Overview

Comprehensive Description

Biology

Elysia chlorotica is a “solar-powered” marine sea slug that sequesters and retains photosynthetically active chloroplasts from the algae it eats and, remarkably, has incorporated algal genes into its own genetic code. It is emerald green in color often with small red or white markings, has a slender shape typical of members of its genus, and parapodia (lateral "wings") that fold over its body in life. This sea slug is unique among animals to possess photosynthesis-specific genes and is an extraordinary example of symbiosis between an alga and mollusc as well as a genetic chimera of these two organisms.

To obtain algal chloroplasts Elysia chlorotica slugs use their radula (tooth) to pierce a filament of the alga Vaucheria litorea and suck out its contents. The ingested algal cytoplasm and nuclei move through the gut but algal chloroplasts are trapped and concentrated in vacuoles along branches of the digestive tract. While inside an algal cell, functional chloroplasts use proteins encoded by their own genes as well as others encoded by genes within the algal nucleus. Within a sea slug, however, isolated chloroplasts can not receive proteins from the algal genome. Remarkably, these chloroplasts remain functional anyway because the slug genome includes the algal genes necessary for plastid function. Elysia chlorotica probably gained these algal genes through lateral (or horizontal) gene transfer. One possible vector is a virus that infects the sea slug and carried pieces of algal DNA (Pierce et al., 2003).

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Description

Original Description of Elysia chlorotica: A. A. Gould, 1870 (Report on the Invertebrata of Massachusetts): "Animal emerald green, dotted with white and red spots; sleder, tapering behind, with broad, lateral expansions, folded and overlapping each other on the back when the animal is in motion; tentacles two, lanceolate, folded beneath; head distinct, obtuse, slightly emarginate; anterior angles of foot widely produced, triangular.

Actæon, Agassiz, Proc. Bost. Soc. Nat. Hist, iii. 191 (1850)

Actæon chloroticus, Agassiz, in MSS.

Animal emerald green, finely dotted with opaque white interspersed with red specks. Body slender, tapering backwards with very broad lateral expansions or wings, which, when folded as they are when the animal is crawling, overlap each other on the back in a roof-like manner, and the while animals has then a lance-shaped form generally acutely pointed behind, but in some attitudes obtuse; when expanded, they have a broad ovate form, like a leaf with the border more or less undulating, and this resemblance is further carried out by the vein-like folds or canals which ramify on its surface from the heart which forms a globular or bulbous eminence in front; the expansion begins at the anterior part of this bulb. In front of this is a well-marked neck and head, on which latter are two delicately lanceolate tentacles, which are furrowed or folded beneath. The eyes are placed a little behind the tentacles. The head is obtuse and slightly emarginate. The organs of generation are just behind the right tentacle, and the male organs is very often protruded, of about the same form and nearly as large as teh tentacle. The anterior angels of the foot are widely produced, of a recurved triangular form, as if another pair of tentacles. Length, about one inch, sometimes an inch and a half; breadth, when folded, about one fifth the length, and height equal to breadth, when fully expanded, equal to three fourths the length.

Found in great numbers in brackish water, on the Cambridge marshes, in the spring of 1848 (Agassiz)."

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Distribution

Gulf of St. Lawrence (unspecified region), and Prince Edward Island (from the northern tip of Miscou Island, N.B. to Cape Breton Island south of Cheticamp, including the Northumberland Strait and Georges Bay to the Canso Strait causeway) Nova Scotia; USA: Massachusetts, Connecticut, New York, New Jersey, Maryland, Florida; Florida: East Florida, West Florida; USA: Texas
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Elysia chlorotica, commonly known as the eastern emerald elysia, is found along the eastern coast of the United States, as far north as Nova Scotia, Canada and as south as southern Florida.

Biogeographic Regions: nearctic (Native ); atlantic ocean (Native )

  • Rumpho, M., E. Summer, B. Green, T. Fox, J. Manhart. 2001. Mollusc/algal chloroplast symbiosis: how can isolated chloroplasts continue to function for months in the cytosol of a sea slug in the absence of an algal nucleus?. Zoology, 104: 303-312.
  • Rumpho, M., K. Pelletreau, A. Moustafa, D. Bhattacharya. 2011. The making of a photosynthetic animal. The Journal of Experimental Biology, 214/2: 303-311.
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The type locality is Massachusetts, USA. Its distribution along the North American Atlantic Coast includes Nova Scotia, Massachusetts, Connecticut, New York, New Jersey, Maryland, and Florida (http://www.malacolog.org).

