The secret surfing skills of hitchhiking remoras

A new study into the fluid dynamics of blue whales has given researchers the first ocean recordings of their famous hitchhiking partner, the remora, revealing the suckerfish’s secret whale-surfing skills as they ride aboard the world’s largest animal.

A remora is filmed whilst attached to a blue whale near the surface, as part of a new study into the the giants fluid dynamics

Sticking to the bodies of sharks, whales and other large marine creatures is a well-known specialty of the remora fishes (Echeneidae), who use the super-strong suction disks on their heads to hitchhike across the world’s oceans. Capable of growing anywhere between 30cm to a metre in length, these ray-finned fish are one of the least understood groups in the ocean, despite spending most of their lives attached to some of the best studied animals in the world. The little we do know about them comes from studying their hosts or removing individuals at the surface (which is no easy task), but what they are up to during their trans-oceanic voyages has largely remained a mystery. However, a new study into the fluid dynamics of blue whales has somewhat accidentally revealed a range of fascinating remora behaviours which take their hitchhiking reputation to the next level.

Studying the fluid dynamics of whales

The intriguing new insight into remoras comes courtesy of a new blue whale research project using state of the art tagging to study the fluid dynamics of their movements. The international research group behind the project, including scientists from New Jersey Institute of Technology, Woods Hole Oceanographic Institute, Stanford University and the Barcelona Supercomputing Centre, wanted to know how the 25m long whales were able to reach such incredible speeds (up to 5 metres per second) with relatively minimal effort.

Despite their enormous size blue whales are able to achieve some of the fastest cruising speeds in the ocean

To do this they attached multi-sensor biologging tags to blue whales off the coast of California at Palos Verdes and San Diego. These specialised tags were attached to the whales using 2-inch suction cups and contained dual cameras, as well as sensitive equipment capable of calculating various measurements such as surface pressure, complex fluid forces, GPS location and traveling speeds. The data they collected was then sent to be analysed at the Barcelona Supercomputer Centre to produce ‘highest-resolution whole-body fluid dynamic analysis of whales to date’.

However, an additional benefit of using such advanced tags, which included a near complete visual coverage of the whales’ surfaces, was that it allowed them to see what the remoras attached to the whales were doing as well. In fact the tags were able to capture video footage of 27 remoras for a total of 211 minutes. This gave us our first real look at what remoras get up to beneath the surface, when nobody is there to watch them. The research team released their new remora findings in a new paper which was recently published in the Journal of Experimental Biology.

Picking the best spot

The first discovery uncovered by this new study is that the remoras who attached themselves to the blue whales strategically stick to points where the forces of drag are lowest. The team identified three such locations on the whale’s body that are favourites of the remora – directly behind the blowhole, next to and behind the dorsal fin and the flank region above and behind the pectoral fin. At the spot behind the blowhole for example, drag can be reduced by up to 84% compared to the rest of the whale’s body. At a casual speed of 1.5 metres per second this produces a force of 0.02 Newtons, compared to 0.14 Newtons at the fluke of the tail which is the most turbulent sport for remoras on the whale.

One of the studies tags gets an up-close look at a remora locked on to a flow-optimal location behind the blowhole

Interestingly these forces are still much lower than the maximum that the remoras can experience whilst still remaining attached to their hosts, which is believed to be somewhere between 11-17 Newtons. Even at the whales top speeds the forces still don’t come close to this threshold, yet remoras still pick the most flow optimal points to attach themselves. “They like to go for the easy ride” claims Erik Anderson, one of the researchers involved in the study speaking in a recent press release, who went on to suggest “this saves them energy and makes life less costly”.

Going with the flow

The idea that remoras select sites on their host with the least drag makes a lot of sense and was something that most researchers probably suspected beforehand. However the next discovery made by the new study was much more unexpected. The video footage of the remoras showed that they can actually move around on their host whilst they are travelling around the ocean. This would seem almost impossible, as the blue whales travel at speeds up to seven times faster than the remora, but the suckerfish are able to do it by surfing through a thin layer of fluid surrounding the whale’s body.

This image. captured by one of the study tags, clearly shows separation between a remora and its blue whale host, highlighting their ability to surf across its body

This is known as a boundary layer and within it drag force is reduced by 72% compared to the much more forceful torrent of water just above. This is a result of the Venturi effect, a principle of fluid dynamics which states that when fluid is funnelled into a narrow space it moves at a higher velocity but has a lower pressure. By utilizing the boundary layer and the Venturi effect, detached remoras can therefore skim around up to 1cm above the whale’s skin without falling off. This allows them to move around their hosts during transit, something which was previously thought to be impossible, which could allow them to feed on the go and also socialise or even mate with other remoras.

What next?

The discovery of these fascinating new remora behaviours has seriously changed what we know about these hitchhiking suckerfish. Rather than just hanging onto their hosts for dear life like we suspected, they are instead strategically picking locations where they can effortlessly stick and even surfing through invisible layers of drag-reduced water. However, this raises even more questions about remoras for researcher to answer in the future. For example, are other remoras hosts large enough to produce their own boundary layer for in-transit movement? And if not, then why do remoras ride around on anything other than large baleen whales? Hopefully researchers can answer these questions using similar camera-based tags in the future and continue to uncover more about these fascinating creatures.

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