''Elysia atroviridis'' is a

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

species of

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 ...

belonging to the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Plakobranchidae Plakobranchidae is a family (biology), family of sea slugs, marine (ocean), marine opisthobranch gastropod mollusks in the superfamily Plakobranchoidea. They superficially resemble nudibranchs but they are sacoglossans, members of the clade Sacog ...

native to the northwest pacific. ''E. atroviridis'' slugs live up to three meters deep, and are generally dark green in skin colour with small, black spots all around the body. ''E. atroviridis'' is a member of the superorder

Sacoglossa Sacoglossa are a superorder of small sea slugs and sea snails, marine gastropod mollusks that belong to the clade Heterobranchia known as sacoglossans. There are 284 valid species recognized within this superorder. Sacoglossans live by ingestin ...

, and thus possesses the ability to perform

kleptoplasty Kleptoplasty or kleptoplastidy is a process in symbiosis, symbiotic relationships whereby plastids, notably chloroplasts from algae, are sequestered by the host. The word is derived from ''Kleptes'' (κλέπτης) which is Greek language, Greek ...

. ''E. atroviridis'' and '' Elysia marginata'' are notable for possessing the ability to completely autotomize their bodies and grow them back within 20 days.

Kleptoplasty

Being a member of the superorder Sacoglossa, ''E. atroviridis'' is capable of

, which is the process of taking

chloroplasts 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 ...

from digested

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 ...

and using them 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 ...

. Kleptoplasty is performed when the slug eats algae and partially digests it, but keeps the chloroplasts intact, incorporating them into their digestive tract's tissues, which allows the chloroplasts to stay alive and contribute energy to the host via photosynthesis.

Autotomy

''E. atroviridis'', along with ''E. marginata'', possess the ability to completely autotomize their bodies, and survive without their

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

kidneys In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and right in the retro ...

reproductive organs A sex organ, also known as a reproductive organ, is a part of an organism that is involved in sexual reproduction. Sex organs constitute the primary sex characteristics of an organism. Sex organs are responsible for producing and transporting ...

, etc. The method they use is unknown, but it is theorized the groove on their neck which is at the location where the head detaches plays a role in the autotomy, and the aforementioned kleptoplasty plays a role in keeping the slugs alive without the digestive system, but researchers are still unsure about how they manage to stay alive without their major organs. The bodies never regrew the head, and responded to tactile stimuli, but eventually stopped and grew pale, presumably from losing chloroplasts. Older individuals after autotomy did not feed and died after 10 days.

Function

The function of the autotomy is unknown. In an experiment, 82 ''E. atroviridis'' specimens were infected with a

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

, and three autotomized. The researchers suggested that the slugs autotomize as a reaction to the infection. Furthermore, all slugs who autotomized were infected with a

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

, and all regrew their bodies back with no parasite. The autotomy could not be a response to predation, as it takes several hours to autotomize. Additionally, researchers attempted to simulate a

predator Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

attack but no autotomy was attempted in the slug. Other theories for the function of the autotomy include escaping algae the slugs are tangled in and removing accumulated toxic chemicals.

Kleptoplasty

Autotomy

Function

References

Further reading