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Physical Description

Morphology

Elysia chlorotica has 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. 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. Elysia chlorotica 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, E. chlorotica 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. Adults normally range in size from 20 to 30 mm but specimens of up to 60 mm have been documented. 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.

Range length: 20 to 60 mm.

Average length: 30 mm.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry

  • Colin, P. 1978. Marine Invertebrates And Plants of the Living Reef. Neptune city, /Nj: T.F.H Publications.
  • Humann, P. 1992. Reef Creature Identification. Jacksonville, Fl: New World Publications.
  • Rumpho, M., E. Summer, J. Manhart. 2000. Solar-powered sea slugs. Mollusc/algal chloroplast symbiosis. Plant Physiology, 123/1: 29-38.
  • Rumpho, M., J. Worful, J. Lee, M. Tyler, D. Bhattacharya, A. Moustafa, J. Manhart. 2008. Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. PNAS, 105/46: 17867-17871.
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Size

Length: approximately 7 mm (Marcus, 1980) to 45 mm (http://www.malacolog.org).

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Ecology

Habitat

infralittoral of the Gulf and estuary
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Elysia chlorotica is found in salt and tidal marshes, shallow creeks, and pools with depths of less than 0.5 m. The eastern sea slug is the most euryhaline osmoconformer known to date. The slug can survive salinity levels ranging from nearly fresh water (~24 mosm) to brackish salt water (~2422 mosm). Elysia chlorotica is generally found close to its main food source, Vaucheria litorea, an intertidal alga. The slug has an obligate relationship with the alga for both nutrients and physical development.

Range depth: 0 to 0.5 m.

Habitat Regions: saltwater or marine

Aquatic Biomes: coastal ; brackish water

Wetlands: marsh

  • Green, B., W. Li, J. Manhart, T. Fox, E. Summer, R. Kennedy, S. Pierce, M. Rumpho. 2000. Mollusc-algal chloroplast endosymbiosis. Photosynthesis, thylakoid protein maintenance, and chloroplast gene expression continue for many months in the absence of the algal nucleus. Plant Physiology, 124/1: 331-342.
  • Pierce, S., S. Edwards, P. Mazzocchi, L. Klingler, M. Warren. 1984. Proline betaine: A unique osmolyte in an extremely euryhaline osmoconformer. The Biological Bulletin, 167/2: 495-500.
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Most often found on Vaucheria spp. in saltwater tidal marshes from 3-32% salinity.

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Depth range based on 5 specimens in 1 taxon.

Environmental ranges
  Depth range (m): 0 - 0
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

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Trophic Strategy

Elysia chlorotica is a kleptoplastic member of the clade Sacoglossa, which are sap sucking sea slugs. This species feeds exclusively on V. litorea, and rarely feed upon Vaucheria compacta. The slug has an obligate relationship with its food source, requiring it for metamorphosis from the veliger to juvenile to the adult stage.

As an adult, E. chlorotica obtains nutrients by consuming chloroplast cells from the alga. Elysia chlorotica 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, E. chlorotica 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.

Plant Foods: sap or other plant fluids

Primary Diet: herbivore (Algivore, Eats sap or other plant foods)

  • Mujer, C., D. Andrews, J. Manhart, S. Pierce, M. Rumpho. 1996. Chloroplast genes are expressed during intracellular symbiotic association of Vaucheria litorea plastids with the sea slug Elysia chlorotica. Cell Biology, 93/22: 12333-12338.
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Associations

Elysia chlorotica has little impact on the environment because they are not predators of animals and are not known to be a prey of choice for any particular species. They interact with Vaucheria litorea, as all juveniles must feed on these plants before metamorphosis can occur.

  • Hoagland, K., R. Robertson. 1988. An assessment of poecilogony in marine invertebrates: phenomenon or fantasy?. The Biological Bulletin, 174/2: 109-125.
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There are no known predators of E. chlorotica. The leaf like structure of its appearance allows it to blend amongst the algae and plants of its marine habitat.

