Small giant clam (Tridacna maxima)
Facts
| Also known as: | rugose clam and maxima clam |
|---|
| Kingdom | Animalia |
|---|---|
| Phylum | Mollusca |
| Class | Bivalvia |
| Order | Veneroida |
| Family | Tridacnidae |
| Genus | Tridacna (1) |
| Size |
Length: 35 – 40 cm (2) |
Status
The small giant clam is classified as Lower Risk – conservation dependent (LR/cd) on the IUCN Red List 2004 (1) and is listed on Appendix II of CITES (3).
Description
At less than a third of the size of the true giant clam (Tridacna gigas), the small giant clam deserves its name. As an adult, it has a large shell that adheres to a rock by its byssus – a tuft of long, tough filaments that protrude from a hole next to the hinge of the shell. When open, the bright blue, green or brown mantle is exposed and obscures the edges of the shell with its prominent and distinctively furrowed edges. The small giant clam is a bivalve mollusc, referring to the two valves on the mantle. These siphon water through the body to extract oxygen from the water using the gills, and to feed on algae. The pigmentation of the mantle is caused by iridophores – pigment cells which are thought to protect the clam from the effects of intense light (5).
Range
The small giant clam has the widest range of all giant clam species. It is found in the oceans surrounding east Africa, India, China, Australia, Southeast Asia and the islands of the Pacific (1).
Habitat
It is found living on the surface of reefs or sand, or partly embedded in coral (2). It occupies well lit areas, due to its symbiotic relationship with photosynthetic algae, which require sunlight for energy production (5).
Biology
A sessile mollusc, the small giant clam attaches itself to rocks and siphons water through its body, filtering it for phytoplankton, as well as extracting oxygen with its gills. However, it does not need to filter-feed as much as other clams since it obtains most of the nutrients it requires from tiny photosynthetic algae known as zooxanthellae (5).
Beginning life as a tiny fertilised egg, the small giant clam hatches within 12 hours, becoming a free-swimming larva. This larva then develops into another, more developed, larva which is capable of filter-feeding. At the third larval stage, a foot develops, allowing the larva to alternately swim and rest on the substrate. After 8 – 10 days, the larva metamorphoses into a juvenile clam, at which point it can acquire zooxanthellae and function symbiotically. The juvenile matures into a male clam after two or three years, becoming a hermaphrodite when larger. Reproduction is stimulated by the lunar cycle, the time of day, and the presence of other eggs and sperm in the water. Hermaphroditic clams will release sperm first, followed by eggs (5).
Threats
The small giant clam is collected in small numbers for the shell trade, and may also be eaten (2).
Conservation
It has been shown that numbers in protected regions are considerably higher than numbers outside these areas. Research into clam farming, as well as into the ecology, growth rates and reproductive behaviour of the small giant clam is necessary to understand the conservation needs of this species. The Tongan Government gives a limit for the minimum size that can be harvested (2).
Further Information
For further information on giant clams and their cultivation see:
Ellis, S. (1998) Spawning and early larval rearing of giant clams (Bivalvia: Tridacnidae). Center for Tropical and Subtropical Aquaculture, 130: 1 – 55. Available online at:
http://www.ctsa.org/upload/publication/CTSA_130631672860873095404.pdf
Authentication
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Alga: A collection of taxonomically unrelated groups that share some common features but are grouped together for historical reasons and for convenience. They are of simple construction, and are mainly photoautotrophic, obtaining all their energy from light and carbon dioxide, and possess the photosynthetic pigment, chlorophyll A. They range in complexity from microscopic single cells to very complex plant-like forms, such as kelps. Algal groups include blue-green algae (cyanobacteria), red algae (rhodophyta), green algae (chlorophyta), brown algae and diatoms (chromista) as well as euglenophyta.
Hermaphrodite: Possessing both male and female sex organs.
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.
Metamorphosis: An abrupt physical change from the larval to the adult form.
Photosynthesis: 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.
Plankton: Aquatic organisms that drift with water movements; may be either phytoplankton (plants), or zooplankton (animals).
Sessile: Fixed and stationary.
Symbiotic relationship: Relationship in which two organisms form a close association, the term is now usually used only for associations that benefit both organisms (a mutualism).
References
- IUCN Red List (February, 2005)
http://www.redlist.org - Collins, N.M., Pyle, R.M. and Wells, S.M. (1983) The IUCN Invertebrate Red Data Book. IUCN, Gland, Switzerland.
- CITES (February, 2005)
http://www.cites.org - Fish Index (February, 2005)
http://species.fishindex.com/species_2649tridacna_maxima_maxima_clam.html - Ellis, S. (1998) Spawning and early larval rearing of giant clams (Bivalvia: Tridacnidae). Center for Tropical and Subtropical Aquaculture, 130: 1 - 55.
