| Eastern emerald elysia | |
|---|---|
 | |
| AnE. chlorotica individual consuming its obligatealgal foodVaucheria litorea | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Gastropoda |
| Family: | Plakobranchidae |
| Genus: | Elysia |
| Species: | E. chlorotica |
| Binomial name | |
| Elysia chlorotica Gould, 1870 | |
Elysia chlorotica (common name theeastern emerald elysia) is a small-to-medium-sizedspecies of greensea slug, amarineopisthobranchgastropodmollusc. This sea slug superficially resembles anudibranch, yet it does not belong to thatclade. Instead it is a member of the cladeSacoglossa, the sap-sucking sea slugs. Some members of this group usechloroplasts from thealgae they eat forphotosynthesis, a phenomenon known askleptoplasty.Elysia chlorotica is one species of such "solar-powered sea slugs". It lives in asubcellularendosymbiotic relationship with chloroplasts of the marineheterokontalgaVaucheria litorea.
Elysia chlorotica can be found along the east coast of theUnited States, including the states ofMassachusetts,Connecticut,New York,New Jersey,Maryland,Rhode Island,Florida, (east Florida and west Florida) andTexas. They can also be found as far north asNova Scotia,Canada.[1]
This species is most commonly found insalt marshes,tidal marshes,pools, and shallow creeks, at depths of 0 m to 0.5 m.[1]
AdultElysia chlorotica are usually bright green in color owing to the presence ofVaucheria litoreachloroplasts in thecells of the slug's digestivediverticula. Since the slug does not have a protective shell or any other means of protection, the green color obtained from the algae also functions as a camouflage against predators.[2] By taking on the green color from the chloroplasts of the algal cells, the slugs are able to blend in with the sea bed, helping them improve their chances of survival andfitness. However, they can occasionally appear reddish or greyish in colour, which is thought to depend on the amount ofchlorophyll in the branches of the digestive gland throughout the body.[3] This species can also have very small red or white spots scattered over the body.[3] Ajuvenile, prior to feeding on algae, is brown with redpigment spots due to the absence of chloroplasts.[4]Elysia chlorotica have a typicalelysiid shape with largelateralparapodia 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.[4]
Elysia chlorotica feeds on theintertidal algaVaucheria litorea. It punctures the algal cell wall with itsradula, then holds thealgal strand firmly in its mouth and sucks out the contents as from a straw.[4] Instead of digesting the entire cell contents, or passing the contents through its gut unscathed, it retains only thechloroplasts, by storing them within its extensive digestive system. It then takes up the live chloroplasts into its own gutcells as organelles and maintains them alive and functional for many months. The acquisition of chloroplasts begins immediately followingmetamorphosis from theveliger stage when thejuvenile sea slugs begin to feed on theVaucheria litorea cells.[5] Juvenile slugs are brown with red pigment spots until they feed upon thealgae, at which point they become green. This is caused by the distribution of the chloroplasts throughout the extensively branched gut.[4] At first the slug needs to feed continually on 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. SomeElysia chlorotica slugs have even been known to be able to use photosynthesis for up to a year after only a few feedings.
The chloroplasts of the algae are incorporated into the cell through the process ofphagocytosis in which the cells of the sea slug engulf the cells of the algae and make the chloroplasts a part of its own cellular content. The incorporation of chloroplasts within the cells ofElysia chlorotica allows the slug to capture energy directly from light, as most plants do, through the process ofphotosynthesis.E. chlorotica can, during time periods where algae is not readily available as afood supply, survive for months. It was once thought that this survival depended on the sugars produced throughphotosynthesis performed by the chloroplasts,[6] and it has been found that the chloroplasts can survive and function for up to nine or even ten months.