Anti-predator Adaptations: cryptic

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Found most commonly on and eating the yellow-green alga Vaucheria litorea and Vaucheria compacta (Rumpho et al., 2000).

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General Ecology

Ecology

Found at depths of 0 to 0.5 m.

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Life History and Behavior

Behavior

Little information is known on the techniques used by this species to communicate. Since the communication techniques of other sea slugs is variable, it is difficult to compare other species with E. chlorotica. The slug's eyes are not very developed.

Communication Channels: tactile

Other Communication Modes: pheromones ; vibrations

Perception Channels: tactile ; vibrations ; chemical

  • Brandly, B. 1984. Aspects of the ecology and physiology of Elysia cf. furvacuda (Mollusca: Sacoglossa). Bulletin of Marine Science, 34/2: 207-219.
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Life Cycle

The blastula of a developing Elysia chlorotica 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.

Elysia chlorotica has a veliger, juvenile, and adult stage of life. As a veliger larva, E. chlorotica 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, E. chlorotica 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.

Development - Life Cycle: metamorphosis

  • Hoffmeister, M., W. Martin. 2003. Interspecific evolution: microbial symbiosis, endosymbiosis and gene transfer. Environmental Microbiology, 5/8: 641-649.
  • Pierce, S., T. Maugel, M. Rumpho, J. Hanten, W. Mondy. 1999. Annual viral expression in a sea slug population: Life cycle control and symbiotic chloroplast maintenance. The Biological Bulletin, 197/6: 1-6.
  • Schmitt, V., N. Anthes, N. Michiels. 2007. Mating behaviour in the sea slug Elysia timida (Opisthobranchia, Sacoglossa): hypodermic injection, sperm transfer and balanced reciprocity. Frontiers in Zoology, 4/17: 1-9.
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Hermaphroditic adults lay eggs in late spring. Larvae hatch out of gelatinous egg clutches after 7-8 days and are free-swimming veligers that eat single-celled algae in the plankton. After metamorphosis, which is induced by the presence of aucheria litorea or Vaucheria compacta, juveniles eat these algae and turn green in color from the sequestration of intact algal chloroplasts into their digestive diverticula (branches of digestive tract). In late spring, following the laying of egg masses, most slugs die (possibly from a virus, see Pierce et al., 1999) (Rumpho et al., 2000).

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Life Expectancy

Elysia chlorotica lives to be approximately 11 months old. Adults experience mass death after laying their string of eggs in the spring of each year both in the wild and when held in captivity. According to research done by Pierce this may be due to a viral expression not a biological clock. That means that although this death is synchronized among all adults it is due to the final stage of a disease that every slug inherits not an internal biological cue. Pierce et al. (1984) were unable to identify the pathogen but did find evidence of virulent DNA in the nucleus of all test subjects.

Average lifespan

Status: wild:
11 months.

Average lifespan

Status: captivity:
11 months.

Average lifespan

Status: wild:
11 months.

Average lifespan

Status: captivity:
11 months.

  • Pierce, S. 1982. Invertebrate cell volume control mechanisms: a coordinated use of intracellular amino acids and inorganic ions as osmotic solute. The Biological Bulletin, 1603: 405-419.
  • Pierce, S., R. Biron, M. Rumpho. 1996. Endosymbiotic chloroplasts in molluscan cells contain proteins synthesized after platid capture. The Journal of Experimental Biology, 199/10: 2323-2330.
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The typical lifespan is 11 months.

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Reproduction

The details of how E. chlorotica 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.

Sexually reproducing hermaphrodites may act only as female or male. Sperm are less costly than eggs, so functioning as a male may be more desirable energetically. 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.

Mating System: polygynandrous (promiscuous)

These slugs are simultaneous hermaphrodites, capable of internal self-fertilization, although this particular species more commonly outcrosses. Out-crossing is essential sexual reproduction with another individual. Eggs are laid in long mucous-laden strings, hatching approximately in a week. The eastern emerald elysia breeds once a year in the early spring.

Breeding interval: Once annually

Breeding season: Early spring

Range gestation period: 7 to 8 days.

