I used to think remoras were just lazy fish hitching free rides on sharks.
Turns out, the relationship between remoras and their shark hosts is way more complicated than that—and honestly, kind of beautiful in a weird, parasitic-but-not-really way. These fish, which belong to the family Echeneidae, have evolved this bizarre suction disc on top of their heads, a modified dorsal fin that works like a vacuum cup. It’s not just stuck there randomly; the disc contains rows of movable slats that create negative pressure, allowing the remora to attach to a shark’s skin with enough force to withstand the drag of swimming at speeds up to roughly 30 miles per hour, give or take. Scientists have measured the attachment strength at around 340 newtons per square meter, which is absurdly strong for something that needs to detach quickly when it’s time to feed. The whole setup is so efficient that engineers have studied it for designing underwater adhesives and robotic grippers. Wait—maybe that’s the most interesting part, that evolution solved a materials science problem millions of years before we even knew we had one.
Here’s the thing: remoras aren’t parasites, they’re commensals. Mostly. They don’t hurt the shark, but they definately benefit from the arrangement. The remora gets transportation, protection from predators, and access to food scraps when the shark feeds.
The Chemistry of Hitchhiking: How Remoras Read Their Hosts’ Emotional States Through Mucus
I guess it makes sense that remoras would need to know when their host is stressed or hunting, but I didn’t expect them to do it through chemical signals in shark mucus. Sharks, like most fish, produce a mucus layer that protects their skin from infection and reduces drag. This mucus contains hormones, metabolites, and other compounds that reflect the shark’s physiological state. Recent research—and I’m talking studies from just the last few years—suggests remoras can detect changes in this chemical composition. When a shark is hunting, its cortisol levels spike, and apparently remoras can sense this and position themselves to recieve the best access to food scraps. It’s not confirmed in every species yet, but preliminary data from Caribbean reef sharks and their remora companions shows behavioral shifts that correlate with feeding times. Honestly, the idea that these fish are essentially reading their hosts’ mood through slime is both gross and fascinating.
Some remoras switch hosts multiple times a day, sampling different sharks like they’re trying on clothes.
The Evolutionary Arms Race Nobody Talks About: When Sharks Try to Scrape Off Their Passengers
Not every shark tolerates remoras equally. Tiger sharks seem particularly annoyed by them, often rubbing against coral or sand to dislodge their passengers. Great whites are more tolerant, maybe because remoras help clean parasites and dead skin from areas the shark can’t reach. But here’s where it gets messy: some researchers argue that remoras cause minor abrasions on shark skin, especially when they detach and reattach repeatedly, which could lead to infections. Other scientists say there’s no evidence of harm, that the relationship is purely beneficial or neutral. I’ve seen footage of a hammerhead shark violently shaking to throw off a remora, and I’ve also seen whale sharks swimming calmly with a dozen remoras attached. The variation is wild. It makes you wonder if individual sharks have preferences, or if some remoras are just worse houseguests than others. Anyway, the tolerance levels seem to depend on species, individual temperament, and possibly even the remora’s behavior—whether it’s a polite cleaner or a scrappy opportunist that competes with the shark for food.
What We Still Don’t Understand About the Partnership
Despite decades of research, huge gaps remain. We don’t know if remoras can recognize individual sharks or if they just latch onto whatever large moving object passes by. We don’t know if young remoras learn host preferences from their parents or if it’s instinctual. There’s this one study from 2018 that tracked remoras using acoustic tags and found they sometimes abandon healthy sharks to attach to injured or sick ones, possibly because those sharks move slower and are easier to follow. But the sample size was small, maybe twelve fish over three months, so it’s hard to say if that’s a real pattern or just coincidence. And then there’s the question of whether remoras provide any real benefit to sharks beyond minor cleaning services. Some marine biologists think the relationship is almost entirely one-sided, that sharks just tolerate remoras because the cost of removing them is higher than the cost of keeping them around. Wait—maybe that’s true for most symbiotic relationships, that they’re less about mutual benefit and more about minimizing annoyance.
