Razor clam

Impossibly good at digging

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Standing firmly upright in the shallows, the sharp shell of the appropriately-named razor clam can be a menace to unwary people wading barefoot or swimming at low tide. But the razor clam has attracted the interest of engineers wondering how it manages to dig itself into the seabed so effectively.

Image source—RazorClam23 - Wikicommonsrazor-clam
Many people enjoy razor clams as a culinary dish, and dig them out of the shallows at low tide for this purpose.

The thing is, engineers thought that it shouldn’t be able to do this. The insertion forces needed to embed jetty pylons and anchoring devices into the sand or mud of the sea floor are huge and costly, because the deeper you go, the harder it gets to penetrate any further.1

Yet the razor clam, capable of producing a force of only 10 Newtons (N),2 manages to pull itself into the sea mud much deeper than engineers assessed should be possible.

Engineers had tried pushing an empty razor clam shell3 packed with epoxy resin (i.e. to match the weight of a live one) into seashore mud, but found that a force of 10 N would merely be sufficient for the clam to submerge to approximately 1–2 cm (0.4–0.8 inch) only. Yet razor clams can burrow as deep as 70 cm (28 inches)!

The razor clam’s engineering ‘solution’

Wanting to understand, with a view to copying the secret of the razor clam’s digging prowess, engineers investigated further.4 They realized that the creature must be manipulating the surrounding soil to reduce burrowing drag and thus lowering the energy required for deeper submersion. And it turns out that is indeed the case—the razor clam is able to agitate the soil locally, and create a region of fluidization around itself. Moving through fluidized, rather than static, soil reduces drag forces on the animal, making digging a whole lot easier.

It happens like this. With its valves (shell) expanded, the razor clam extends its foot downwards into the soil beneath it, and pushes its valves up (yes, up!). It then contracts its valves (i.e. closes its shell), which pushes blood into its foot. This expanded foot now serves as a great terminal anchor for the next step, pulling downwards on the valves.

Note this key point. Of course there is an energy cost to the razor clam associated with pushing up and contracting its valves—motions that do not directly contribute to downward progress. But those two movements are precisely what causes fluidization of the surrounding soil at that moment—the secret to the razor clam’s ability to burrow deeper than its strength would theoretically allow. Who’d-a-thought?!

Having now penetrated deeper than before, the razor clam expands its valves—accomplished simply through elastic rebound of the hinge ligament, thus requiring no additional energy input by the creature—and so is ready for the next digging cycle.

razor-clam-digs
How the razor clam digs: First it extends its foot downwards, then it pushes up its shell,before closing its shell suddenly (4th pic) which inflates its foot, thus helping its foot to act as an ‘anchor’ for the next stage of pulling its shell downwards. It then opens its shell again (far right), thus being ‘reset’ to repeat the digging cycle.

Inspired by nature

With its secret discovered, the razor clam becomes the latest in a long string of organisms to have inspired engineering solutions—a discipline known as biomimicry or biomimetics.5 Listen to the engineers ‘rave’ about the razor clam:

E. directus is an attractive candidate for biomimicry when judged in engineering terms: its body is large (approximately 20 cm [8 inches] long, 3 cm [1.2 inches] wide); its shell is a rigid enclosure with a one degree of freedom hinge; it can burrow over half a kilometer [0.3 miles] using [no more than] the energy in an AA battery; it can dig quickly, up to 1 cm/s−1 [0.4 inches per second], and it uses a purely kinematic event to achieve localized fluidization, rather than requiring additional water pumped into the soil.4

They go on to point out numerous industrial applications that could benefit from having such a compact, low-energy, reversible burrowing system, such as anchoring, installing subsea cables, and mining for oil. E.g. a marine anchor based on the razor clam “should be able to provide more than ten times the anchoring force per insertion energy as existing products.”4

As to the question, Who’d-a-thought?, well, it should be obvious (Romans 1:20). It was the same One in whose image we are made, who has endowed man with the capacity to delve into the things He created as we try to work out how they do so well what they were designed to do. As the Psalmist noted:

“O Lord, how manifold are your works! In wisdom have you made them all; the earth is full of your creatures.” (Psalm 104:24)
Hans Hillewaert -Wikicommonsempty-razor-clam-shell
An empty razor clam shell lies on the beach at Wilmereux, France. This species, Ensis directus, was the one studied by biomimetics engineers but there are numerous other species of razor clams. Coastal peoples around the world are generally familiar with razor clams of one form or another.

References and notes

  1. The insertion force F(z) increases linearly with depth z, resulting in an insertion energy, E = ∫F(z) dz, that scales with depth squared. Return to text.
  2. The SI unit for force; for example if you were standing on the earth a force of 10 Newtons is the force you’d need to apply to hold aloft a weight of approximately 1 kg (2.2 pounds) against the force of gravity. Return to text.
  3. Specifically, the Atlantic razor clam, Ensis directus. Return to text.
  4. Winter, A. and 4 others, Razor clam to RoboClam: burrowing drag reduction mechanisms and their robotic adaptation, Bioinspiration & Biomimetics 9(3):036009, 2014. Return to text.
  5. For a selection of earlier articles on this topic, see creation.com/biomimetics. Return to text.
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