However further study on several similar species showed these sea slugs do just as well when they are deprived of light.[7][8] 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 ofP. ocellatus for 55 days, keeping two in the dark, treating two with chemicals that inhibited photosynthesis, 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 ofE. timida and kept them in complete darkness for 88 days — and all survived.[9]
In another study, it was shown thatE. chlorotica definitely have a way to support the survival of their chloroplasts. After the eight-month period, despite the fact that theElysia chlorotica were less green and more yellowish in colour, the majority of the chloroplasts within the slugs appeared to have remained intact while maintaining their fine structure.[5] By spending less energy on activities such as finding food, the slugs can invest this precious energy into other important activities.AlthoughElysia chlorotica are unable to synthesize their own chloroplasts, the ability to maintain the chloroplasts in a functional state indicates thatElysia chlorotica could possess photosynthesis-supporting genes within its own nucleargenome, possibly acquired throughhorizontal gene transfer.[6] Since chloroplastDNA alone encodes for just 10% of theproteins required for proper photosynthesis, scientists investigated theElysia chlorotica genome for potential genes that could support chloroplast survival and photosynthesis. The researchers found a vital algal gene,psbO (anuclear gene encoding for amanganese-stabilizing protein within thephotosystem II complex[6]) in the sea slug's DNA, identical to the algal version. They concluded that the gene was likely to have been acquired throughhorizontal gene transfer, as it was already present in the eggs and sex cells ofElysia chlorotica.[10] It is due to this ability to utilize horizontal gene transfer that the chloroplasts are able to be used as efficiently as they have been. If an organism did not incorporate the chloroplasts and corresponding genes into its own cells and genome, the algal cells would need to be fed upon more often due to a lack of efficiency in the use and preservation of the chloroplasts. This once again leads to a conservation of energy, as stated earlier, allowing the slugs to focus on more important activities such as mating and avoiding predation.
More recent analyses, however, were unable to identify any activelyexpressed algal nuclear genes inElysia cholorotica, or in the similar speciesElysia timida andPlakobranchus ocellatus.[11][12]These results weaken support for the horizontal gene transfer hypothesis.[12] A 2014 report utilizing fluorescent in situ hybridization (FISH) to localize an algal nuclear gene, prk, found evidence of horizontal gene transfer.[13] However, these results have since been called into question, as FISH analysis can be deceptive and cannot prove horizontal gene transfer without comparison to theElysia cholorotica genome, which the researchers failed to do.[14]
The exact mechanism allowing for the longevity of chloroplasts once captured byElysia cholorotica despite its lack of active algal nuclear genes remains unknown. However, some light has been shed onElysia timida and its algal food.[15] Genomic analysis ofAcetabularia acetabulum andVaucheria litorea, the primary food sources ofElysia timida, has revealed that their chloroplasts produceftsH, another protein essential forphotosystem 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 offtsH could in principle contribute greatly to the observedkleptoplast longevity inElysia cholorotica andElysia timida.[15]
AdultElysia chlorotica are simultaneoushermaphrodites. When sexually mature, each animal produces bothsperm andeggs at the same time. However,self-fertilization is not common within this species. Instead,Elysia chloroticacross-copulate. After the eggs have beenfertilized within the slug (fertilization is internal),Elysia chlorotica lay their fertilized eggs in long strings.[4]
In the life cycle ofElysia chlorotica, cleavage isholoblastic and spiral. This means that the eggs cleave completely (holoblastic); and each cleavage plane is at anobliqueangle to theanimal-vegetal axis of theegg. The result of this is that tiers of cells are produced, each tier lying in thefurrows between cells of the tier below it.At the end of cleavage, theembryo forms astereoblastula, meaning ablastula without a clear centralcavity.[4]
Elysia chloroticagastrulation is byepiboly: theectoderm spreads to envelope themesoderm andendoderm.[4]
After theembryo passes through atrochophore-like stage during development, it then hatches as a veliger larva.[4] 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 feeds onphytoplankton 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 thestomach. In the stomach, food is sorted and then moved on to the digestive gland, where the food is digested and thenutrients are absorbed by the epithelial cells of the digestive gland.[4][16][17]
