Solar-Powered Sea Slugs

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Elysia subornata

Solar-powered sea slugs refers to two groups of sea slugs — marine opisthobranch gastropods — that are able to use photosynthesis to supply the animals with food: 1)Sacoglossa, which incorporate functioning chloroplasts into their tissues using kleptoplasty; and 2) species of aeolid nudibranchs in the genera Phyllodesmium and Pteraeolidia, which incorporate living zooxanthellae in their tissues. [Source: Wikipedia]

Sacoglossa sea slugs break a cardinal rule of living things — that only plants are able to employ photosynthesis to get nutrition — and they do it almost as a form of green thievery. They can take energy from the sun and, using only parts of their cells, turn it into chemical food.. Utilizing what is called an ‘endosymbiotic’ relationship, these sea slug feed on algae by slicing or puncturing the algae cells and then ingesting the algae’s chloroplasts (in-cell structures) unharmed, into the slugs’ own body and carry out photosynthesis, This is why they are named “solar powered sea slugs.” They usually are vivid green in color, but can also be red or brown depending on the type of algae they eat. [Source: Wildlife Trust]

Solar-powered sea slugs live in the Northern Atlantic, from Britain to the Mediterranean in the east and Nova Scotia to Florida in the west. They live between 12-15 months and unusually for a sea slug, can survive in low salinity (low salt) waters. They reach lengths of five centimeters and are usually vivid green, brown or red in color. They look similar to sea hares — their floppy ‘rhinophores’ sort of resemble rabbit bunny ears. These sea slugs have distinctive turquoise-colored chloroplasts visible on their wings, foot and head. When the “wings” are unfolded they look like beautiful green leaves, which serves as camouflage amongst the algae where they live and avoid predators.

The Leaf Sheep (Scientific name: Costasiella Kuroshimae) is a species of sacoglossan sea slug that feeds almost exclusively on algae and then uses that algae to perform photosynthesis and supply the by using the chloroplasts of the algae. Also known as a "leaf slug" or "salty ocean caterpillar" and discovered in 1993 off the coast of the Japanese island Kuroshima, it practices the chemical process of kleptoplasty, in which they retain the chloroplasts from the algae they feed on. Absorbing the chloroplasts from algae then enables them to indirectly perform photosynthesis.

Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) coris.noaa.gov ; International Coral Reef Initiative icriforum.org ; Coral Reef Alliance coral.org ; Global Coral reef Alliance globalcoral.org ; Global Coral Reef Monitoring Network gcrmn.net

Solar-Powered Sea Slug Chloroplasts

Katherine J. Wu wrote in The Atlantic: These sea slug steal photosynthesizing machinery— the chloroplasts—from the algae they eat, and store the green, light-converting blobs in their body for extended periods. Some species can reap the nutritional benefits of these self-replenishing snack packs for months, perhaps for longer than a year. The slugs’ felonious feat, known as kleptoplasty, is so remarkable that it’s been held up by creationists as proof of intelligent design. (It is, to be clear, evolution.) And researchers still aren’t sure how the plant-pantomiming animals pull it all off. [Source: Katherine J. Wu, The Atlantic, September 28, 2021]

“Most sacoglossan sea slugs aren’t of the solar-powered ilk; they digest the chloroplasts along with everything else. That certain species among them manage to keep the little structures operational for days, weeks, or months is, frankly, bonkers—and seems impossible at first, because of how dependent chloroplasts are on their native host cells. Chloroplasts were, millions of years ago, free-living bacteria that were ultimately engulfed by bigger cells; in exchange for room and board, the microbes pumped out energy for their hosts, forging what became a permanent codependency.

Nowadays, plant and algae chloroplasts can’t get by without protein cargo that’s manufactured exclusively out of genes in the nucleus, which doesn’t survive the sea slug’s discerning digestion. Sticking a chloroplast into a sea-slug cell and expecting it to run is like asking a car to gun indefinitely down a highway with no gas or oil-change stations. (It’s also why we humans can’t just stick chloroplasts into test tubes and profit.) And yet, even stripped of their nuclear entourage, the chloroplasts persist—and work. “It seems like a biotechnological marvel,” Debashish Bhattacharya, of Rutgers University, told me. “How the hell do they keep the chloroplasts alive?”