Average gestation period: 7 days.

Key Reproductive Features: seasonal breeding ; simultaneous hermaphrodite; sexual ; fertilization (Internal ); oviparous

There are no documented incidents of parental care or investment in this species. Adults experience mass death both in natural and laboratory environments at approximately eleven months old. Pierce et al. (1984)suggest this is due to a viral expression, but little evidence exists.

Parental Investment: no parental involvement; pre-fertilization (Provisioning)

  • Pierce, S., S. Edwards, P. Mazzocchi, L. Klingler, M. Warren. 1984. Proline betaine: A unique osmolyte in an extremely euryhaline osmoconformer. The Biological Bulletin, 167/2: 495-500.
  • Pierce, S., T. Maugel, M. Rumpho, J. Hanten, W. Mondy. 1999. Annual viral expression in a sea slug population: Life cycle control and symbiotic chloroplast maintenance. The Biological Bulletin, 197/6: 1-6.
  • Rumpho, M., E. Summer, J. Manhart. 2000. Solar-powered sea slugs. Mollusc/algal chloroplast symbiosis. Plant Physiology, 123/1: 29-38.
  • Rumpho, M., K. Pelletreau, A. Moustafa, D. Bhattacharya. 2011. The making of a photosynthetic animal. The Journal of Experimental Biology, 214/2: 303-311.
  • Schmitt, V., N. Anthes, N. Michiels. 2007. Mating behaviour in the sea slug Elysia timida (Opisthobranchia, Sacoglossa): hypodermic injection, sperm transfer and balanced reciprocity. Frontiers in Zoology, 4/17: 1-9.
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Like all sacoglossans, this species is a simultaneous hermaphrodite. Copulation occurs by the penis via the vaginal opening/ female aperture, often reciprocally (Jensen, 1999). There is no penal stylet (Marcus, 1980).

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Molecular Biology and Genetics

Genetics

GenBank sequences from NCBI for Elysia chlorotica: here.

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Molecular Biology

Barcode data: Elysia chlorotica

The following is a representative barcode sequence, the centroid of all available sequences for this species.


There are 2 barcode sequences available from BOLD and GenBank.

Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.

See the BOLD taxonomy browser for more complete information about this specimen and other sequences.

TTGCGTTGACTCTTTTCAACAAACCATAAAGATATTGGTACTTTGTATGTAATTTTTGGTATGTGATGTGGATTAGTGGGGACTGGCTTA---AGTCTACTAATTCGATTTGAGCTCGGAACTTCCGGTGCTTTCTTAGGTGAT---GACCACTTTTATAATGTTATTGTTACAGCACACGCCTTTGTGATAATTTTTTTCATAGTTATGCCTCTAATAATTGGAGGATTCGGAAATTGAATGGTTCCTATTCTT---ATTGGTGCTCCCGATATAAGGTTCCCTCGTATAAATAATATAAGGTTCTGGTTACTTCCCCCTTCTTTTATTTTTCTTCTATGTTCTAGCCTCGTAGAAGGAGGTGCTGGGACAGGATGGACTGTGTACCCTCCACTAAGGGGGCCAATCGGCCACGGAGGGGCTTCCGTGGACTTG---GCAATTTTTTCACTCCATCTTGCCGGGATGTCTTCTATTCTAGGTGCAGTAAACTTTATTACTACGATTTTTAACATACGCTCTCCAGGAATAACATTTGAGCGGTTAAGGTTATTCGTCTGGTCTGTTCTTGTGACTGCCTTTTTATTACTTTTATCGCTTCCCGTTCTAGCTGGT---GCAATTACTATGTTATTAACTGATCGAAATTTCAATACTAGATTTTTTGATCCAGCTGGGGGCGGTGACCCTATCCTGTATCAACATCTTTTCTGATTTTTTGGTCACCCTGAAGTATATATTCTTATTTTACCAGGTTTTGGGATAATTTCTCATATCTTAAGTAATTTTTCTTCTAAGCCT------GCATTTGGAACACTAGGTATGATTTACGCTATGGTTTCTATTGGGTTATTAGGATTTATTGTATGAGCTCACCATATGTTTACTGTGGGAATGGATGTGGACACTCGGGCATACTTTACGGCTGCTACAATGGTAATTGCGGTGCCTACTGGTATTAAGGTGTTTAGTTGAATAATA---ACT
-- end --

Download FASTA File

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Statistics of barcoding coverage: Elysia chlorotica

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 3
Specimens with Barcodes: 3
Species With Barcodes: 1
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Conservation

Conservation Status

Elyisa chlorotica has no special status at this time. Populations are not in decline.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

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Relevance to Humans and Ecosystems

Benefits

There are no known negative effects to humans from Elyisa chlorotica.

