Horse’s hoof clam  (Hippopus hippopus)

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).

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).

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).

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).

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).

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).

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.

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

• 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