How and Why Solar-Powered Sea Slugs Photosynthesize

Lettuce leaf sea slug

Katherine J. Wu wrote in The Atlantic: In general, these marine slugs, “latch on to a straw-shaped stretch of algae, puncture it with a tooth, and slurp out its contents. The resultant sludge then floods the slug’s über-branched gut, where it’s captured by cells that hoard the chloroplasts intact, while breaking down or discarding everything else that’s algal. Solar-powered sea slugs tend to hatch translucent or whitish. But the chloroplasts swathe large portions of their flat, billowy bodies in a startling verdigris. In the 1970s, one pioneering biologist who espied Eastern emerald elysia’s emerald hues dubbed them the “leaves that crawl.” [Source: Katherine J. Wu, The Atlantic, September 28, 2021]

Several potential explanations have been put forth over the years. In one, the sea slugs use their own in-house accoutrement to jerry-rig the chloroplasts, making them more durable. In another, the animals manage to ransack algal nuclei, co-opting chloroplast-fortifying genes, though most of the researchers I talked with characterized the evidence for this idea as scant or mixed. A few years ago, a group of scientists proposed another work-around: Perhaps the kidnapped chloroplasts are important to the slug less as photosynthetic factories, and more as self-contained food stores—mini, cell-intrinsic calorie caches that could be digested by the animal in times of nutritional need, like a camel’s fat-rich hump. In that scenario, chloroplast maintenance could fall to the wayside.

Pierce told me that idea doesn’t have much support. (It also doesn’t negate the possibility that the sea slugs are solar-powered: Some species, for instance, could be tapping into those reserves after milking the chloroplasts’ photosynthesizing chops for weeks.) And many experts are wholly convinced that, for Elysia chlorotica and several of its closest kin, the biggest benefit of harboring algal contraband centers totally on photosynthesis, especially because “it’s dangerous business to steal a chloroplast,” Paulo Cartaxana, who studies the slugs at the University of Aveiro, in Portugal, told me. Chloroplasts are fragile and fussy; they emit toxic compounds while they work. The structures must be providing big perks, or they’d have been booted long ago. And proof of this has indeed racked up.

Several chloroplast-hoarding sea-slug species will live longer and grow larger when allowed to soak up sunlight. Karen Pelletreau, of the University of Maine,, collaborated on prior work showing that Eastern emerald elysia slugs in particular seem to be totally dependent on chloroplasts; without them, the plant wannabes simply perish in their youth. One recent study proposed that the chloroplasts’ energetic oomph may even be powerful enough to sustain certain sea-slug species after they purposefully decapitate themselves and begin the arduous work of sprouting a new body from their severed head.

“Spawning is a huge reproductive investment,” Sónia Cruz, also of the University of Aveiro, and another author on the new E. timida study, told me. “It takes a lot of energy out of them.” Each slug has to manufacture hundreds of eggs, each packed with enough nutrients to sustain its offspring during early development. The chloroplasts appear to offer an energetic boon, in some cases doubling the slugs’ output.

Applications and What Has Been Learned from Solar-Powered Sea Slugs

Pierce told Katherine J. Wu of The Atlantic: “To this day I get questions from little kids in their science classes” who have stumbled upon these slugs and want to know if they could help “end world hunger.” The answer, Pierce said is no. But the proposal isn’t totally outlandish. [Source: Katherine J. Wu, The Atlantic, September 28, 2021]

Anna Karnkowska, an evolutionary biologist at the University of Warsaw, in Poland, said that lessons could be learned from the other members of the chloroplast-stealing club, most of which are single-celled creatures such as dinoflagellates (though at least a couple of marine worms seem to briefly hijack the structures too). These unicellular pirates are thought to have an especially intense relationship with their chloroplasts; for them, kleptoplasty might be an intermediate step toward fixing the structures permanently into their cells and making them heritable from generation to generation.

Sea slugs, with their multicelled anatomy and complex lifestyle, would have a much harder time passing pilfered chloroplasts down. As far as scientists can tell, what the slugs accomplish is akin to black-market organ theft, but little more: When the animal dies, its chloroplast cargo dies with it. But even if the chloroplasts’ tenancy is a dead end, it’s a fascinating push to rethink the strange and category-defying ways in which organisms interact, Karnkowska said. The slugs offer the chloroplasts a home, and get to, for a while, masquerade as pseudo-plants; the chloroplasts, in turn, become the sole survivors of carnage, enduring where the rest of their algal comrades could not.

Further evidence that the animals pack a substantial photosynthetic punch comes from studies that have tracked chloroplast-produced chemical food packets on an atomic scale, as they migrate into a menagerie of sea-slug tissues, where they can presumably facilitate all sorts of sea-sluggy things. Cartaxana’s recent work showcases something new: In Elysia chlorotica’s close cousin Elysia timida, what comes out of gut-cell chloroplasts can end up in reproductive tissues and boost the number of eggs new sea-slug parents lay. (E. timida slugs, while greedy burglars at the dinner table, are very reciprocal lovers. All of them are hermaphrodites, and they mate by colliding head-to-head and mutually inseminating each other with penises that unspool from beneath their right eye.)

