The largest of bivalves this week we are looking at the giant clam. These giant shelled organisms are iconic parts of coral reefs and are eagerly desired for aquariums and display cases around the world.
Taxonomy and Evolution
There are ten species of giant clam and all belong to the genus Tridacna; we’ll mostly be looking at two of these species, the giant clam (T. gigas) and southern giant clam (T. derasa). The giant clams belong to a long history of bivalve evolution with the first bivalves being some of the first complex animals to evolve over 530 million years ago. Tridacna, however, is much more recent with their fossils appearing around 20 million years ago in the Miocene epoch. They also belong to the family Cardiidae, which evolved in the early Triassic, which means that giant clams are closely related to the cockels you eat at the beach. The ten species of giant clam are as follows: T. crocea; T. derasa; T. elongatissima; T. gigas; T. maxima; T. mbalavuana; T. noae; T. rosewateri; T. squamosa; and T. squamosina. DNA tests on ocean samples do point to an unidentified species being out there, waiting to be discovered.
Biology
As expected by their name, the giant clam is gigantic. In fact, the giant clam is the second largest bivalve in natural history, with the Cretaceous Platyceramus only exceeding it in size. On average, giant clams reach 1 to 1.5 metres (or 3-5 feet) in length and 199.6 kg (440 pounds), although very few reach the 1.5 metres. The 1.5 metre maximum comes from the largest ever giant clam which was discovered in the early-1800s off the coast of Sumatra. In life this clam would have weighed a staggering 250 kg (or 550 pounds). Each clam has several folds in its shell – the giant clam having five and the southern giant clam having around seven. The clam is made of two parts – the shell and inner mantle. They can be incredibly colourful and it is not unexpected to see members of the same species being different colours. Some species can be especially bright with the maxima clam (T. maxima) having a mantle that shines a beautiful blue and green. Mantles also contain hundreds of photoreceptors that allow the clams to detect light and shadows, and then act accordingly – a sudden shadow could indicate the arrival of a predator.
Giant clams go through an astonishing life cycle. Bivalves are hermaphrodites but the giant clam is not capable of self-fertilisation. Instead, they snychronise spawning with quarters of the moon when the tide is coming in, where they will expel a cloud containing sperm and eggs in the literal millions. Very few of these will get successfully fertilised, so the clams will expel lots over two hours. Eggs and compatible sperm will form a larvae which will drift in the ocean as zooplankton – these larvae are so small that they can only be measured in micrometres. After around a week the larvae will grow a foot which will eventually latch onto a rock, the seafloor, or anything sturdy where it will eventually grow into the clam. At a growth rate of 12 centimetres the larvae will grow its hard shell and become a juvenile clam at around 20 centimetres in length. Very few fertilised eggs will reach this stage. Many will be lost before they grow a foot, many will not be able to latch onto anything, many will then be lost as they latched onto a place too inhospitable for the clams, and many will not survive their muscles changing into the adult form. When fully grown the clams will slowly get larger, and are capable of living for a century!
When a giant clam becomes latched to something it will remain there for the rest of its life. Just like corals, giant clams become the backbone of its habitat and forms a mini-ecosystem around it. Seaweed often grows on their shells which attracts a whole range of reef animals. The clams themselves live symbiotically with a group of single-cell algae called zooxanthellae that grows on the clam’s mantle. Clams rely on the zooxanthellae for energy, and in return the clam gives the zooxanthellae protection and a source of carbon dioxide, phosphates, and nutrients. The mantle is a powerful muscle which sucks in water and filters out oxygen and plankton. As mentioned earlier, the clams are capable of closing their shells for protection, although it is a myth that they can clamp down on a human. This myth was so pervasive that US navy personnel were issued guides advising how to avoid giant clams and get out of their grasp if clamped down on during the Pacific theatre of the Second World War. T. gigas isn’t actually capable of fully closing its shell, and those who are capable of doing so close their shell too slowly to capture a human off-guard. Instead, if a human causes the clam to close its shell it is because the human scared the clam who retreated into its shell for safety.
