and shallow zones where it lives on algae and seagrass meadows such as sebadales or posidonias.
The green sea slug typically measures between 2 and 5 cm in length, although some specimens can reach up to 6 cm. Its body is elongated and flattened, with two lateral extensions called parapodia, which it uses to swim or glide gently over the substrate. The predominant color of its body is bright green, with whitish or yellowish spots or lines along the edges of the parapodia, allowing it to camouflage among the algae. This green coloration is due to the chloroplasts it incorporates from the algae it consumes, especially from genera like Codium and Cladophora, which it can keep alive inside its body for weeks, enabling it to perform photosynthesis.
Elysia viridis is herbivorous, feeding mainly on green algae such as Codium and Cladophora, from which it extracts chloroplasts through a process called kleptoplasty. By this mechanism, the slug ingests the algae and stores the chloroplasts in its own cells, keeping them functional for a time. This allows it to obtain additional energy through photosynthesis, almost like a plant. Although it can survive solely on nutrients obtained from algae, this ability to photosynthesize gives it an advantage in environments with scarce food, making it one of the most fascinating traits of this species.
The reproductive cycle of Elysia viridis is interesting because, like many other sea slugs, it is a simultaneous hermaphrodite, meaning each individual has both male and female reproductive organs. During reproduction, two individuals mate by exchanging sperm, allowing them to fertilize each other. Subsequently, both lay small egg masses shaped like ribbons on algae. The eggs hatch in a few weeks, giving rise to planktonic larvae that, after a brief period in the water column, settle on the seabed and develop into juveniles.
A fascinating curiosity about Elysia viridis is its ability to keep chloroplasts active for weeks or even months, giving it a unique capacity for photosynthesis. This symbiogenesis phenomenon has attracted much scientific attention, suggesting an unusual evolutionary relationship between animals and plants. Additionally, this type of slug can change color depending on the algae it feeds on, offering camouflage advantages. Though small and seemingly fragile, Elysia viridis is a remarkable example of evolutionary adaptation in the animal kingdom.
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