Elysia Chlorotica
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''Elysia chlorotica'' (
common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often con ...
the eastern emerald elysia) is a small-to-medium-sized
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of green
sea slug Sea slug is a common name for some Marine biology, marine invertebrates with varying levels of resemblance to terrestrial Slug, slugs. Most creatures known as sea slugs are gastropods, i.e. they are Sea snail, sea snails (marine gastropod moll ...
, a marine opisthobranch
gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...
mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...
. This sea slug superficially resembles a
nudibranch Nudibranchs () are a group of soft-bodied marine gastropod molluscs, belonging to the order Nudibranchia, that shed their shells after their larval stage. They are noted for their often extraordinary colours and striking forms, and they have b ...
, yet it does not belong to that
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
. Instead it is a member of the clade Sacoglossa, the sap-sucking sea slugs. Some members of this group use
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s from the
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
they eat for
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
, a phenomenon known as kleptoplasty. ''Elysia chlorotica'' is one species of such "solar-powered sea slugs". It lives in a subcellular endosymbiotic relationship with chloroplasts of the marine
heterokont The stramenopiles, also called heterokonts, are protists distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, an ...
alga '' Vaucheria litorea''.


Distribution

''Elysia chlorotica'' can be found along the east coast of the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, including the states of
Massachusetts Massachusetts ( ; ), officially the Commonwealth of Massachusetts, is a U.S. state, state in the New England region of the Northeastern United States. It borders the Atlantic Ocean and the Gulf of Maine to its east, Connecticut and Rhode ...
,
Connecticut Connecticut ( ) is a U.S. state, state in the New England region of the Northeastern United States. It borders Rhode Island to the east, Massachusetts to the north, New York (state), New York to the west, and Long Island Sound to the south. ...
, New York,
New Jersey New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...
,
Maryland Maryland ( ) is a U.S. state, state in the Mid-Atlantic (United States), Mid-Atlantic region of the United States. It borders the states of Virginia to its south, West Virginia to its west, Pennsylvania to its north, and Delaware to its east ...
,
Rhode Island Rhode Island ( ) is a state in the New England region of the Northeastern United States. It borders Connecticut to its west; Massachusetts to its north and east; and the Atlantic Ocean to its south via Rhode Island Sound and Block Is ...
,
Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...
, (east Florida and west Florida) and
Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
. They can also be found as far north as
Nova Scotia Nova Scotia is a Provinces and territories of Canada, province of Canada, located on its east coast. It is one of the three Maritime Canada, Maritime provinces and Population of Canada by province and territory, most populous province in Atlan ...
,
Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
.


Ecology

This species is most commonly found in
salt marshes A salt marsh, saltmarsh or salting, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open Seawater, saltwater or brackish water that is regularly flooded by the ti ...
, tidal marshes, pools, and shallow creeks, at depths of 0 m to 0.5 m.


Description

Adult ''Elysia chlorotica'' are usually bright green in color owing to the presence of ''Vaucheria litorea''
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s in the cells of the slug's digestive
diverticula In medicine or biology, a diverticulum is an outpouching of a hollow (or a fluid-filled) structure in the body. Depending upon which layers of the structure are involved, diverticula are described as being either true or false. In medicine, t ...
. 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. 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 and fitness. However, they can occasionally appear reddish or greyish in colour, which is thought to depend on the amount of
chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
in the branches of the digestive gland throughout the body. This species can also have very small red or white spots scattered over the body. A juvenile, prior to feeding on algae, is brown with red
pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...
spots due to the absence of chloroplasts. ''Elysia chlorotica'' have a typical elysiid shape with large
lateral Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...
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.