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Although Elysia chlorotica does not directly benefit humans, members of the scientific community are very interested in this sea slug. Many studies about how this animal not only obtains the chloroplast from its algal food supply but also 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.

  • Kim, E., J. Archibald. 2010. Plastid evolution: Gene transfer and the maintenance of ‘stolen’ organelles. BMC Biology, 8/73: 1-3.
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References

Gould, A. A. 1870. Report on the Invertebrata of Massachusetts Second Edition, Comprising the Mollusca. Wright and Potter: Boston, pp. 524.

Marcus, E. 1980. Review of Western Atlantic Elysiidae (Opisthobranchia Ascoglossa) with a description of a new Elysia species. Bulletin of Marine Science 30(1): 54-79.

Pierce, S.K., Massey, S.E., Hanten, J.J., and N.E. Curtis. 2003. Horizontal transfer of functional nuclear genes between multicellular organisms Biol. Bull. 204: 237–240.

Rumpho, M. E., Summer, E. J., and J. R. Manhart. 2000. Solar-Powered Sea Slugs. Mollusc/Algal Chloroplast Symbiosis1 Plant Physiology 123: 29–38.

Rumpho, M. E., Summer, E. J., Green, B.J., Fox, T.C., and J. R. Manhart. 2001. Mollusc/algal chloroplast symbiosis: how can isolated chloroplasts continue to function for months in the cytosol of a sea slug in the absence of an algal nucleus? Zoology 104(3-4): 303-12.

Rumpho, M. E., Worfula, J.M., Leeb, J., Kannana, K., Tylerc, M.S., Bhattacharyad, B., Moustafad, A., and J. R. Manhart. 2008. Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica PNAS 105 (46): 17867–17871.

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Wikipedia

Elysia chlorotica

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.

Distribution[edit]

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.[1]

Ecology[edit]

This species is most commonly found in salt marshes, tidal marshes, pools and shallow creeks, at depths of 0 m to 0.5 m.[1]

Description[edit]

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.[2] By taking on the green color from the chloroplasts of the algal cells, the slugs are able to blend in with the green sea bed beneath them, helping them improve their chances of survival and fitness. 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.[3] This reddish-brown color is most often associated with juveniles since they are usually this color before they begin feeding on algae. This reddish color in turn could potentially harm the juvenile since they can easily be seen by predators, making them less likely to make it to adult-hood. This species can also have very small red or white spots scattered over the body.[3] A juvenile, prior to feeding, is brown with red pigment spots due to the absence of chloroplasts.[4] 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.[4]

Feeding[edit]

(A) A defined tubule of the digestive diverticula extending into the parapodial region of the animal (arrow). The digestive system consists of densely packed tubules that branch throughout the animal's body. Each tubule is made up of a layer of single cells containing animal organelles and numerous algal plastids. This cell layer surrounds the lumen. (B) Magnified image of the epidermis of E. chlorotica showing densely packed plastids. The animals are light grey in color without their resident plastids, which contribute chlorophyll to render the sea slugs bright green.

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.[4] 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.[5] 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.[4] 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. Some Elysia 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 a process known as phagocytosis in which the cells of the sea slug engulf the cells of the algae and make the chloroplasts apart of its own cellular content. 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. Photosynthesis is a chemical process that harnesses sunlight and allows it to be used as an energy source for organisms. This process was once believed to be exclusive to plants, but the discovery of organisms such as Elysia chlorotica has challenged that theory. 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.[6] Since then 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. 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.[7]

