Horse’s hoof clam (Hippopus hippopus)

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Horse's hoof clam
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Horse’s hoof clam fact file

Horse’s hoof clam description

KingdomAnimalia
PhylumMollusca
ClassBivalvia
OrderVeneroida
FamilyTridacnidae
GenusHippopus (1)

A member of the giant clam family (Tridacnidae), the horse’s hoof clam (Hippopus hippopus) is named for the shape of its shell. Reddish bands on the outer side of the shell give it its alternative common name of ‘strawberry clam’ (2).

As in all clams, the body of the horse’s hoof clam is enclosed within a shell made from two separate hinged sections, known as valves (4). The valves of the horse’s hoof clam are thick, heavy, and often covered with encrustations (5). Located immediately beneath the valves is a flap of muscular tissue, called the mantle (4), which in this species is a mottled greenish or brownish (2) (5).

Also known as
bear paw clam, strawberry clam.
Size
Length: 40 cm (2)
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Horse’s hoof clam biology

Remarkably, all horse’s hoof clams spend the first two to three years of life as males, but then develop reproductive organs that enable them to produce both sperm and eggs (2) (6).

The horse’s hoof clam spawns in June (7), releasing sperm and then eggs into the surrounding waters, where fertilisation takes place (2). The fertilised eggs quickly develop into a swimming larval stage before settling on the substrate, where they develop into the adult clams and are unable to move from their position on the coral reef (6) (8). The horse’s hoof clam reaches maturity at about three to five years old, when it measures between 13 and 15 centimetres (5).

The horse’s hoof clam has a special relationship with photosynthetic algae, which live inside the clam’s fleshy mantle tissue, and provide it with the majority of its nutrition (6) (8). The algae use sunlight, like plants, to produce sugars, which are then released into the bloodstream of the clam (6) In return, the algae may receive some nutrition from the clam’s waste products (4).

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Horse’s hoof clam range

The horse’s hoof clam is found throughout the Indo-Pacific, from the coast of Thailand, east to Vanuatu. It used to be found in the waters surrounding Tonga, Fiji and Samoa, but is now extinct in these locations (2).

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Horse’s hoof clam habitat

The horse’s hoof clam occurs on the ocean floor in sandy and rubble areas on outer reef flats, in water up to ten metres deep. It may also occur on seaweed beds (2) (5).

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Horse’s hoof clam status

The horse’s hoof clam is classified as Least Concern (LC) on the IUCN Red List (1) and listed on Appendix II of CITES (2).

IUCN Red List species status – Least Concern

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Horse’s hoof clam threats

A major concern for the horse’s hoof clam, and all giant clams, is consumption by locals. In the island of Kiribati, local folklore says that the fully grown clams leave their shells, turn into rays and swim away. This belief has caused fishermen to harvest giant clams before they are mature. Not only is the meat eaten, but the shell is used in a variety of ways (2), such as for tourist souvenirs (9). As giant clam numbers are depleted, the prices for them increase, leading to increased exploitation (2).

It is not certain whether exploitation is the cause of the horse’s hoof clam’s extinction in Tonga, Fiji and Samoa. However, those countries do have a history of overfishing other species of giant clams (10).

Like other giant clam species, the horse’s hoof clam may be particularly vulnerable to over-collection due to its late maturity, relatively short larval lifespan, and the ease with which it can be harvested from shallow reef habitats (11).

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Horse’s hoof clam conservation

There are some measures in place in order to protect the horse’s hoof clam from over-exploitation (9). For example, export of horse’s hoof clam meat from Palau is prohibited (12), and commercial harvesting is prohibited in the Marshall Islands (9). The horse’s hoof clam is also listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), meaning that international trade in this species should be carefully controlled (3).

In addition, a number of countries breed the horse’s hoof clam in captivity (9), which may help lessen the pressure on wild populations.

View information on this species at the UNEP World Conservation Monitoring Centre.
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Find out more

Find out more about the conservation of clams and other molluscs:

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Authentication

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

This species information was authored as part of the ARKive and Universities Scheme.
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Glossary

Algae
Simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
Fertilisation
The fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.
Larval
Of the stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
Mantle
In molluscs, a fold of skin that encloses a space known as the mantle cavity, which contains the gills. The mantle is responsible for the secretion of the shell.
Photosynthetic
Capable of photosynthesis, a metabolic process characteristic of plants in which carbon dioxide is broken down, using energy from sunlight absorbed by the green pigment chlorophyll. Organic compounds are made and oxygen is given off as a by-product.
Reef flat
The shoreward, flat, broadest area of a coral reef.
Spawning
The production or depositing of large quantities of eggs in water.
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References

  1. IUCN Red List (May, 2011)
    http://www.iucnredlist.org/
  2. Knop, D. (1996) Giant Clams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams. Kraft Druck GmbH, Ettlingen, Germany.
  3. CITES (May, 2011)
    http://www.cites.org/
  4. Burton, M. and Burton, R. (2002) International Wildlife Encyclopedia. Marshall Cavendish, New York.
  5. Isamu, T. (2008) Palau Case Study - Tridacnidae. International Expert Workshop on CITES Non-Detriment Findings, Cancun, Mexico. Available at:
    http://www.conabio.gob.mx/institucion/cooperacion_internacional/TallerNDF/wg9.html
  6. Ellis, S. (1997) Spawning and Early Larval Rearing of Giant Clams (Bivalvia: Tridacnidae). Publication Number No. 130, Center for Tropical and Subtropical Aquaculture, Hawaii.
  7. Jameson, S. (1976) Early life history of the giant clams Tridacna crocea (Lamarck), Tridacna maxima (Roding), and Hippopus hippopus (L.). Pacific Science, 30: 219-233.
  8. Ruppert, E.E., Fox, R.S. and Barnes, R.D. (2004) Invertebrate Zoology. Thomson Brooks/Cole, Belmont, USA.
  9. CITES (2006) Review of Significant Trade in Specimens of Appendix II Species: Species Selected Following CoP12, 10.2-8b. Twenty-second Meeting of the Animals Committee, Lima, Peru. Available at:
    http://www.cites.org/eng/com/ac/22/index.shtml
  10. Munro, J. (1993) Giant Clams. Pacific Islands Forum Fisheries Agency, Honiara.
  11. Kay, E.A. (1995) The Conservation Biology of Molluscs. IUCN, Gland, Switzerland. Available at:
    http://data.iucn.org/dbtw-wpd/edocs/SSC-OP-009.pdf
  12. Heslinga, G., Perron, F. and Orak, O. (1984) Mass culture of giant clams (F. tridacnidae) in Palau. Aquaculture, 39: 197-215.
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Horse's hoof clam  
Horse's hoof clam

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