Amos Winter from the Massachusetts Institute of Technology, USA, wants to build self-burying machines. ‘There are many applications that could benefit from a self-burrowing reversible anchor that embeds efficiently as far as energy consumption is concerned’, explains Winter. He adds, ‘When I started the project our hypothesis was that there is probably an animal that has figured out a pretty good way of digging into soil, so I looked around at animals that dig into the ocean bottom and razor clams stood out.’ According to Winter, the 15–20 cm long molluscs can burrow as far as 70 cm down in mud or sand soils beneath the sea, yet muscle force measurements by E. R. Trueman in the 1960s had found that the mollusc's muscles were not strong enough to heave them through that much soil. Intrigued, Winter and his thesis advisor, Anette Hosoi, decided to find out how razor clams burrow (p. 2072).

But first Winter wanted to know how far a clam could force its way through mud at its seashore home propelled by its muscular foot alone. Packing an empty razor clam shell with epoxy resin and pushing it into exposed seashore mud, Winter measured the resistance encountered by the shell and found that the clam could burrow no deeper than 2 cm. They had to be doing something else to burrow farther, but what? Winter had to get his hands on some animals and successfully reproduce the clam's environment in the laboratory to analyse their burrowing technique.

Having obtained an official permit and after being taught how to collect the molluscs by the Shellfish Constable of Gloucester, Massachusetts, Winter recalls how difficult it was to build a transparent simulation of the mollusc's environment. Hitting on transparent 1 mm diameter soda-lime glass particles as a good substitute for one of the clam's natural homes, coarse sand, Winter filled a narrow chamber with the water-saturated particles and blasted it with two 1 kW halogen bulbs to visualise the clams' descent as they burrowed.

Analysing the clam's burrowing technique, Winter saw that the animal initially extended the foot before lifting the shell up. Then, the clam contracted the shell rapidly, inflating the foot with the blood expelled from its body. Having inflated the foot to anchor itself in place, the clam pulled on the secured appendage to drag the shell further down into the simulated soil. But this still couldn't explain how the clam was able to burrow so far through static soil.

Turning his attention to the glass-particle-simulated sandy soil, Winter eventually discovered that the key to the mollusc's burrowing technique was the moment when it contracted the shell. ‘As soon as it starts contracting the shell it relieves the pressure it is exerting on the soil and that sucks more water towards its body so that you get increased unpacking of the soil particles’, he explains. Essentially, the clam fluidised the surrounding soil – turning it into quicksand – which dramatically reduced the drag on the shell, allowing the mollusc to pull itself down before the surrounding sand particles slid back into place and the soil resolidified. And when Winter analysed the amount of energy required to move through the temporarily fluidised glass sand, he realised that it was a fraction of the energy required to move through a static soil.

Having discovered how razor clams burrow, Winter has successful designed and built a machine that burrows using the razor clam's quicksand energy-saving mechanism, which he hopes to develop into a self-contained gadget that can dig into, and out of, the ocean floor.

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R. L. H.
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Localized fluidization burrowing mechanics of Ensis directus
J. Exp. Biol.