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Entovalva lessonothuriae

By Jingchun Li

Geographic Range

This species is known only from Japan. It was first discovered and described from the Ryukyu Archipelago: Kasari Bay on Amami Island and in Urazoko Bay on Ishigaki Island. (Kato, 1998)

Habitat

This clam species has an extraordinary life style: it lives inside the esophagus of a sea cucumber, Holothuria pardalis. These sea cucumbers are normally found in sand under boulders on coral-reef flats within the intertidal zone. The clams have been found nowhere else, they seem to occur only in this host species. (Kato, 1998)

Physical Description

Species in the genus Entovalva all have very distinct morphologies compared to other bivalve species, presumably because of adaptations to an endosymbiotic life style. Their shells are small, thin and completely enclosed in expanded mantle tissue. The gills are well developed and extend outside the shell. In females, one part of the mantle cavity (the suprabranchial chamber) is greatly enlarged and used to brood larvae. The foot is very well developed and is usually bent to the left to attach the esophagus wall of the host. Middelfart 2011 is a link to a short video clip of a live Australian Entovalva. (Kato, 1998; Middelfart and Craig, 2004; Middelfart, 2011)

  • Range length
    ~0.7 to ~3.7 mm
    to in

Development

The fertilized eggs are brooded in a suprabranchial chamber of the female until they develop into D-shell veliger stage larvae. The larvae are then released and are thought to pass through the host's intestine and out. To survive they must then be ingested by a new host during the host's deposit-feeding activities. (Kato, 1998)

Reproduction

This species is a sequential protandric hermaphrodite. Clams are mostly found in pairs inside one host: normally one big female individual and a small male. Interestingly, the male is found attached to the female clam, not the host sea cucumber. In occasions when only one individual is present in the host, that individual, if sexually mature, is always a female. (Kato, 1998)

Entovalva lessonothuriae are normally found in pairs: One female and one dwarf male. Very little is known of reproductive modes. Fertilization is apparently internal, females retain their fertilized embryos until the larvae reach the D-shell veliger stage. Clutch size, breeding cycles, and age at reproduction are unknown. (Kato, 1998)

This species is a brooder: the female keeps fertilized eggs inside her suprabranchial chamber until they develop into veliger larvae. Males provide no parental investment. (Kato, 1998)

Lifespan/Longevity

The life span of Entovalva lessonothuriae is currently unknown, but it probably lives for multiple years.

Behavior

Entovalva lessonothuriae has a bilaterally flattened foot that bends to the left. This is thought to be an adaptation for attachment to the host holothurian's esophagus. The bivalve has also retained autonomous locomotion, respiration and feeding abilities, which is unusual compared to other endosymbiotic taxa. (Kato, 1998)

Communication and Perception

Detailed studies about communication and perception of Entovalva lessonothuriae have not been done so far. However it is documented that sensory organs such as papillae on the mantle (found in related species) are almost lacking in Entovalva lessonothuriae. This could be a result of the species' endosymbiotic life style. It's very likely that individuals communicate chemically, so that males and females can locate each other within a host. (Kato, 1998)

Food Habits

The well-developed ctenidia (“gills”) suggest that Entovalva lessonothuriae is still a filter feeder, even though it lives within the digestive tract of its host holothurian. (Kato, 1998)

Predation

Specific predators on Entovalva lessonothuriae are unknown. Its endosymbiotic life style probably protects it from many predators, though also makes it vulnerable to predators that attack its host.

Ecosystem Roles

In Kato (1998)'s study, 19%~33% of the holothurian hosts examined harbored Entovalva lessonothuriae and did not contain any other symbionts. This suggests that Entovalva lessonothuriae might discourage competitors, but no other evidence of this has been discovered. It is not known whether Entovalva lessonothuriae has any effect on the host's nutrition. The size of the bivalve was very weakly correlated with host biomass. So far, Entovalva lessonothuriae has only been found from one host species, but only one other holothurian species in its range, Patinapta ooplax, is known to have been examined. (Kato, 1998)

Species Used as Host
  • Holothuria pardalis

Economic Importance for Humans: Positive

Entovalva lessonothuriae has no known economic value for humans.

Economic Importance for Humans: Negative

There are no known adverse effects of Entovalva lessonothuriae on humans.

Conservation Status

The population size of this species is unknown, and it has not been given any special legal protections.

Contributors

Jingchun Li (author), Special Projects, George Hammond (editor), Animal Diversity Web Staff.

Glossary

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

detritus

particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

ovoviviparous

reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.

planktivore

an animal that mainly eats plankton

polyandrous

Referring to a mating system in which a female mates with several males during one breeding season (compare polygynous).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

zooplankton

animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)

References

Kato, M. 1998. Morphological and ecological adaptations in montacutid bivalves endo- and ecto-symbiotic with holothurians. Canadian Journal of Zoology, 76: 1403–1410.

Middelfart, P. 2011. "Bizarre clam: Entovalva sp." (On-line). Youtube. Accessed July 31, 2011 at http://www.youtube.com/watch?v=5hPGUozETN0.

Middelfart, P., M. Craig. 2004. Description of Austrodevonia sharnae n. gen., n. sp. (Galeommatidae: Bivalvia), an ectocommensal of Taeniogyrus australianus (Stimpson, 1855) (Synaptidae: Holothur- oidea). Molluscan Research, 24: 211–219.

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