friends, foes, and followers of fishes -- 8/13/26

13 Aug 2026

friends, foes, and followers of fishes -- 8/13/26

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Today's encore selection -- from Intertwined by Michael Gross. Remoras and manta rays:

“Larger fish species are often seen accompanied by smaller species—which may be anything from helpful cleaners, via unobtrusive fellow travelers, through to flesh-eating parasites. Some of these ecological relations have been studied in detail, but many, especially the more dynamic ones, remain poorly understood. These connections may be vital for our understanding of the fate of marine species in a changing oceanic environment.

“Manta rays, with their vast wingspan of up to seven meters (23 ft.), are among the most charismatic fish species one can observe in the warmer parts of the oceans and thus a key attraction for scuba-diving destinations such as the Maldives or Hawaii. Of the

two species now merged into the genus Mobula (together with the devil rays), the smaller, more coastal reef manta ray (Mobula alfredi) can readily be observed near coral reefs, while the giant ocean manta ray (Mobula birostris) is slightly more elusive.

“Both species had been listed as vulnerable on the IUCN (International Union for Conservation of Nature) Red List of Threatened Species, but M. birostris has now been moved to endangered.

“Countries such as Indonesia have realized that the long-lived animals are much more economically valuable alive as a tourist attraction than dead on the fish market and have protected them accordingly. However, a growing demand for the cartilage of the manta ray's gill plates is still fueling an unsustainable level of killing, which often happens either illegally or in international waters.

“The Manta Trust, an international charity based in the United Kingdom, is working to improve the protection and appreciation of manta rays. Its experts can even identify thousands of individual manta rays. While the rays are mostly black from above (the dorsal side), their ventral side is typically white with a pattern of black spots that is unique in each individual.

M. alfredi group in the Maldives

“Working with the dive and snorkel tourism industry in the Maldives, the Manta Trust has established an image database identifying individual resident reef manta rays by their spots; the database had passed 5,000 entries in December 2020. As with the starlike patterns of whale sharks and the whisker spots of lions, the rays' spots are like fingerprints that identify individuals, thus enabling researchers to better understand the life cycle of the animals, including their social network within their own species and with other species.

“Manta rays are often observed aggregating in groups of several dozen individuals, and they are often seen accompanied by smaller fish species, including some that attach themselves to the rays. These fellow travelers obviously use the large rays for their own benefit, but are they parasites, mutualists, or commensalists (i.e., do they harm, benefit, or have no effect on the rays)? As it is difficult to monitor the contacts of a fast-moving species, surprisingly little is known about the interspecies connection of manta rays.

“Aimee Nicholson-Jack from the Manta Trust and colleagues expanded the scope of knowledge on the manta ray ‘hitchhikers’ with what the authors say is only the second systematic analysis of such species to be published in a peer-reviewed format. Using the Maldives database and photographic evidence accumulated in three decades worth of citizen science engagement, the researchers evaluated the associations observed for 4,901 M. alfredi individuals identified in 353 sites.

“They found 12 species associated with the reef manta rays, including, for the first time, species that don't belong to the well-known family Echeneidae (remoras). The most observed companion was the sharksucker remora (Echeneis naucrates), which was observed in 10% of the sightings. It was most likely to be present when the rays were visiting cleaning stations, which are nearshore locations where populations of cleaner fish feed on crustacean parasites present on the rays. Near-term pregnant female rays were more likely to be seen with E. naucrates, but this may be causally related with the fact that they spend more time at cleaning stations. Interestingly, juvenile remoras tended to be found with juvenile rays.

“In addition to filter feeding near the surface, manta rays dive to depths below 200 meters (660 ft.) to hunt. It is thought that E. naucrates is unable to survive at that depth, so the movement pattern of the ray explains why the hitchhikers aren't always with them. A tagging study of M. alfredi diving behavior found that all tagged individuals went to depths beyond 300 meters (980 ft.), with a maximum recorded depth of 672 meters (2,205 ft.).

“By contrast, the most common companion of the giant oceanic manta ray, the giant remora (Remora remora), seems to stay with the ray for longer and often attaches itself. Based on records of 663 identified M. birostris individuals, R. remora was present in just over half the sightings.

“While the remoras provide some benefits to the manta ray host, including removal of crustacean parasites, the habit of attaching to the ray can also come with a fitness cost. The researchers observed examples of scars due to remora attachment and of remoras attached in unsuitable places, such as inside the gills or the anus.

“A third remora species, the white suckerfish (Remora albescens), was only rarely seen on the images analyzed, but it has been discovered within the mouths of rays examined more closely, which leads the authors to suggest that these remoras are more commonly associated with rays than the observation data suggest. …

“Among the hitchhiking species, the remoras have attracted attention for the efficiency of the reversible suction mechanism they use to attach to their hosts, which apart from rays also include other marine megafauna from sharks to whales (animals with more than 44 kg, or 97 lb., body weight are considered megafauna). The attachment survives remarkable drag forces on fast-swimming animals but can be fastened or released in a fraction of a second. They achieve this with an adhesive disc that evolved from dorsal fin elements and consists of a series of parallel lamellae surrounded by a fleshy lip. When the lip makes contact, the lamellae rotate to expand the enclosed space and thus reduce the pressure.

“Brooke Flammang's group at New Jersey Institute of Technology in Newark has studied the suction apparatus for clues to remora behavior, especially when, where, and why they attach or let go. Investigating the anatomy of the lip in E. naucrates, the researchers discovered densely innervated structures that they propose to be push-rod mechanoreceptors that allow the remoras to sense when they have made contact with a host and quickly trigger attachment. Once attached, the mechanism would enable the remoras to sense shear forces and ensure they remain fixed.

“While entirely plausible in terms of what the remoras need to hitch a ride, this was a sensational discovery because it is unprecedented among fishes. The only other comparable mechanism the researchers could find in the literature is in monotremes (platypus and echidnae).

“Regarding the question where remoras attach and how they behave on their host surface, Flammang had a lucky break when she saw a conference talk from whale researcher Jeremy Goldbogen of Stanford University, who had tagged blue whales with video cameras and ‘inadvertently gotten hundreds of hours of remora footage.’ After seeing the movies of remoras skating on the surface of the whales, Flammang set out to analyze their behavior. Until then, all the information available was based on photos and short-term observations.

“Working with experts on fluid dynamics and the Barcelona Supercomputing Center in Spain, Flammang and colleagues obtained models of the flow conditions around the whale. They identified various areas where the anatomy of the whale reduces the drag forces and makes it easier for the remoras to remain attached, such as behind the blowhole and behind the fins. In these areas, the drag force on a whale swimming at 1.5 meters (5 ft.) per second is reduced by up to 80%. Sure enough, the videos show the remoras preferentially settling in these areas, thus saving energy. Calculations showed, however, that they could maintain attachment anywhere on the whale, even on the highly agile tail fluke.

“While the remoras had their favorite spots, they didn't always stay in one place. Footage shows them gliding across the surface of their host, to get from one sweet spot to another, or to meet with fellow passengers. Knowing the best spot to attach to animals like whales, rays, and shark will prove helpful to conservation research, as it can facilitate tagging the animals. With applications like tagging in mind, Flammang's group has developed biomimetic suction devices based on the remora studies.”