Eastern Emerald Elysia— a Photosynthesizing Solar-Powered Sea Slugs

Elysia chlorotica body

Eastern emerald elysia (Scientific name: Elysia chlorotica) are small-to-medium-sized green sea slugs that unusually utilize chlorophyll and photosynthesis to obtain food and nutrition. They superficially resembles nudibranchs but are not nudibranchs. Instead they are members of the clade Sacoglossa, the sap-sucking, solar-powered sea slugs described above. Their average lifespan is around 11 months. Adults experience mass death both in the wild and in labs at approximately 11 months. [Source: Chelsea Blanchet, Animal Diversity Web (ADW) /=]

Eastern emerald elysia are found in the western Atlantic Ocean off the coast of North American from Nova Scotia in the north to southern Florida in the south in salt and tidal marshes, shallow creeks, and pools, generally in depths of less than 50 centimeters. These sea slugs are more tolerant to a wide range of salinities than any creature known. They are able to survive salinity levels ranging from nearly fresh water (~24 mosm) to brackish salt water (~2422 mosm). Eastern emerald elysia is generally found close to its main food source, Vaucheria litorea, an intertidal algae. The slug has an obligate relationship with the alga for both nutrients and physical development./=\

Eastern emerald elysia are diurnal (active mainly during the daytime), motile (move around as opposed to being stationary), sedentary (remain in the same area) and solitary. They spend most of their time floating in the water or among their preferred algae to obtain optimal sun exposure. These slugs generally only gather with others for mating. They sense using touch, vibrations and chemicals usually detected by smell and communicate with touch, vibrations and pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species). /=\

Eastern emerald elysia have no known predators of Eastern emerald elysia. Their leaf like structures allow them to blend in almost perfectly amongst the algae and plants of their marine habitat. They have little impact on their environment as the feed only on one type of algae, are not predators of animals and are not known to be a prey of any particular species. They have not been evaluated for the International Union for Conservation of Nature (IUCN) Red List. They have no special status according to the Convention on the International Trade in Endangered Species (CITES). /=\

Eastern Emerald Elysia Characteristics

Eastern emerald elysia range in length from two to six centimeters (0.79 to 2.36 inches), with their average length being three centimeters (1.18 inches). They are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature), heterothermic (have a body temperature that fluctuates with the surrounding environment) and have bilateral symmetry (both sides of the animal are the same). [Source: Chelsea Blanchet, Animal Diversity Web (ADW) /=]

Eastern emerald elysia have two main life stages: a juvenile stage which is defined as the time before the slug begins feeding on V. litorea, and an adult stage. According to Animal Diversity Web: The stages of development are distinguishable based on the slug’s morphology and coloring. The slugs start as veliger larva, meaning they are equipped with a shell and ciliated vellum used for swimming and obtaining food. After metamorphosing to juveniles, the slugs are normally brown with ventrally-located spots of red pigmentation. Eastern emerald elysia only undergoes metamorphosis into the adult phase after exposure to and consumption of V. litorea, at which time its coloring and morphology also change.

After the initial feeding, Eastern emerald elysia sequesters chloroplasts obtained from the plant into its specialized digestive tract. The presence of the chloroplasts turns the slug from brown to bright green. Most adults lose the red spots. The green color persists only as long as the slug has functional chloroplasts in its cells. When the chloroplasts are expelled, the slug loses its bright green color and reverts to a gray color.

The eastern emerald elsyia obtains its name from its adult structure. Elysid refers to the adult slug’s leaf-like shape which is caused by two large lateral parapodia on either side of its body. This morphology is beneficial as both camouflage and allowing the slug to be more efficient at photosynthesis. Other members of this family are distinguished by their parapodia in addition to bright coloring.

