''Entovalva nhatrangensis'' is a

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 small marine

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

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 ...

in 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 ...

Lasaeidae. It was first described in 2010 and its specific name ''"nhatrangensis"'' derives from the locality where it was originally found,

Nha Trang Nha Trang ( or ; ) is a coastal city and capital of Khánh Hòa Province, on the South Central Coast of Vietnam. It is bounded on the north by Ninh Hoà town, on the south by Cam Ranh city and on the west by Diên Khánh District. The city had ...

Bay in

Vietnam Vietnam, officially the Socialist Republic of Vietnam (SRV), is a country at the eastern edge of mainland Southeast Asia, with an area of about and a population of over 100 million, making it the world's List of countries and depende ...

. It lives inside the

oesophagus The esophagus (American English), oesophagus (British English), or œsophagus ( archaic spelling) ( see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), colloquially known also as the food pipe, food tube, or gullet, ...

of certain species of sea cucumbers. It is considered to be an

endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

rather than 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 ...

because it does not harm its

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County * Host Island, in the Wilhelm Archipelago, Antarctica People * ...

Description

''Entovalva nhatrangensis'' can grow to about in length. It has a very small, delicate shell which is internal, being entirely enclosed by large folds of the mantle which fuse above the hinge. The

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s are small but their structure is similar to those of other free-living bivalves. The foot extends permanently from the ventral side of the animal. This is relatively large compared to other bivalves and contains the visceral mass and the gonads. ''Entovalva nhatrangensis'' can be distinguished from the other three previously described species in the genus ''Entovalva'' by the different shape of its body and foot, and by the fact that its outer body epithelium is distinctively folded.

Biology

''Entovalva nhatrangensis'' is an endosymbiont living inside the

of a

sea cucumber Sea cucumbers are echinoderms from the class (biology), class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number be ...

. In a research study undertaken in 2010, 23 sea cucumbers known as brown sandfish ('' Holothuria spinifera'') were gathered from shallow waters in Nha Trang Bay in Vietnam. Of these, 22 were found to harbour the bivalve in their oesophagus. The average number of molluscs per host was 84 with a range of 1 to 167. Another sea cucumber, '' Holothuria leucospilota'', also acts as a host to the bivalve but of 30 specimens collected from the same locality, only one was found to harbour any of them, and that one contained just 5 molluscs. It was at first surmised that ''Entovalva nhatrangensis'' might absorb nutrients through its epithelium. This idea was rejected however because the surface of the mantle is covered by a thin cuticle, which would make absorption difficult. The stomach was found to contain

diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

s, therefore it is likely that the bivalve uses its gills to filter them and other fine organic particles from the contents of the sea cucumber's gut in which it is immersed. ''Entovalva nhatrangensis'' is a protandric hermaphrodite. This means that it starts life as a male and later becomes a female. Fertilization takes place when a male inserts

spermatophore A spermatophore, from Ancient Greek σπέρμα (''spérma''), meaning "seed", and -φόρος (''-phóros''), meaning "bearing", or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especiall ...

s (bundles of sperm) into the gill cavity of a female. It is unclear how this is done but it seems likely that the male, always smaller than the female, makes its way into the siphon of the female to deposit them in the required position. The eggs are fertilized as they emerge from the genital pore into the siphon. The resulting larvae are brooded in the siphon until they are released as D-larvae with their rudimentary shells already formed. It is surmised that the larvae pass through the sea cucumbers gut and out through its anus before becoming

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

ic for a period.

References

{{Taxonbar, from=Q25938376 Lasaeidae Molluscs of the Pacific Ocean Molluscs described in 2010