Anyone who has tried to pull a razor clam from a sandy beach knows that they can dig fast. These edible animals can bury themselves at around one centimetre per second, and they go deep. A clam the length of a hand can create a burrow up to 70 centimetres down.
Like all molluscs, the clam has a muscular foot, but it’s not that muscular. Based on measurements of the foot’s strength, Amos Winter from Massachussetts Institute of Technology calculated that it should only be able to dig a couple of centimetres into the mud. It shouldn’t be able to submerge its body, much less create a burrow five times longer.
But Winter knows the razor clam’s secret: it doesn’t just rely on raw power. The clam adds water to the soil just below it, making it softer and easier to penetrate. It digs by turning part of a beach into quicksand.
The clam’s digging equipment couldn’t be simpler: a pair of long valves that run the length of its body and open or close its shell; and a foot that sits beneath the them. It extends the foot downwards and pushes against it to lift the shell up slightly. Then, it contracts the valves, sending blood into the foot and inflating it. This foot becomes an anchor. By pulling against it, the clam can drag its shell downwards.
To understand how these motions create a burrow despite the foot’s weedy nature, Winter had to capture several clams. And to do that, he had to become a licensed clam digger. Back in the lab, studying the clams wasn’t easy. As Winter writes in a wonderfully deadpan academic way: “The adage ‘clear as mud’ is used to describe the difficulty of visually investigating burrowing animals.” He finally saw what the clams were doing when he created a homemade “visualiser”— a repurposed ant farm. The animal was trapped between two transparent plates, filmed with high-speed cameras, and surrounded by a ‘beach’ of glass beads.
Winter’s videos revealed that when the clam contracts its valves, it does more than just pump the foot with blood. The contraction closes its shell, which relieves the pressure on the surrounding soil. The soil starts to crumble, and mixes with water pulled into the gap from above. The water “fluidises” the soil, making it soft and loose like quicksand. It offers far less resistance, and the clam can move through it with around ten times less energy. It does so quickly, before the soil has a chance to solidify again.
Winter isn’t just studying clams for the sake of it. The list of sponsors for his study is telling: Battelle Memorial Institute, a science and technology development company; Bluefin Robotics; and the Chevron Corporation, an energy company that explores for oil and gas.
Winter used his newfound knowledge to create RoboClam: a robot that duplicates the clam’s burrowing technique. It’s about the size of a lighter, but it comes with a much larger supportive frame of pistons and regulating elements. After further development, RoboClam could act as a lightweight anchor that could be easily set and unset. It could tether small robotic submarines for studying the ocean floor; help to install undersea cables or deep-water oil rigs; or even detonate buried underwater mines.
Reference: Winter, Deits & Hosoi. 2012. Localized fluidization burrowing mechanics of Ensis directus. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.058172
Photo by Arne Huckelheim
In the 1940s, visitors watching football games at Berkeley’s Californian Memorial Stadium would often be plagued by beetles. The insects swarmed their clothes and bit them on the necks and hands. The cause: cigarettes. The crowds smoked so heavily that a cloud of smoke hung over the stadium. And where there’s smoke, there’s fire. And where there’s fire, there are fire-chaser beetles.
While most animals flee from fires, fire-chaser beetles (Melanophila) head towards a blaze. They can only lay their eggs in freshly burnt trees, whose defences have been scorched away. Fire is such an essential part of the beetles’ life cycle that they’ll travel over 60 kilometres to find it. They’re not fussy about the source, either. Forest fires will obviously do, but so will industrial plants, kilns, burning oil barrels, vats of hot sugar syrup, and even cigarette-puffing sports fans.
The beetles find fire with a pair of pits below their middle pair of legs. Each is only as wide as a few human hairs, and consists of 70 dome-shaped sensors. They look a bit like insect eyes. In the 1960s, scientists showed that the sensors detect the infrared radiation given off by hot objects. Each one is filled with liquid, which expands when it absorbs infrared radiation. This motion stimulates sensory cells and tells the beetle that there’s heat afoot.
Several neural diseases, including chronic pain and epilepsy, involve a lack of restraint. That is, damage to nerves in the spine reduces the levels of a signalling chemical called GABA, which silences excitable neurons. The result: too much neural activity.
There are drugs that can restore GABA, but they don’t always work, they are only temporary and they have unwanted side effects like sedation. There is another option: transplant GABA-producing neurons directly into the spine. Scientists have now done this in mice, with successful results.
I covered the story for The Scientist. Check it out.
Photo by Nanny Snowflake
In a small office north of London, Stephanie Pierce from the Royal Veterinary College is watching a movement that hasn’t been seen for 360 million years. On her computer, she has resurrected the long-extinct Ichthyostega – one of the earliest four-legged animals to creep about on land. By recreating this iconic beast as a virtual skeleton, Pierce has shown that while it looked like a giant salamander, it couldn’t possibly have walked like one. It had some of the planet’s earliest bony legs, but they weren’t very good at taking steps.
Ichthyostega hails from the Devonian period, a time in Earth’s history when swimming transformed into walking. Fish invaded the land and evolved into the first tetrapods—four-limbed animals that include amphibians, reptiles, birds and mammals. Muscular fins used for steering and balance evolved into legs for walking.
Since its discovery over 50 years ago, Ichthyostega has been an icon of this pivotal transition. Some 300 specimens have been found but many are incomplete, flattened or distorted. Pierce’s new model provides the best look yet at the animal’s skeleton. “It makes Ichthyostega a bit more tangible,” she says. “It’s not just a fossil laying in a rock now. It’s an animal that’s coming to life.”
Pierce built her virtual skeleton by putting dozens of Ichthyostega specimens in powerful CT-scanners, choosing only the best preserved ones out of the 300 or so in existence. “The front end of the animal was mainly composed from one beautifully preserved specimen called ‘Mr Magic’,” she says.
