Disco clam: A poster child for weird bivalves

In the face of danger, most bivalves protect their squishy bodies by snapping their hard shells shut. It’s a tried and tested tactic that’s worked well for millions of years.

But not all bivalves play by the same rules. In fact, some seem to have thrown the entire rulebook out the window. Ctenoides ales – also known as the electric clam, disco clam or electric flame scallop – is just one of the species doing life its own way.

What is a disco clam?

Bivalves are invertebrate animals that belong to a group known as molluscs. They have a shell that’s made up of two halves, known as valves, which are joined by a hinge. Bivalves will usually clamp these two sides together to protect themselves from hungry predators.

The disco clam, however, does the complete opposite. At a whiff of danger, it opens its valves wide and produces hypnotic, lightning-like flashes.

“It’s aposematic – it’s saying, ‘I taste nasty, go away, go away, go away’,” our Curator of Benthic Molluscs Dr Katie Collins animatedly explains.

We often think of light in the ocean as bioluminescence. This is light that an organism creates inside its body and radiates out, such as anglerfishes’ infamous luminous lures. There are a few bioluminescent bivalves, but once again, the disco clam does things its own way. Instead of producing light, it reflects it.

The edge of its red, tentacled mantle has two distinct sides. The front acts like a mirror and the back is barely reflective. By furling and unfurling their mantle mind-blowingly fast, letting light bounce off one side, the bivalve makes it look like their flesh is flashing with white light. It’s just like a disco ball.

What makes it even more unique, is that this reflective surface is packed with silica spheres, the same material as quartz or glass. The spheres are tiny, each only about 340 nanometres wide. A single nanometre is just 0.0000001 centimetres.  

“Ctenoides ales makes a whole mineral in its flesh that, as far as I can find out, no other bivalve now or before has ever done,” says Katie delightedly. “It’s quartz – the second most common mineral that Earth is made of – and they’re synthesising it in their bodies!”

“Most species in a genus behave in similar ways. But the disco clam has red tissue, which its nearest relatives don’t, and it makes this flashing display, and its relatives can’t do that either.”

The disco clam might be using its dazzling display to put off creatures such as mantis shrimps. These crustaceans have amazing eyesight and are formidable predators – some species have punches so powerful that they can break aquarium glass. 

Who discovered disco clams?

Ctenoides ales was named in 1927 by Harold Finley, a scientist from New Zealand. But it’s only in recent years that we’ve come to understand the mollusc’s disco-ball behaviour. Dr Lindsey Dougherty has been at the forefront of much of that research. She was a graduate student at the University of California, Berkeley, USA, in 2010 when she came across the bivalve flashing away on the ocean floor. Fascinated, she went on to study it for her PhD.

“It was done by observation and really careful electron microscopy – she went in and took really tiny samples of tissue, and she found the silica,” Katie explains. “What we know about the disco clam we owe to someone who had the skills, time and funding as a PhD student to do really good, solid work.”

PhD projects are often the result of a strange observation or a fanciful story and a scientist that wants to get to the bottom of it.

“The disco clam was just a species. It could have been completely identical to the next Ctenoides along. But now we know that bivalves can produce silica and have aposematic displays,” Katie continues.

“What species will we randomly pick tomorrow because someone has a hunch? What will it turn out to be able to do?”

Bizarre bivalves

Disco clams are undeniably strange. But if you consider them alongside other members of their class Bivalvia, they might not even make the top 10 for weirdness.  

Bivalves have been around for a very long time. They turn up in the fossil record in the early Cambrian, more than 500 million years ago, but their roots go even deeper than that. Since evolving, bivalves have figured out how to dig themselves into the seafloor and some have even learned to see. Scallops, which are the sister group to the disco clam’s family Limidae, have exceptional vision, for example. 

They have taken a lot of forms over time, so defining what a bivalve is isn’t particularly easy. Apart from having a shell that has two valves, there isn’t really a unifying set of features that says bivalve.

“Their shells are very important,” explains Katie. “Some of them do really weird things with their shells, but there’s no bivalve without two shells.”

We think that all bivalves have gills and generally they have a muscular foot, although in some species it’s very reduced. They don’t have a head, a brain or, universally, any other organ.

Like other invertebrate animals, bivalves have an open circulatory system filled with haemolymph. This is the equivalent to vertebrates’ blood. However, some bivalves have both haemolymph and blood.

