Lampreys suck at swimming, but it helps them move faster

The long parasitic fish generate spinning vortices in the water that essentially pull its body forward. Jana Howden reports.

Lampreys are – unsurprisingly – parasites of other fish. But the physics of their swimming style is the topic of a new paper.

Looking at a lamprey’s toothy sucker mouth, you’d be forgiven for thinking it was a creature straight out of a horror film. But these jawless fish also exploit suction to help them swim.

Marine biologist Sean Colin from Roger Williams University in the US and his team tracked swimming sea lampreys (Petromyzon marinus) and found controlled bending of their entire body helped “slingshot” them through the water.

This method of swimming, published in the Journal of Experimental Biology, could enhance robot design.

As animals swim, they push and pull on the surrounding water. This creates regions of high and low pressure around their body which can help move them along.

Most animals – including humans – swim by pushing on water. This generates positive pressure, which propels them forwards.

But limbless lampreys also create little whirlpools – vortices of negative pressure – in front of their body as they snake their way through water.

This “sucking” motion pulls them along. It generates most of their forward movement.

Colin and his team knew from previous work that lampreys rely mostly on negative pressure zones to propel themselves, but they didn’t know how they made them.

Did the power come from the front half of the animal – in the muscles behind the head – or was the whole body involved?

To find out, Colin’s team compared healthy lampreys to lampreys that had a small cut in their spinal cord, paralysing them from the middle of their body down. These semi-paralysed lampreys could swim, but their bottom half followed passively.

Both groups of lampreys were recorded swimming through a 1.5-metre-long tank and their body movements compared.

The researchers also measured the movement of fluid around the swimming lampreys using a technique known as high-speed particle image velocimetry.

This involved peppering tiny glass beads throughout the water tank and tracking them with a laser to measure the whirls and eddies around the animals.

It turns out lampreys must actively control the bending of their bodies to build negative pressure zones that whiz them through the water.

While both groups of lampreys could create negative pressure zones, the healthy cohort’s vortices grew as they swam and bestowed more suction.

Vortices generated by semi-paralysed lampreys, on the other hand, collided with each other and fizzled out. They had to rely on positive pressure zones to push them along.

All in all, healthy lampreys travelled twice as fast as their semi-paralysed counterparts.

Colin says this research could be key for companies looking to design underwater machines and vehicles.

“Without understanding the important features of how these animals move, you’re not going to be able to design these vehicles well,” he said.

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