Elysia chlorotica, commonly known as the green leaf sea slug, is a sea mollusk with unique features that are part animal and part plant. It belongs to the phylum Mollusca (mollusks like clams, oysters, or octopuses) and the class Gastropoda (like snails and sea slugs), but it has the amazing ability to photosynthesize like plants.

It is part of the order Sacoglossa, commonly known as “sea slugs”. It is found on the eastern coast of the United States and Canada, from Texas and Florida in the south to Nova Scotia in the north. It lives in shallow waters, in the crevices of rocks and algae beds, where it can glide through thanks to its soft and flexible body.

Its coloration is bright green, resembling a leaf, providing effective camouflage among the algae it feeds on. Sometimes, depending on the amount of chlorophyll it contains, it can have a reddish or grayish color. They usually measure between 2 and 3 centimeters in length, though some specimens reach up to 6 centimeters.

Another specimen of Elysia chlorotica
Another specimen of Elysia chlorotica. Credit: Karen N. Pelletreau et al. / Wikimedia Commons

Like other mollusks, Elysia chlorotica does not have bones or an external shell, just a muscular foot used for movement, and a mantle that secretes a sticky substance. It has a pair of sensory tentacles on its head, simple eyes, and a mouth with a radula for scraping food.

What makes this mollusk unique is its symbiotic relationship with green algae of the species Vaucheria litorea, which grow in filamentous form. After ingesting them, Elysia chlorotica can retain the chloroplasts (organelles in plant cells where photosynthesis takes place) from these algae within its own cells for months.

Thus, they continue photosynthesizing, providing energy to the slug and supplying it with nutrients. This ability to maintain chloroplasts in a functional state suggests, according to scientists, that it likely has genes for photosynthesis in its nuclear genome.

Digestive Tubules of Elysia chlorotica, with Algal Plastids
Digestive Tubules of Elysia chlorotica, with Algal Plastids. Credit: Karen N. Pelletreau et al. / Wikimedia Commons

In this way, Elysia chlorotica gets energy from the sun just like plants. The chloroplasts use sunlight to convert carbon dioxide and water into sugars and oxygen.

This supplements its diet and allows it to survive for long periods without much food. There have been reports of specimens surviving for months (between nine and ten) in aquariums without any other food source than sunlight.

Even more fascinating is the fact that Elysia chlorotica not only retains them but has developed the ability to pass these functional chloroplasts to its offspring. The young hatch with chloroplasts ready for photosynthesis, without needing to feed on algae after being born.

A specimen of Elysia chlorotica
A specimen of Elysia chlorotica. Credit: Smithsonian Environmental Research Center / Wikimedia Commons

It is believed that this symbiotic relationship between the mollusk and the algae evolved as an adaptation by Elysia chlorotica to take advantage of the limited resources in its habitat. By obtaining energy from the sun through photosynthesis, it relies less on feeding and can survive periods of scarcity.

Elysia chlorotica‘s ability to retain functional organelles from another species, known as kleptoplasty, is a unique feature of most sacoglossan mollusks. Scientists continue studying this unusual mechanism in the hopes of better understanding the evolutionary adaptation of species and the biological interactions between different organisms.

Elysia chlorotica is a hermaphroditic organism that inhabits the boundary between the animal and plant kingdoms and survives by feeding on sunlight.


This article was first published on our Spanish Edition on October 20, 2023. Puedes leer la versión en español en El molusco marino que es mitad animal, mitad planta, y se alimenta de luz solar

Sources

Mary E. Rumpho, Elizabeth J. Summer, James R. Manhart, Solar-Powered Sea Slugs. Mollusc/Algal Chloroplast Symbiosis,, Plant Physiology, Volume 123, Issue 1, May 2000, Pages 29–38, doi.org/10.1104/pp.123.1.29 | Christa G, de Vries J, Jahns P, Gould SB. Switching off photosynthesis: The dark side of sacoglossan slugs. Commun Integr Biol. 2014 Jan 1;7(1):e28029. doi: 10.4161/cib.28029 | José Ramón Alonso Peña, Botánica Insólita | Wikipedia


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