It was painstaking work. These fossils are so old that chemically, they are almost identical to the rocks around them. By eye, the bones stand out. To the scanners, they blend in. Pierce spent over two years scanning the specimens and building her model, but the results were worth it. “This has been on the wish-list for years,” says Michael Coates, who studies tetrapod evolution at the University of Chicago.
Those boots weren’t made for walking…
The model showed that Ichthyostega’s shoulders and hips were oddly restricted. They could move back and forth, and up and down, but they couldn’t rotate about their long axis. Hold your arm out and rotate your palm so it faces up then down—Ichthyostega’s shoulder couldn’t do that.
Most modern tetrapods need long-axis rotation in order to walk. Without it, their legs can’t be thrown forward or pulled backward. Ichthyostega’s limitations meant that despite having four limbs, it probably couldn’t have taken a step. It hind feet would never have been planted flat against the ground or supported its weight. It had invaded the land, but it wasn’t striding across it.
“It highlights the fact that the earliest tetrapods are not just ‘gigantic salamanders’, despite a vague similarity in outline,” says Per Ahlberg from Uppsala University. “The limbs and girdles are very different from anything now living.”
Pierce thinks that Ichthyostega moved by paddle with its front limbs, using powerful muscles and flexible elbows to make rowing motions. The closest living analogue is probably the mudskipper – a fish that drags itself along muddy land with its front fins (as in the video below).
Pierce also compared Ichthyostega’s joints and limbs to those of other living animals with sinuous bodies and interesting gaits, including a salamander, crocodile, seal, otter and platypus. Compared to these modern species, Ichthyostega’s hips and shoulders were similarly flexible in most planes of movements, but along their long axis, they could barely rotate.
Some scientists think that the tetrapods evolved limbs before they could walk, and their first members lived in shallow water. Others think that it’s the other way round, and that muscular limbs, hips and shoulders evolved while fish still had fins. The virtual Ichthyostega supports the former idea, since it had limbs but couldn’t walk. But Coates cautions against “fitting a smooth transition from swimmers to walkers.” He says, “Evolutionary transitions needn’t follow linear routes. Ichthyostega probably represents one of multiple experiments among the first tetrapods with limbs, trying-out life in the shallows.”
So… what made those tracks?
Other early tetrapods had similar shoulders and hips, so they probably had the same limitations too. John Hutchinson, who led the new study, plans to find out. His lab is busy reconstructing other early tetrapods including Acanthostega, one of Ichthyostega’s contemporaries, and Pederpes, a later model.
But Ahlberg notes that Ichthyostega had a very unusual and rigid spine, and may not have been representative of other early tetrapods. “Other tetrapods are known to have had more flexible spines” he says, “and this probably allowed them to overcome the limitations of their shoulders and hips”.
This might explain why Ahlberg and others have discovered tracks that pre-date Ichthyostega by around 20 million years, and had become fairly common by the time it evolved. Many of these tracks showed precisely the kind of salamander-like movements that Ichthyostega was apparently incapable of making. They were clearly made by early four-legged tetrapods, and to this date, we don’t know what made them.
Pierce agrees that the final word on Ichthyostega’s movements will have to wait until she can animate its entire skeleton. “The ultimate goal would be to try and create some sort of dynamic movement,” she says. She has applied for a grant to do just that, to model the motions of the entire animal, and compare them to salamanders or crocodiles. “That’s going to take so much time, but it’ll be very interesting,” she says.
PS: I want to point out that in researching this story, I spent a good minute on my living room floor trying to walk without long-axis rotation. It was really hard, and I looked like an idiot. I did a similar thing when I was writing about hummingbird wing movements for Nature. I’m going to christen this Method Science Journalism.
Reference: Pierce, Clack & Hutchinson. 2012. Three-dimensional limb joint mobility in the early tetrapod Ichthyostega. Nature http://dx.doi.org/10.1038/nature11124
Image by Julie Molnar
More on tetrapods:
The world’s largest animals have been hiding something. The bodies of the giant rorqual whales—including the blue, fin and humpback—have been regularly displayed in museums, filmed by documentary makers, and harpooned by hunters. Despite this attention, no one noticed the volleyball-sized sense organ at the tips of their lower jaws. Nicholas Pyenson from the Smithsonian Institution is the first, and he thinks that the whales use this structure to coordinate the planet’s biggest mouthfuls.
The Open Notebook has a series called Natural Habitat, which looks at the space in which science writers work. I, perhaps foolishly, agreed to take part in it. You can find the resulting video and photos here, featuring the local pub, treelancing (TM), and a cuddly giant squid.
A blind man sees his fiancée’s smile for the first time. Another walks around at night, navigating via streetlamps and headlights. Yet another reads his own name (and spots a typo). All three had lost their sight years before, as an inherited disorder destroyed the light-sensing cells of their retinas. But they had since been fitted with retinal implants that took over from the broken cells, sensing incoming light, and converting it into electrical impulses delivered to the brain. The devices are a long way from 20/20 vision, but they have nonetheless restored sight to those who had lived without it for years.
These retinal implants seem miraculous, but they have a major drawback: they rely upon a working optic nerve. This is the main communication line between the eye and the brain. If it’s damaged, no amount of retinal techno-wizardry will help. And that’s bad news for people with glaucoma, the world’s second leading cause of blindness, which wrecks the optic nerve.
But even for those people, there is hope. Silmara de Lima from the Children’s Hospital in Boston has found a way of regenerating the optic nerve in adult mice, partly restoring their vision. Although his techniques cannot be used directly in humans, they provide an important proof of principle that optic nerve injuries can be reversed. We just need to figure out how.