Feeding

''Elysia chlorotica'' feeds on the intertidal alga '' Vaucheria litorea''. It punctures the algal cell wall with its radula, then holds the algal strand firmly in its mouth and sucks out the contents as from a straw. Instead of digesting the entire cell contents, or passing the contents through its gut unscathed, it retains only the
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s, by storing them within its extensive digestive system. It then takes up the live chloroplasts into its own gut cells as organelles and maintains them alive and functional for many months. The acquisition of chloroplasts begins immediately following
metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...
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 Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
, at which point they become green. This is caused by the distribution of the chloroplasts throughout the extensively branched gut. 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. 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 the process of
phagocytosis Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...
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 of ''Elysia chlorotica'' allows the slug to capture energy directly from light, as most plants do, through the process of
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. ''E. chlorotica'' can, during time periods where algae is not readily available as a
food Food is any substance consumed by an organism for Nutrient, nutritional support. Food is usually of plant, animal, or Fungus, fungal origin and contains essential nutrients such as carbohydrates, fats, protein (nutrient), proteins, vitamins, ...
supply, survive for months. It was once thought that this survival depended on the sugars produced through
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
performed by the chloroplasts, 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. 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 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 of ''E. timida'' and kept them in complete darkness for 88 days — and all survived. 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 while maintaining their fine structure. 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 in a functional state indicates that ''Elysia chlorotica'' could possess photosynthesis-supporting genes within its own nuclear
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
, possibly acquired through
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
. Since chloroplast DNA alone encodes for just 10% of the
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s 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 Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...
-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 Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
, as it was already present in the eggs and sex cells of ''Elysia chlorotica''. 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 actively expressed algal nuclear genes in ''Elysia cholorotica'', or in the similar species '' Elysia timida'' and '' Plakobranchus ocellatus''. These results weaken support for the horizontal gene transfer hypothesis. A 2014 report utilizing fluorescent in situ hybridization (FISH) to localize an algal nuclear gene, prk, found evidence of horizontal gene transfer. However, these results have since been called into question, as FISH analysis can be deceptive and cannot prove horizontal gene transfer without comparison to the ''Elysia cholorotica'' genome, which the researchers failed to do.Rauch C, J de Vries, S Rommel, LE Rose, C Woehle, G Christa, EM Laetz, H Wägele, AGM Tielens, J Nickelsen, T Schumann, P Jahns, and SB Gould. 2015. Why it is time to look beyond algal genes in photosynthetic slugs. Genome Biology and Evolution Advance Access 7:2602–2607. 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''.


Life cycle

Adult ''Elysia chlorotica'' are simultaneous
hermaphrodite A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...
s. When sexually mature, each animal produces both
sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...
and eggs at the same time. However,
self-fertilization Autogamy or self-fertilization refers to the Cell fusion, fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a Reproduction, reproductive mechanism employed by many flowering pl ...
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.


Cleavage

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 In Euclidean geometry, an angle can refer to a number of concepts relating to the intersection of two straight Line (geometry), lines at a Point (geometry), point. Formally, an angle is a figure lying in a Euclidean plane, plane formed by two R ...
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 An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
forms a stereoblastula, meaning a
blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development, the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (fr ...
without a clear central cavity.


Gastrulation

''Elysia chlorotica'' gastrulation is by epiboly: the
ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...
spreads to envelope the
mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...
and endoderm.


Larval stage

After the
embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
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 feeds 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 The stomach is a muscular, hollow organ in the upper gastrointestinal tract of Human, humans and many other animals, including several invertebrates. The Ancient Greek name for the stomach is ''gaster'' which is used as ''gastric'' in medical t ...
. In the stomach, food is sorted and then moved on to the digestive gland, where the food is digested and the
nutrients A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
are absorbed by the epithelial cells of the digestive gland.


See also

* '' Elysia clarki'' * '' Elysia viridis'' * Karyoklepty


References


External links


Solar-powered Sea Slug Harnesses Stolen Plant Genes
''New Scientist'', 2008-11-24
Half Plant, Half Animal
Videos w/additional info.
Science News



The animal that wanted to be a plant
(in Spanish)
Video showing Elysia chlorotica
{{taxonbar, from=Q133568 Plakobranchidae Gastropods described in 1870 Taxa named by Augustus Addison Gould