Humans love a tale of terrifying sea monsters and often depictions of ferocious marine creatures are unfair and unwarranted.
The giant clam is a good example of this. While sitting on the seabed, sunbathing and generally minding their own business, giant clams have somehow drummed up a reputation for being man-eating terrors, probably because people assumed their immense size (they can grow more than one metre long) meant they were capable of munching a person.
- It might look like a clam, but don’t be fooled – this is no clam. It walks on two legs and tucks its home under its arms wherever it goes
- It’s the oldest individual living creature in the world, survives at 400m deep underwater and uses its ‘foot’ to move along the seafloor
While clams were quietly anchored on the seafloor not bothering anybody, people were spreading the word that these epic molluscs were waiting for the right opportunity to pounce on an unsuspecting diver and guzzle them down.
This has never been proven and is rather ridiculous when you consider their taste for much smaller planktonic prey, their on-tap energy source provided by the algae living inside their bodies and the fact they can’t even close their shells all the way.
What is a giant clam?
Giant clams are the largest bivalves (two shelled molluscs) on the planet. There are several different species but Tridacna gigas, sometimes known as the true giant clam, is the largest. It can grow more than one metre long.
Their stonking size led to unfounded rumours that giant clams can snap shut in an instant, trapping poor divers who snuck their arm inside to steal a pearl (giant clams can produce pearls but they don’t have mother-of-pearl, or nacre, so they don’t have the same pretty iridescent sheen as the pearls prized in jewellery).
Living on shallow tropical reefs, giant clams have lots in common with corals, including their symbiotic relationship with microalgae that provide them with food through photosynthesis. So, you could say giant clams are solar-powered!
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What do giant clams look like?
In a word? Massive. Their huge, thick shells protect their soft insides from predators while having the added benefit of keeping them in one place despite turbulent ocean currents that could wash them away or tip them over if they were lighter.
When their shell is open, basking in the sunlight, peek inside to glimpse a trippy rainbow of blues, pinks and purples. This colourful tissue is the clam’s outer mantle, which is packed full of tiny algae called zooxanthellae, which live in harmony with the clam and provide it with energy.
Some of these colours come from the clam’s own tissue but the little algae add their own hues to this picturesque pallet, making it even more astonishingly beautiful.
Where do they live?
Giant clams live in relatively shallow waters – down to about 20 metres deep – where the sunlight can still reach, allowing their symbiotic algae to photosynthesise and create energy. They love warm, tropical waters and can be found in the Indian Ocean and South Pacific.
Their larvae are free swimming but as they grow larger, they settle in a spot for the rest of their lives. Their enormous shells, which can tip the scales at around 200kg, weigh them down so they aren’t washed away by strong currents or waves.
These thick shells (which continue getting thicker after they’ve stopped growing longer) are also important in capturing and storing carbon.
What do giant clams eat?
Despite their gargantuan size, these mammoth creatures feed on tiny prey, filtering seawater and eating plankton. But this is only a tiny proportion of their nutrient intake. The majority of their energy (as much as 90%) is provided by tiny houseguests living inside their bodies so the clams are, essentially, solar-powered!
How do they feed?
Like corals, giant clams have tiny squatters – microscopic algae called zooxanthellae – living inside their bodies. These little lodgers share the energy they create as a result of photosynthesis with their enormous hosts, which enables giant clams to survive even in waters with very little nutrients.
The giant clams give their algae a little helping hand: they have special cells that reflect harmful UV rays (so they don’t get sunburn) while reflecting the other rays like a hall of mirrors so it reaches the
The zooxanthellae are the reason giant clams have such psychedelic mantles that are marbled with different colours. The algae produce vibrant colours which make the clams’ bodies look even more dazzling.
How do they reproduce?
When you’re locked to one spot by the sheer weight of your hefty body, finding another clam to mate with is impossible. And, although they are hermaphrodites, giant clams don’t want to reproduce with themselves.
So, what do they do? These molluscs are broadcast spawners: when they’re ready to reproduce, they release eggs and sperm into the water in the hope that the currents will sweep them into the path of another giant clam (to prevent inbreeding, they release their eggs and sperm at different times rather than simultaneously).
Fertilised eggs drift through the water until they find a place to settle on the seafloor and start developing.
Are they dangerous to humans?
With a nickname like ‘man-eating clams’, you’d think that these enormous molluscs might terrorise the seas. And people once thought giant clams did ‘eat’ humans.
Some legends tell of these massive molluscs snapping shut and gobbling down an unsuspecting human or trapping their limbs until they drowned. But this ferocious reputation is undeserved. There are no proven records of human fatalities caused by giant clams.
“Reputable marine manuals from centuries before warned against giant claims, detailing how a trapped diver could sever the clam and escape without drowning,” writes Great Barrier Reef Foundation on its website. “In truth, giant clams are harmless to people. Their shells move far too slowly to trap any unsuspecting diver.” Not only that but they can’t even fully close their massive shells.
Are they under threat?
Yes. The gigantic shell that was designed to protect them has put giant clams in peril as these animals have been overharvested for their shells – to be kept as ornaments or turned into jewellery – for their meat (their adductor muscles – the part which closes the two shells together – is seen as a delicacy) or taken into the aquarium trade.
They are also struggling because of climate-induced warming waters and human threats such as habitat destruction and pollution. Like corals, when giant clams lose their symbiotic algae, they can bleach and die. Those that survive the bleaching are often less able to resist other stressors in the future.
The largest and, arguably, most famous species of giant clam – Tridacna gigas – is categorised as critically endangered by the IUCN Red List of Threatened Species.
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