In another study, it was shown that "E. chlorotica" definitely have a way to support the survival of their chloroplasts. 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.[5] By spending less energy on activities such as finding food, the slugs can invest this precious energy into other important activities. 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, possibly acquired through horizontal gene transfer.[6] 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[6]) 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.[8] 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 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 actively expressed algal nuclear genes in Elysia cholorotica, or in the similar species Elysia timida and Plankobranchus ocellatus. [9][10] These results weaken support for the horizontal gene transfer hypothesis.[10] However a 2014 report utilizing fluorescent in situ hybridization to localize an algal nuclear gene, prk; confirmed horizontal gene transfer.[11]

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.[12] 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. [12]

Life cycle[edit]

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.[4]

Cleavage[edit]

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.[4]

Gastrulation[edit]

Elysia chlorotica gastrulation is by epiboly: the ectoderm spreads to envelope the mesoderm and endoderm.[4]

Larval stage[edit]

After the embryo passes through a trochophore-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 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.[4][13][14]

See also[edit]

References[edit]

  1. ^ a b Rosenberg, G. (2009). "Malacolog 4.1.1: A Database of Western Atlantic Marine Mollusca". Elysia chlorotica Gould, 1870. Retrieved 5 April 2010. 
  2. ^ name="Rumpho,Summer, and Manhart. "Solar-Powered Sea Slugs. Mollusc/Algal Chloroplast Symbiosis." Plant Physiology.May 2000.
  3. ^ a b Rudman, W.B. (2005). Elysia chlorotica Gould, 1870. [In] Sea Slug Forum. Australian Museum, Sydney
  4. ^ a b c d e f g h i Rumpho-Kennedy, M.E., Tyler, M., Dastoor, F.P., Worful, J., Kozlowski, R., & Tyler, M. (2006). Symbio: a look into the life of a solar-powered sea slug. Retrieved June 8, 2014, from https://web.archive.org/web/20110918070141/http://sbe.umaine.edu/symbio/index.html
  5. ^ a b Mujer, C.V., Andrews, D.L., Manhart, J.R., Pierce, S.K., & Rumpho, M.E. (1996). Chloroplast genes are expressed during intracellular symbiotic association of Vaucheria litorea plastids with the sea slug Elysia chlorotica. Cell Biology, 93, 12333-12338
  6. ^ a b c Rumpho ME; Worful JM; Lee J et al. (November 2008). "Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica". Proc. Natl. Acad. Sci. U.S.A. 105 (46): 17867–17871. doi:10.1073/pnas.0804968105. PMC 2584685. PMID 19004808. Retrieved 2008-11-24. 
  7. ^ Solar-Powered Slugs Are Not Solar-Powered, National Geographic
  8. ^ Green Sea Slug Is Part Animal, Part Plant, Wired
  9. ^ Wägele H, Deusch O, Händeler K, Martin R, Schmitt V, Christa G et al. (2011). "Transcriptomic evidence that longevity of acquired plastids in the photosynthetic slugs Elysia timida and Plakobranchus ocellatus does not entail lateral transfer of algal nuclear genes.". Mol Biol Evol 28 (1): 699–706. doi:10.1093/molbev/msq239. PMC 3002249. PMID 20829345. 
  10. ^ a b Bhattacharya D, Pelletreau KN, Price DC, Sarver KE, Rumpho ME (2013). "Genome analysis of Elysia chlorotica Egg DNA provides no evidence for horizontal gene transfer into the germ line of this Kleptoplastic Mollusc.". Mol Biol Evol 30 (8): 1843–52. doi:10.1093/molbev/mst084. PMC 3708498. PMID 23645554. 
  11. ^ Schwartz, J. A.; Curtis, N. E.; Pierce, S. K. (2014). "FISH Labeling Reveals a Horizontally Transferred Algal (Vaucheria litorea) Nuclear Gene on a Sea Slug (Elysia chlorotica) Chromosome". The Biological bulletin 227 (3): 300–12. PMID 25572217.  edit
  12. ^ a b de Vries J, Habicht J, Woehle C, Huang C, Christa G, Wägele H et al. (2013). "Is ftsH the key to plastid longevity in sacoglossan slugs?". Genome Biol Evol 5 (12): 2540–8. doi:10.1093/gbe/evt205. PMC 3879987. PMID 24336424. 
  13. ^ Mature Veliger (schema)
  14. ^ Video
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