The ‘ship-holding’ capabilities of the family of fish known as remora (Echeneidae), also known as the sharksucker or suckerfish, have long held strong fascination for both sailors and natural historians. Gripping tales of the fish latching on to and impeding human vessels go back centuries. Now roboticists, by successfully replicating the fish’s unique anatomy, believe future autonomous underwater vehicles can use the same ‘hitchhiking’ technique to extend their range and efficiency.
A “biorobotic adhesive disc for underwater hitchhiking”, developed by a team of robotics researchers mostly from Beihang University in Beijing and Harvard University, mimics what the scientists describe as “one of the most extraordinary adaptations within the vertebrates” – one giving the remora the remarkable ability to latch on to a wide range of biological and nonbiological surfaces, including sharks and dolphins, cetaceans, sea turtles and boat hulls.
“We consider hitchhiking as an effective strategy for reducing energy expenditure during transport or movement of small underwater robots,” the scientists say in their paper describing their invention, published in Science Robotics.
The energy required to hitch a ride is a fraction of what a robot (or fish) would require to propel itself. “Coupled with a platform of streamlined shape, such a system could markedly reduce transport and movement costs and increase mission durations for autonomous underwater vehicles.”
Lead researcher Li Wen, whose work at Beihang University mainly focuses on bio-inspired robotics, biomechanics and soft robotics, sees several applications for the underwater adhesive technology. “For example, we can use this biorobotic disc as a tag to monitor marine animals,” he says. “We can also use this disc to grip large, flat objects underwater.”
Wen first became fascinated by the attributes of the remora in 2012, while working on a project at Harvard led by George Lauder to create artificial shark skin using 3D-printing techniques. That research showed how tooth-like scales help sharks to cruise efficiently. It also hooked Wen on the ‘sharksucker’ as a result of looking at so many images of sharks with remora stuck to them. “Then we decided to start this project.”
Historically, though, it has not been the remora’s ability to latch onto sharks but ships that has most stuck in the minds of men. In the third century BCE, Aristotle – who besides being an eminent Greek philosopher was also a pioneering biologist – mentions the fish in his History of Animals, noting the charming reputation of the ‘ship-holder’ for bringing “luck in affairs of law and love”. Four centuries later, Pliny the Elder, who besides being a natural historian was also a naval commander, was less generous. In The Natural History, he dwells on the fish’s reputation for retarding the progress of ships, and suggests the “evil properties” of the ship-holder are compensated by a single merit: “it is good for staying fluxes of the womb in pregnant women, and preserves the foetus up to birth.”
What makes the remora so successful in latching on and holding fast to sharks and ships alike is the unique dual action of the adhesive disc that has evolved from its dorsal fin on the top of its cranium.
Animals use a range of adhesive biomechanisms to stick to surfaces. Geckos use van der Waals forces – close-distance attraction between molecules – courtesy of tiny hairs on the pads of their feet. Frogs use capillary forces – the ‘stickiness’ of liquid – to climb trees. These forces, however, don’t work well underwater.
Marine animals mostly rely on suction forces, using suckers and muscles to create a pressure seal.
The remora goes one better by combining suction with frictional force. Its adhesive disc has an outer ‘lip’ to create the suction seal; the frictional force comes from what’s within that lip – arrays of tiny spines, or spinules, along ridges of tissue, called lamellae, that the fish can flex up and down. These make the disc look something like the sole of a fishy sneaker.
The spinules, once raised into contact with the surface of whatever the remora has suckered itself to, act in a similar way to the crampons that give a mountain climber traction on a sheer cliff. Frictional force means the climber doesn’t slip down the mountainside, and that the remora doesn’t slip off a surface moving through the water at speed. Particular species have evolved spinules spaced to specially mesh with the gaps between scales of particular hosts, such as sharks. One of these, Remora australis, specialises in only attaching to cetaceans. Others are more generalist.
How did the remora get stuck with these highly adhesive attributes? The greatest advantage is the reduced energy expenditure on movement. Indeed, the remora has evolved so that it now depends on the hitchhiking way of life to live, lacking its own swim-bladder and needing the swift movement of a host to drag water through its gills. Attachment to the likes of a shark also offers a high degree of personal security from predation by other creatures, and access to the ample leftovers courtesy of a host that doesn’t waste time carefully chewing its food.
The robotic version of this remarkable biological adaptation was engineered through analysing one particular type of remora, the slender sharksucker (Echeneis naucrates) and creating a 3D-printed multi-material prototype containing a soft peripheral lip and composite lamellae lined with about one thousand carbon-fibre spinules fabricated through laser machining. The prototype, measuring 127 by 72 millimetres, and weighing 129 grams, was tested by mounting it on an underwater remotely operated vehicle generating different thrust forces and then seeing how it fared in staying attached to both smooth plexiglass and rough shark skin.