An international study reveals the secret life of remora surfers hitchhiking blue whales

Attaching to the bodies of sharks and other larger marine life is a well-known specialty of remora fish (Echeneidae) and their super-powerful suction discs on their heads. But a new study has now fully documented the “sucker fish” in hitchhiking action below the ocean’s surface, uncovering a much more refined skill set the fish uses to navigate the intense hydrodynamics that comes from attempting to ride a edge of a 100 foot. blue whale (Balaenoptera musculus).

In a study published on October 28 in Journal of Experimental Biology, an international team of researchers studying the unique fluid environments of blue whales traveling off the coast of Palos Verdes and San Diego, California, reported that they have captured the first-ever continuous record of remora behavior on a host organism. , using advanced biosensor tags with video recording capabilities.

The study reveals the secrets behind the fish’s success in hitchhiking aboard whales more than 30 times their size to safely cross the ocean: they select the most optimal regions for flow over the whale’s body to attach to, such as behind the whale blowhole, where drag resistance for fish is reduced by up to 84%. The team’s results also show that remora can roam freely to feed and socialize during their run even as their whale host reaches blast speeds of more than 5 meters per second, using previously unknown surf and skimming behaviors along special lanes. low resistance travel that exist just outside the surface of the whale’s body.

The researchers say the study represents the highest resolution whole-body fluid dynamics analysis to date, whose insights could potentially be used as a basis for better understanding the species’ behavior, energy use, and overall ecological health. , as well as improve the labeling and monitoring of whales and other migratory animals in future studies.

“Whales are like their floating island, basically like their little ecosystems … Taking a look at the flow environment of blue whales in pinpoint resolution through this study is extremely exciting,” said Brooke Flammang, assistant professor of biology at the New Jersey Institute of Technology and the corresponding author of the study. “By lucky coincidence, our recordings have captured how the remora interact in this environment and are able to use the distinct flow dynamics of these whales to their advantage. It’s amazing because we know next to nothing about how they behave. i remora about their hosts in the wild for an extended period of time. “

Until now, scientists studying the symbiotic relationships between remoras and their hosts in their natural ocean habitat have relied mostly on still images and anecdotal evidence, leaving much of the way they behave regarding their famous attachment behavior underneath. the surface a mystery.

In their recent investigation, the researchers used dual-camera multisensor biologging tags that they connected to the whales via four 2-inch suction discs. The tags were able to calculate various measurements within the whale’s ecosystem, such as surface pressure and complex fluid forces around the whales, as well as GPS location and travel speeds through the tag’s vibrations, all while recording the hesitation at 24 frames per second and 720p resolution.

“Fortunately, the drag on the dimple-shaped aircraft cockpits has been measured many times, and we have been able to apply this knowledge to help understand the drag that these obstacles were experiencing,” said Erik Anderson, co-author, researcher of biofluid dynamics at Grove City College and a guest investigator at the Woods Hole Oceanographic Institute. “But our study still required us to calculate, for the first time ever, the flow on a blue whale using computational fluid dynamics … it took an international team of biologists, programmers, engineers and a supercomputer to do so.”

The team’s 211 minutes of video footage and whale tag data processed by researchers at the Barcelona Supercomputing Center captured a total of 27 hesitations in 61 overall whale locations, finding that remora were most often podding and traveling between three of the most hydrodynamically beneficial points where parting flow and wakes are caused by the distinct topographical features of the whale: directly behind the blowhole, near and behind the dorsal fin and the flank region above and behind the pectoral fin.

According to the team’s measurements, Anderson says that the shear force experienced by a medium-sized remora in the wake behind the blowhole of a whale swimming at a random speed of 1.5 m / s can be as low as 0.02 Newton. half the strength. in the free stream above. However, Anderson notes that the remora’s average suction force of 11-17 Newton is more than equal to even the whale’s most intense parking spot, its tail, where the remora experiences about 0.14 Newton shear force. . And although the forces are greater, the same is also true for large remora riding whales that swim at much higher speeds.

“We learned that the remora’s suction disc is so strong that they could stick anywhere, even the tail flick where the strongest drag was measured, but they like to go for the easy ride,” said Erik Anderson. “This saves them energy and makes life less expensive as they hitchhike and skim the surface of whales like a NASA probe on an asteroid or a mini-world.”

Remoras Go Surf’s Up

The tags showed that to conserve energy as they move to their floating island, the remora harness the physics of the whale by navigating within a thin layer of fluid surrounding the whale’s body, known as the boundary layer, where the team discovered. that drag force is reduced by up to 72% compared to the much more powerful free stream just above. Flammang states that fish can rise within 1 cm of their host in this layer to feed or join their mates at other low-resistance social spots on the whale, occasionally changing direction by skimming, or repeatedly attacking and releasing their discs. suction on the whale’s body.

Flammang suspects that the remora are able to move freely without being completely detached from their fast hosts, who can move nearly seven times faster than the remora, through something called the Venturi effect.

“The skimming and sailing behavior is surprising for many reasons, mainly because we think that by standing within an inch of the whale’s body, they are harnessing the Venturi effect and using suction forces to maintain their proximity,” Flammang explained. “In this narrow space between the remora and the whale, when fluid is channeled into a confined space, it moves at a higher speed but has lower pressure, so it won’t push the remora away but can actually suck it towards the host. it can swim in free flow to grab a mouthful of food and back down to the boundary layer, but it takes a lot more energy to swim in free flow.

In addition to uncovering new details about the remora’s hitching ability, the team says they will continue to explore both the flow environments around whales and the mechanisms by which specifically adapted organisms such as remora successfully attach themselves to hosts in order to improve animal labeling technologies and projects for extended periods of behavioral and ecological monitoring. The team is also using their new insights into the remora’s preferred low-resistance attachment positions to better inform where they might tag whales in studies to come.

“It’s an extremely arduous process to study whales with permits, search regulations, and the game of finding animals, all because the tags usually drop within 48 hours,” Flammang said. “If we can find a better way to collect long-term data through better tag placement or better technology, it could really advance our learning of the species and many other animals that remoras attach to.”