“Some of them have a weird organ, or just a space in their mantle, full of haemoglobin blood that is, on a molecular structure, so much larger than the haemoglobin you or I have, or any other mammal. It doesn’t connect with their circulatory system at all, and it’s found in several different groups that aren’t closely related. We don’t know why they have it, we don’t know why the haemoglobin is so big – it’s a big mystery.”

Some bivalves live for a really long time. At up to 1.2 metres across, giant clams are the world’s largest bivalves. They can reach the ripe old age of 70, maybe even 100. But that’s nothing compared to the tiny Arctica islandica, which can live for more than 500 years. The most famous is a clam called Ming that lived from the 1490s until 2006. It was named after China’s Ming dynasty, which is when it hatched.

Then there are their diets. Most bivalves are filter feeders, some are deposit feeders and others rely on symbiotic bacteria in their bodies to make food for them. There are also bivalves that have turned carnivore, modifying their breathing apparatus called the siphon to reach out and grab unsuspecting prey, such as copepods.

But the list of bizarre adaptations doesn’t end there.

“Freshwater mussels – unionids – have a larval stage that’s parasitic,” Katie adds. “They modify their mantle into a fish shape and then wait on the river floor with their valves open. When a fish comes along and tries to bite them, they grab onto its face and squirt their larvae into its gills.”

The mussel’s offspring live in the fish’s gills until they’re big enough to leave and find a new home on the riverbed.

“It’s like something out of Alien – the facehugger – but the fish lives and goes on to carry baby clams somewhere else,” Katie jokes.

“The disco clam is a good poster child for bivalves because the second you scratch the surface of Bivalvia, you’re going to find something you don’t believe in the slightest!”

The bivalve family tree

Conservative estimates suggest there are around 6,500 living bivalve species, plus 10,000 or so fossil ones. But most have never been studied in detail and many have been named from dead shells rather than living animals.

“For most of them we don’t know what the soft tissue looks like. We have a family level phylogenetic tree, but it’s still got some mysteries in it,” says Katie.

To create a fully fleshed-out family tree, you need examples of the species. But even vast museum collections have gaps.

Katie uses the example of Pholadomya candida, a clam now so rare that we currently don’t know if it’s extinct or still hanging on in the wild. Globally, only two specimens with soft tissues exist in museum collections, but that’s more than we have for many other much more common species.

“Lots of things that are in collections are there because they were noted as being weird or worth picking up in some way. That leaves you with a gap of common things.”

But even if we had the specimens, mollusc DNA can be tricky to untangle. In bivalves specifically there are factors that make DNA work particularly challenging.

“We’re in this situation where we don’t have most of their anatomy, we know there are lots of them and we don’t have a phylogenetic tree, which these days is a fundamental first thing if you want to study a group of animals.”

Why do we need bivalves?

“All this would be fine – there are lots of animal groups where this is the case – except millions of people around the world subsist on bivalves for their main source of protein,” Katie continues. 

This is a poignant topic, as how bivalves will cope with climate change is a pressing issue. Katie has been studying how these animals might fare in our warming oceans by looking at fossil bivalves and how they coped with changes in their prehistoric environments.

“The fossil record can show you that if you change the temperature in a given area, the bivalves will not change themselves – they will move,” Katie explains. “So, if you change the whole climate, they’re going to die and when they do, there will be a trophic cascade – they’re going to take a lot with them.”

Bivalves arguably don’t have a huge presence in the public eye. They’re an underappreciated yet vital part of our watery world.

“Bivalves are ecosystem engineers – they build places for other things to live, they provide food that other things eat and they clean water.”

They also help us with our plastic problem by removing microplastics from the water column. They’re clever filter feeders and can tell what’s food and what isn’t. Instead of eating the plastic, they can turn the particles into a lump of pseudofaeces that they dump out of their bodies, burying it in the seafloor. This helps prevent other animals from eating it and passing plastic up the food chain to the fish we and other animals eat.

In some places, people use bivalves as sensors – for example, the shutting of their shells tells us that the water quality in a facility has declined. This offers a clear signal that something is wrong and that we need to act.

“They are one of the best possible biosensors you can have,” Katie praises.

“Plus, when you think about bang for buck, the amount of protein you get from the amount of food you put in, bivalves are incredibly economical as a food source. They just filter water. All you need for aquaculture is for people to continue living and dying on the planet – and preferably not throwing lead into the water.”

“They’re just good like that.”