Remoras, a type of fish commonly known as the “suckerfish”, are the ultimate hitchhikers of the sea. On the top of the remora’s head is a huge suction cup that is used to latch onto the bodies of larger marine animals. The bigger animal, usually a shark or whale, swims in search of a meal with the remora hanging on for a piggyback ride. Once the host animal finds its meal, the remora can detach from the host to snag a bite, and then rejoin for another ride. What’s the deal with that? Remoras are actually operating in a very clever manner called commensalism, which means they gain a benefit without harming the other animal in the relationship. In addition to stealing meals, remoras also gain the protection of the larger animal, and traverse farther faster. But in order to understand how this relationship truly operates, you have to dive a little deeper and take a look at the anatomy of the remora fish.
The remora fish undoubtedly has a unique look, with a ridged suction cup almost similar in appearance to a type of curtain blinds, that can then create a suction-like vacuum to stick to the host (See Figure 1). Adhesion is accomplished by the remora rotating its lamellae, a thin layer of tissues, which then generates friction and creates a pressured seal. However, not a lot is known about the suction cup aside from the visible layout. For example, under what conditions could the remoras stay attached to the host? Can the remora tell whether it had made contact with the host animal? With these types of questions in mind, a group of scientists led by Karly Cohen sought out to determine whether the remora could tell when it was attached to the host animal through the utilization of mechanoreceptors (a sensory cell that can respond to stimuli like touch) in a series of sample collections and morphological observations.
The team originally suspected that there would have to be a type of touch receptor that could pick up on changes in pressure or posture on the suction cup of remora, but specialized touch receptor complexes had not yet been found in fish. This meant the researchers were essentially starting from nothing but the knowledge that the remora should be able to pick up on a stimulus in order to determine whether it is attached to a host. The researchers first decided to take a look at frozen specimens of Echeneis naucrates (suckerfish) from the New Jersey Institute of Technology Department of Biological Sciences to see what they could find through microscopy. The suction cup discs were observed under microscope where they saw that in order to see that there were actually push-rod mechanoreceptors along the disc lip. The mechanoreceptors resembled a column of tight cells that elevated the surface of the skin, and the nerves from each mechanoreceptor extended from a central group of nerves. As a supplement to this, researchers also looked at the differences and similarities between calcified tissue and stained soft tissue, and then compared the abundance of push-rod mechanoreceptors in different regions. As they dissected portions of the suction cup lip, they used a general stain to identify the separate cell types. In the end, they determined that there were mechanoreceptors in varying, scattered amounts along the entire lip of the disc.
The type of push-rod-like mechanoreceptor found on the lip was a complex found previously in only monotremes (an order of egg-laying mammals), which shows there is a lot more research that can be done in comparing similar taxas for similarities in mechanoreception. This was also the first time mechanoreceptors were found in fish in general, which opens up a new range of possibilities on its own. With this new information, scientists are able to better understand the mechanics of remoras latching onto host animals, in addition to discovering new starting points of research to come in the study of sensory systems.
References
E., Cohen Karly, et al. “Knowing When to Stick: Touch Receptors Found in the Remora Adhesive Disc.” Royal Society Open Science, 15 Jan. 2020, Knowing when to stick: touch receptors found in the remora adhesive disc
