Elysia chlorotica

Half Plant Half Animal

  

 by :

            Name                 : Ridlo Firmansyah

          NIM                  : 130210103078

             Study Program   : Biology Education

BIOLOGY EDUCATION

FACULTY OF TEACHER TRAINING AND EDUCATION

JEMBER UNIVERSITY

                                                           2013

Geographic Range

            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 (Rumpho, et al., 2011)

Habitat

            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 (Green, et al., 2000).

Physical Description

            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 (Colin, 1978).

  

Development

            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 (Schmitt, et al., 2007)

Reproduction

            In a similar species, the mating behaviors of Elysia chlorotica 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. 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 (Rumpho, et al., 2011).

Lifespan

            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).

Food Habits

            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 (Brandly, 1984).

Special Unique

            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, plant and animal 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. So, that’s why this animals has the ability to does a photosynthesis processes and make a plant cell continue functioned on an animal cells (Pierce et al., 2003). 

References

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

Colin, P. 1978. Marine Invertebrates And Plants of the Living Reef. Neptune city: T.F.H Publications

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., T. Maugel, M. Rumpho, J. Hanten, W. Mondy. 2003. Annual viral expression in a sea slug population: Life cycle control and symbiotic chloroplast maintenance. The Biological Bulletin, 197/6: 1-6

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.