''Entovalva nhatrangensis'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

of small

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hing ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is est ...

in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Lasaeidae Lasaeidae is a family of very small saltwater clams, marine bivalve molluscs in the order Galeommatida. These bivalves are sometimes called "kelly clams", because one of the genera in this family is ''Kellia''. Genera and species Genera and ...

. 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 ha ...

Bay in

Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making ...

. It lives inside the

oesophagus The esophagus ( American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to ...

of certain species of

sea cucumbers Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian ...

. It is considered to be an

endosymbiont An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...

rather than a

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson h ...

because it does not harm its host.

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 A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

which fuse above the hinge. The

gill A gill () is a respiratory organ that many 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 respiration on land provided they ar ...

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 Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuri ...

. In a research study undertaken in 2010, 23 sea cucumbers known as brown sandfish (''

Holothuria spinifera ''Holothuria spinifera'', the brown sandfish, is a species of sea cucumber in the family Holothuriidae. It is placed in the subgenus ''Theelothuria'', making its full name ''Holothuria'' (''Theelothuria'') ''spinifera''. In India it is known as ...

'') 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 ''Holothuria leucospilota'', commonly known as the black sea cucumber or black tarzan, is a species of marine invertebrate in the family Holothuriidae. It is placed in the subgenus ''Mertensiothuria'' making its full scientific name ''Holothuria ...

'', 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''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising se ...

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 In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have sep ...

. This means that it starts life as a male and later becomes a female. Fertilization takes place when a male inserts

spermatophore A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially salamanders and arthropods, and transferred in entirety to the female's ovipore during reproduction. Spermatophore ...

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 found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

ic for a period.

References

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