Not evaluated (IUCN 3.1)
Elysia chlorotica, common name the eastern emerald elysia, is a small-to-medium-sized species of green sea slug, a marine opisthobranch gastropod mollusc. This sea slug superficially resembles a nudibranch, yet it does not belong to that clade of gastropods. Instead it is a member of the clade Sacoglossa, the sap-sucking sea slugs. Some members of this group use chloroplasts from the algae they eat; a phenomenon known as kleptoplasty. Elysia chlorotica is one of the "solar-powered sea slugs", utilizing solar energy via chloroplasts from its algal food. It lives in a subcellular endosymbiotic relationship with chloroplasts of the marine heterokont alga Vaucheria litorea.
Elysia chlorotica can be found along the east coast of the United States, including the states of Massachusetts, Connecticut, New York, New Jersey, Maryland, Florida (east Florida and west Florida) and Texas. They can also be found as far north as Nova Scotia, Canada.
Adult Elysia chlorotica are usually bright green in colour, due to the presence of Vaucheria litorea chloroplasts in the cells of the slug's digestive diverticula. Since the slug does not have a protective shell or any other means of protection, the slug also uses the green color obtained from the algae as a camouflage against predators. However, they can occasionally appear reddish or greyish in colour, thought to depend on the amount of chlorophyll in the branches of the digestive gland which ramify throughout the body. This species can also have very small red or white spots scattered over the body. A juvenile, prior to feeding, is brown with red pigment spots due to the absence of chloroplasts. Elysia chlorotica have a typical elysiid shape with large lateral parapodia which can fold over to enclose the body. Elysia chlorotica can grow up to 60 mm in length but are more commonly found between 20 mm to 30 mm in length.
Elysia chlorotica feeds on the intertidal algae Vaucheria litorea by puncturing the algal cell wall with its radula. The slug then holds the algal strand firmly in its mouth and, as though it were a straw, sucks out the contents. Instead of digesting the entire cell contents, or passing the contents through its gut unscathed, it retains only the algal chloroplasts, by storing them within its own cells throughout its extensive digestive system. The acquisition of chloroplasts begins immediately following metamorphosis from the veliger stage when the juvenile sea slugs begin to feed on the Vaucheria litorea cells. Juvenile slugs are brown with red pigment spots until they feed upon the algae, at which point they become green. This is caused by the distribution of the chloroplasts throughout the extensively branched gut. Initially, the slug needs to continually feed upon algae to retain the chloroplasts, but over time the chloroplasts become more stably incorporated into the cells of the gut enabling the slug to remain green without further feeding.
The incorporation of chloroplasts within the cells of Elysia chlorotica allows the slug to capture energy directly from light, as most plants do, through the process known as photosynthesis. It was once thought that Elysia chlorotica could, during time periods where algae is not readily available as a food supply, survive for months on the sugars produced through photosynthesis performed by their own chloroplasts. Since then it has been found that the chloroplasts can survive and function for up to nine or even ten months. However further study proved these sea slugs do just as well when they are deprived of light. Sven Gould from Heinrich-Heine University in Düsseldorf and his colleagues showed that even when photosynthesis was blocked, the slugs could survive without food for a long time, and seemed to fare just as well as food-deprived slugs exposed to light. They starved six specimens of P. ocellatus for 55 days, keeping two in the dark, treating two with the drug, and providing two with appropriate light. All survived and all lost weight at about the same rate. The authors also denied food to six specimens of E. timida and kept them in complete darkness for 88 days — and all survived. (E. timida slugs are too small to be weighed reliably, but at the end of the test, those that were light-deprived seemed to be as healthy as the controls. After the eight-month period, despite the fact that the Elysia chlorotica were less green and more yellowish in colour, the majority of the chloroplasts within the slugs appeared to have remained intact and also maintaining their fine structure.
Although Elysia chlorotica are unable to synthesize their own chloroplasts, the ability to maintain the chloroplasts acquired from Vaucheria litorea in a functional state indicates that Elysia chlorotica must possess photosynthesis-supporting genes within its own nuclear genome, most likely acquired through horizontal gene transfer. Since chloroplast DNA alone encodes for just 10% of the proteins required for proper photosynthesis, scientists investigated the Elysia chlorotica genome for potential genes that could support chloroplast survival and photosynthesis. The researchers found a vital algal gene, psbO (a nuclear gene encoding for a manganese-stabilizing protein within the photosystem II complex) in the sea slug's DNA, identical to the algal version. They concluded that the gene was likely to have been acquired through horizontal gene transfer, as it was already present in the eggs and sex cells of Elysia chlorotica.
More recent analyses, however, were unable to identify any actively expressed algal nuclear genes in Elysia cholorotica, or in the similar species Elysia timida and Plankobranchus ocellatus.  These results weaken support for the horizontal gene transfer hypothesis.
The exact mechanism allowing for the longevity of chloroplasts once captured by Elysia cholorotica, despite its lack of active algal nuclear genes, remains unknown. However, some light has been shed on Elysia timida and its algal food. Genomic analysis of Acetabularia acetabulum and Vaucheria litorea, the primary food sources of Elysia timida, has revealed that their chloroplasts produce ftsH, another protein essential for photosystem II repair. In land plants, this gene is always encoded in the nucleus, but is present in the chloroplasts of most algae. An ample supply of ftsH could in principle contribute greatly to the observed kleptoplast longevity in Elysia cholorotica and Elysia timida. 
Adult Elysia chlorotica are simultaneous hermaphrodites. When sexually mature, each animal produces both sperm and eggs at the same time. However, self-fertilization is not common within this species. Instead, Elysia chlorotica cross-copulate. After the eggs have been fertilized within the slug (fertilization is internal), Elysia chlorotica lay their fertilized eggs in long strings.
In the life cycle of Elysia chlorotica, cleavage is holoblastic and spiral. This means that the eggs cleave completely (holoblastic); and each cleavage plane is at an oblique angle to the animal-vegetal axis of the egg. The result of this is that tiers of cells are produced, each tier lying in the furrows between cells of the tier below it. At the end of cleavage, the embryo forms a stereoblastula, meaning a blastula without a clear central cavity.
After the embryo passes through a trochophore-like stage during development, it then hatches as a veliger larva. The veliger larva has a shell and ciliated velum. The larva uses the ciliated velum to swim as well as to bring food to its mouth. The veliger larva feed on phytoplankton in the sea-water column. After the food is brought to the mouth by the ciliated velum, it is moved down the digestive tract to the stomach. In the stomach, food is sorted and then moved on to the digestive gland where the food is digested and the nutrients are absorbed by the epithelial cells of the digestive gland.
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- Mature Veliger (schema)
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