Eastern Emerald Elysia Feeding

Eastern emerald elysia feed exclusively on V. litorea, and rarely feed upon Vaucheria compacta, another kind of algae. They have an obligate relationship with their food source, requiring it for metamorphosis from the veliger to juvenile to the adult stage. [Source: Chelsea Blanchet, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web: As an adult, Eastern emerald elysia obtains nutrients by consuming chloroplast cells from the alga. Eastern emerald elysia removes the chloroplast cells from the plant by projecting its radula, a scraping structure into the alga’s cell walls, and then sucking out the contents of V. litorea cells. The contents of these cells pass through the slug’s highly specialized digestive tract. Over time the chloroplast cells are sequestered into the diverticula of the slug’s digestive system, causing it to turn bright green. After the digestive tract projects green coloration, Eastern emerald elysia is fully capable of photosynthesis for up to 10 months. Due to the slug’s photosynthetic nature, this species can often be found “sun bathing”, or laying with their parapodia extended to obtain maximum sunlight exposure.

Scientists study Eastern emerald elysia to find out how they how they obtains the chloroplast from its algal food supply and how they are able to maintain and utilize the complex structures. This species contains the blueprints to many of the required components of photosynthesis in their genome before even ingesting the chloroplasts of Vaucheria litorea.

Sidney K. Pierce, of the University of South Florida has studied Eastern emerald elysia. He told The Atlantic they can go the rest of its life without eating after just one algae-rich binge in its youth. “We collect them in the field,” he said, “and we never feed them again.” [Source: Katherine J. Wu, The Atlantic, September 28, 2021]

Eastern Emerald Elysia Mating and Reproduction

Eastern emerald elysia are simultaneous hermaphrodites in which individuals have sex organs of both sexes and can produce both sperm and eggs even in the same breeding season and engage in internal reproduction in which sperm from the male parent fertilizes an egg from the female parent. Eastern emerald elysia are oviparous (young are hatched from eggs) and engage in once-a-years seasonal breeding. The breeding season is in early spring. The gestation period ranges from seven to eight days. Eggs are laid in long mucous-laden strings, hatching approximately in a week. Parental care after the laying off eggs has never been observed.

Eastern emerald elysia are capable of internal self-fertilization, although this particular species more commonly engage in sexual reproduction with another individual. Sexually reproducing hermaphrodites may act only as female or male. Sperm take less energy to produce than eggs, so functioning as a male may be more desirable. Many species of sea slugs within the clade Sacoglossa practice hypodermic insemination, in which the sperm of one slug is injected directly into the surface of another slug. They penetrate directly into the mate’s body in the general area of the others gonads and release the sperm directly inside their partner. [Source: Chelsea Blanchet, Animal Diversity Web (ADW) /=]

Eastern emerald elysia are polygynandrous (promiscuous), with both males and females having multiple partners. According to Animal Diversity Web: The details of how Eastern emerald elysia initiates mating and the techniques used during mating are not well known. In a similar species, the mating behaviors of Elysia timida are dependent on the responses generated by the potential partner. These slugs will approach each other head to head and feel the other’s head with their own. Then, one (no way of telling how they decide which begins to move) will proceed downward moving their head down along the other slug’s body. If the partner accepts the invitation to mate the slugs will align head to tail. When the proper alignment is established, mating begins where both slugs insert their penes into the other’s genital area.

Eastern Emerald Elysia Development

The life cycle of Eastern emerald elysia egg is characterized by metamorphosis — a process of development in which individuals change in shape or structure as they grow. According to Animal Diversity Web: The blastula of a developing Eastern emerald elysia egg is holoblastic and spiral, meaning the eggs completely divide. At division, each plane is at an oblique angle to the animal's vegetal axis. Cells produce multiple tiers of cells with no clear center; this is referred to as a stereoblastula. Movements of cells occur by a process referred to as epiboly. Epiboly means that during development the ectoderm cells spread out to cover both the mesoderm and endoderm cell layers. [Source: Chelsea Blanchet, Animal Diversity Web (ADW) /=] /=\

Eastern emerald elysia has a veliger, juvenile, and adult stage of life. As a veliger larva, Eastern emerald elysia has a shell and ciliated vellum, a common feature among a sea slug's developmental cycle. During the larval stage these cilia help the larva to swim in its aquatic environment. Coloration in the larva is different due to the lack of retained chloroplasts in their diverticula. Diverticula are essentially openings along the digestive tract that result in small pocket in which an animal can store food, or in this case stolen chloroplasts.

Veligers will metamorphose into juveniles in one to two days after exposure to V. litorea. After 14 days of exposure to V. litorea and an additional two days of constant contact with this plant, Eastern emerald elysia metamorphoses into the adult leaf-shaped sea slug. The adult sea slug is bright green in color due to chloroplast cells that have been sequestered into the complex diverticula of the animal. Adults die shortly after they lay their string of eggs. Researcher Sidney Pierce suggests mass death is due to the expression of an unknown retro acting virus but no firm evidence to say this for sure exists.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023