Giant clams (subfamily Tridacninae) are the world’s largest living marine bivalves. The largest of all species, Tridacna gigas can grow up to 1 m long, and weigh over 250 kg! Highly conspicuous with their vibrant mantle colours and patterns, these animals are mostly found within the shallow tropical coral reefs of the Indo-Pacific.
Currently, there are 12 described species classified into two genera: Hippopus and Tridacna (see Table 1 for 11 of them; see Fig. 2 for 10 of them). Tridacna ningaloo was recently synonymised with T. noae (Borsa et al. 2015). In 2016, a new species – Tridacna lorenzi was described from the outlying territories of Mauritius (Monsecour 2016).
According to Neo et al. (2017), many of the tridacnine species are facing endangerment from over-exploitation (for food and shells) and loss of habitats. A recent example of extreme exploitation is from the South China Sea.
Ten interesting facts about the Giant Clams:
- Historically, giant clams were notoriously believed to be man-eaters due to their large gape size and ability to clam up! However, there have been no records of human mortality.
- Giant clams are very long-lived animals in the wild! Some say they can live up to 100 years or more!
- Giant clams are simultaneous hermaphrodites – possessing both male and female sexual organs (when mature).
- Giant clams are highly fecund. A single mature Tridacna gigas can produce up to 500 million eggs in one spawning!
- Giant clams share a unique symbiotic relationship with a type of microalgae: zooxanthellae (Symbiodinium spp.). The clams host the zooxanthellae shelter and protection from UV, while zooxanthellae provides food to the clam hosts.
- Although giant clams remain largely sessile throughout their life history, the large animals have displayed an array of behaviour patterns!
- Giant clams can ‘see’ using their light sensitive photo-receptors to detect changes in light intensity.
- Giant clams can squirt an accurate jet of water at a target!
- Giant clams are highly valued by Asian gastronomes, particularly for their adductor muscles.
- Giant clams play numerous ecological roles in the coral reef ecosystems, such as providers of food and shelters, contributors to reef productivity, and builders and shapers of reefs.
Selected further readings:
- Heslinga, GA and WK Fitt (1987) The domestication of reef-dwelling clams. BioScience 37(5): 332-339.
- Huang, D et al. (2007) Movement and aggregation in the fluted giant clam (Tridacna squamosa L.). Journal of Experimental Marine Biology and Ecology 342: 269-281.
- Lucas, JS (1994) The biology, exploitation, and mariculture of giant clams (Tridacnidae). Reviews in Fisheries Science 2(3): 181-223.
- Mingoa-Licuanan, SS and ED Gomez (2002) Giant clam conservation in Southeast Asia. Tropical Coasts: 24-31.
- Monsecour, K (2016) A new species of Giant Clam (Bivalvia: Cardiidae) from the Western Indian Ocean. Conchylia 46 (1-4): 69-77.
- Neo, ML and PA Todd (2011) Quantification of water squirting by juvenile fluted giant clams (Tridacna squamosa L.). Journal of Ethology 29: 85-91.
- Neo, ML et al. (2015) The ecological significance of giant clams in coral reef ecosystems. Biological Conservation 181: 111-123.
- Neo, ML et al. (2017) Giant clams (Bivalvia: Cardiidae: Tridacninae): A comprehensive update of species and their distribution, current threats and conservation status. Oceanography and Marine Biology: An Annual Review 55: 87-388.
- Shang, YC et al. (1991) Report on a market survey of giant clam products in selected countries. Center for Tropical and Subtropical Aquaculture Publication Number 107. 24pp.
- Soo, P and PA Todd (2014) The behaviour of giant clams (Bivalvia: Cardiidae: Tridacninae). Marine Biology 161(12): 2699-2717.
- Trench, RK et al. (1981) Observations on the symbiosis with zooxanthellae among the Tridacnidae (Mollusca: Bivalvia). Biological Bulletin 161(1): 180-198.
- Wilkens, LA (1986) The visual system of the giant clam Tridacna: Behavioural adaptations. Biological Bulletin 170: 393-408.
- Yamaguchi, M (1977) Conservation and cultivation of giant clams in the tropical Pacific. Biological Conservation 11: 13-20.