Hi. It’s time for a new Seabeast of the Week. This one is the giant clam.
The other day, I realised I had no idea what a giant clam is. I’m pretty sure I grew up thinking that clams were large undersea mouths that would occasionally trap people and digest them. I may have been misinformed by a terrifying 1967 Batman TV series episode.
I was pretty scared of clams.
It turns out that that clams are pretty interesting. They are the biggest molluscs in the world (see below for more on molluscs) and they often live for over 100 years. Sadly, they don’t quite deserve the dangerous reputation they’ve been given by sailors’ tales. But instead of eating people, or much of anything, they make up their diet in a fascinating and unusual way for an animal: using photosynthesis, like a plant. Read on and I’ll get to how.
Molluscs include slugs, snails, octopuses and squid. Mussels, oysters and scallops are also molluscs, and along with clams are all “bivalves”. That just means “two shells”, because their shells have two hinged parts. Giant clams have all the normal mollusc body parts, but no real head, brain or eyes. All their bits are hidden, because they do their breathing, eating and excretion in water sucked into their bodies through two “siphons” (holes – in one, out the other).
The giant clam, Tridacna gigas, can be longer than a very tall person and weigh hundreds of kilos. Other clams in the family Tridacna look similar and are sometimes called giant clams, but aren’t as big (though often still big by clam standards). Some are pictured here. They come from the coral reefs of the south Pacific and Indian oceans. Their range is huge, but in many areas giant clams have disappeared. The species has become endangered due to excessive human hunting, mostly for expensive food.
Newborn giant clams float around the ocean as part of the plankton, which is everything that drifts in the water at the whim of the currents. When they’re about a week old, they sink to the sea floor and start swimming from point to point, looking for a good place to settle.
Young clams get food only by filter feeding. This involves straining little edible creatures and debris out of the water – i.e. eating the plankton they just left behind. They do this by taking water into their bodies and pushing it over their gills, which are covered in sticky mucus. The gills trap things, and the clam moves them over to the mouth to eat them. Giant clams filter feed throughout their lives, but it provides less and less of their food as they grow up.
After a while, a growing clam has to pick a spot and stay there for the rest of its life. By now, it has picked up some passengers who will become its best friends and closest allies. Among the plankton a clam filter feeds on are algae: simple, usually single-celled organisms that get their energy from photosynthesis, like plants. (Seaweeds and the green stuff on ponds are also algae.) Certain algae can avoid getting digested by the clam and move out of the stomach into a network of tiny tubes that extend across the mantle – the part of the clam that makes the shell, and which we can see around its “lips”. There, just under the skin, the algae do what they do best. The two organisms enter into a partnership (a “symbiosis”) in which the clam provides a safe home and nutrients while the algae provide energy from the sun.
The giant clam grows, allowing it to catch more sunlight, and the algae provide more and more of its energy. During the day, the clam pushes out its mantle, and at night it closes up for protection. Perhaps it’s a shame to debunk the stories, but in truth giant clams can’t shut fast enough to catch someone. If they could, they wouldn’t actually be able (or want) to digest them. But if it’s any consolation, apparently they have a pretty good grip if you do leave your arm in there for a while.
The mantle’s iridescent patterns are created by crystalline pigments in the skin. The algae actually have a brownish colour, so don’t add much to the look. Instead, the clam pigments have a few functions, including working as a sunscreen. Very intense light is bad for photosynthesisers as well as red-heads. Giant clams also have eyespots, which can sense light and dark, and clear “windows” in their skin that let light through for the algae. It’s said that every clam has a distinctive pattern, different even from its parents and siblings.
Giant clams reproduce by “broadcast spawning”, releasing both eggs and sperm out into the sea, where they meet among the plankton. Spawning is the most energetic thing a giant clam does. They are hermaphrodites, so they can each release either eggs or sperm, but they only do one at a time to avoid fertilising themselves. All nearby clams spawn at the same time, because they can both release and smell a chemical announcing when they are ready to go.
Finally, in case you’re wondering, a giant clam actually produced the largest pearl in the world, but Tridacna pearls aren’t “true pearls” and don’t look particularly pretty. They get valued pretty highly though.
That is probably all you will ever need to know about the giant clam. Do come back for next week’s Seabeast.