New Recordings Show Remoras' Secret Surfing Life Onboard Blue Whales
A new study published in the Journal of Experimental Biology has captured the first-ever continuous recording of remora behaviour while travelling with its host organism. The study into the 'fluid environments' of blue whales travelling off the coast of California used advanced biosensing tags with video recording capabilities to capture the remoras 'hitchhiking' on the whales.
Remoras – from the Echeneidae family of fish – are renowned for sticking themselves to the bodies of sharks, manta rays, turtles and other large marine life, including, occasionally, scuba divers. Powerful suction disks on their heads allow the 'suckerfish' to ride as passengers through the ocean, the constant movement of their hosts keeping water flowing across the remora's gills without having to expend energy swimming. In return for the marine taxi service, the remoras remove parasites from the surface of their hosts, and are rewarded with a never-ending supply of faecal matter on which to feed.
Travelling onboard a 30m-long blue whale (Balaenoptera musculus) for any distance, however, is not quite as simple as sucking on to the host animal's body. The movement of the whales through the water creates 'intense hydrodynamics' in the surrounding environment, meaning the remoras have to employ a 'much more refined skillset' in order to make the most of their ride.
The study shows how the remoras select the 'most flow-optimal regions' of the whales' bodies on which to attach themselves, including behind the blowhole, where drag resistance is reduced by as much as 84 per cent. The recordings also show the remoras moving freely around their hosts' bodies, using 'previously unknown surfing and skimming behaviours along special low-drag travelling lanes that exist just off the surface of the whale’s body'.
'Whales are like their own floating island, basically like their own little ecosystems, ' said Brooke Flammang, assistant professor of biology at the New Jersey Institute of Technology and lead author of the study. 'Through lucky coincidence, our recordings captured how remoras interact in this environment and are able to use the distinct flow dynamics of these whales to their advantage. It is incredible because we’ve really known next to nothing about how remoras behave on their hosts in the wild over any prolonged period of time.'
The footage recorded during the study captured a total of 27 remoras using 61 different locations on the whales. It turns out that the remoras could stick themselves anywhere they wanted to, but preferred to travel in the most 'hydrodynamically beneficial' spots on the whale's surface – those places where the remoras would expend the least energy remaining in position – which included behind the blowhole, next to and behind the dorsal fins, and a region of the whale's flank above and behind the pectoral fins.
'We learned that the remora’s suction disk is so strong that they could stick anywhere, even the tail fluke where the drag was measured strongest, but they like to go for the easy ride,' said Erik Anderson, co-author of the study and biofluid dynamics researcher at Grove City College. 'This saves them energy and makes life less costly as they hitchhike on and skim over the whale surface like a NASA probe over an asteroid or some mini-world.'
In order to conserve energy while moving around the whale, the remoras 'surf' inside a thin layer of fluid – known as the boundary layer – around the whale's body, where drag force is reduced by up to 72 per cent compared to the free stream just above. The fish repeatedly attached and released their suction disks to move and were seen 'skimming' over the whale's body. The researchers suspect that the remoras' ability to remain close to their hosts is aided by the Venturi effect – the same principle by which scuba regulators work – whereby moving fluid creates a lower pressure than the surrounding environment, meaning the remora are effectively pushed back against their host whale's body when they move, instead of being completely pulled away from its surface.
'The skimming and surfing behaviour is amazing for many reasons, especially because we think that [the remoras] are taking advantage of the Venturi effect and using suction forces to maintain their close proximity,' said Flammang. 'In this narrow space between the remora and whale, fluid moves at a higher velocity but has lower pressure, so it is not going to push the remora away but will actually suck it toward the host. They can swim up into the free stream to grab a bite of food and come back down into the boundary layer, but it takes a lot more energy to swim in the free stream flow.'
Along with uncovering new details of the remora’s hitchhiking prowess, the team says they will continue to explore both the flow environments around whales and the mechanisms by which specifically adapted organisms like remoras successfully attach to hosts in order to improve animal tag technologies and designs for extended periods of behavioural and ecological monitoring. The team is also using their new insights into the remora’s preferred low-drag attachment locations to better inform where they might tag whales in studies to come.
'It's an extremely arduous process to study whales, with permitting, research regulations and the game of chance of finding animals, all for the tags to usually fall off within 48 hours,' said Flammang. 'If we can come up with a better way to collect longer-term data through better tag placement or better technologies, it could really advance our learning of the species, and many other animals that remoras attach to.'
The complete study 'Remoras pick where they stick' by Brooke E. Flammang et al can be found at www.jeb.biologists.org/content/223/20/jeb226654