Diet and Threats
Giant clams rely on the zooxanthellae algae for sustenance, so are capable of living in low nutrient environments as long as the algae has enough light for photosynthesis. They still do filter feed as a way to supplement their diet, and until the algae grows on their mantle they are entirely reliant on filter feeding. The various microscopic organisms that we call plankton sustain the clams until algae spores grow, and then supplements the diet. Fully grown giant and southern giant clams have very few predators, where only a few crustaceans, fish, or other animals can nibble on the mantle. Smaller species and younger clams have a wider range of predators, with some fish having powerful enough beaks to break the shell of the clams. As a larva the youngest clams are vulnerable to any filter feeders which can range from the tiniest of krill and sponges to the largest whales. Humans do eat giant clams with part of the mantle being a delicacy in Japan, Southeast Asia, and the Pacific. The presence of the algae means that most of the clam is inedible as it has such a strong seaweed-like taste.
Habitat and Distribution
All ten species of giant clam live in the Indian and Pacific Oceans, with seven of the species being found in the South China Sea. The clams are found in reefs, and, as mentioned above, form the backbone for many reefs like the corals themselves. Lagoons also form a good habitat for giant clams. Relying on photosynthesis for the zooxanthellae means that giant clams have to live closer to the surface to get enough sunlight. Many young clams will die because their larvae land in too deep water so cannot get enough sunlight. As also mentioned above, when giant clams take root they are stuck there for life.
Conservation
The IUCN has not evaluated the giant clams since 1996 – potentially upsetting some readers that is the same year I was born – where the giant and southern giant clams were listed as ‘Vulnerable’. A big part of why they became rare was due to overexploitation for fine dining, their shells for decoration, and for usage in aquariums. As with any other animal, the exotic pet trade leaves more dead than alive at the end, and the long time it takes for giant clams to develop this can devastate clam populations. Efforts have been put into place to prevent overexploitation, with CITES forbidding the international trading of clam shells. Several institutes in the US, Australia, Palau, Marshall Islands, and Malaysia have been successful in raising aquacultures of the clams for aquariums and release into the wild to help undercut the pet trade, and to repopulate wild numbers. However, there is still a continued poaching of wild clams to sustain an illicit demand for jewelry, with clam shells often being used as a substitute for ivory. In 2019 the Philippines’s government managed to seize a staggering 132,000 tonne stockpile of clam shells – roughly five times the weight of the Statue of Liberty. On top of poaching, rising sea temperatures due to climate change has been decimating clam numbers, and rising sea levels alters the habitat which prevents the clams from safely growing. Just like corals, climate change is wiping out giant clams. Between overfishing and global warming, the future of giant clams may be much grimmer compared to the IUCN’s evaluation 26 years ago.
Bibliography:
- ‘Giant Clam’, National Geographic, [Accessed 20/11/2022]
- Dina Fine Maron, ‘Criminals are stealing giant clams – and carving them like ivory. Here’s why’, National Geographic, (06/10/2021), [Accessed 20/11/2022]
- Deep Marine Scenes, ‘Facts: The Giant Clam’, YouTube, (28/09/2021), [Accessed 20/11/2022]
- S. Wells, ‘Tridacna gigas’, IUCN, (1996), [Accessed 20/11/2022]
- S. Wells, ‘Tridacna derasa’, IUCN, (1996), [Accessed 20/11/2022]
- Pamela Soo and Peter Todd, ‘The behaviour of giant clams [Bivalvia: Cardiidae: Tridacninae]’, Marine Biology, 161:12, (2014), 2699-2717
- Thomas Huelsken, Jude Keyse, Libby Liggins, Shane Penny, Eric A. Treml, and Cynthia Riginos, ‘A Novel Widespread Cryptic Species and Phylogeographic Patterns within Several Giant Clam Species (Cardiidae: Tridacna) from the Indo-Pacific Ocean’, PLoS ONE, (2013)
- Lon Wilkens, ‘The Visual System of the Giant Clam Tridacna: Behavioral Adaptations’, Biological Bulletin, 170:3, (1986), 393-408
- B.K. Baillie and D. Yellowlees, ‘Characterization and Function of Carbonic Anhydrases in the Zooxanthellae-Giant Clam Symbiosis’, Biological Sciences, 265:1395, (1998), 465-473
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