The Fish That Wears a Suction Cup as a Hat

Yeah, that's the top of the remora's head. It's as surprised as you are!

Pliny the Elder, the legendary Roman naturalist who was nevertheless wrong about, oh, somewhere around 98 percent of the things he claimed, once wrote of the echeneis—a small, slug-like fish with a habit of sticking to ships and bringing them to a dead stop. The emperor Caligula supposedly met echeneis the hard way on a return journey to Antium. Slowed to a halt, he ordered his men to inspect the belly of the ship, and sure enough they found the fish stuck to the rudder. After they removed it and eventually reached land, Caligula’s men assassinated him, perhaps a bit upset about being clowned by a fish.

Today, biologists know that fish as the remora, literally meaning “hindrance.” And you’ve no doubt seen it before. It’s the critter that sticks to sharks, hitching a free ride and hoovering up its host’s scraps all the while. It can’t stop ships, but it is in possession of one of the most striking adaptations in the sea: a suction cup it wears for a hat.

When you see a remora stuck to the top of a shark, it’s not suctioning on with its mouth. In fact, it’s flipped upside down, using a specialized structure on the top of its head to get a grip. And it doesn’t just target sharks. Remoras go after all kinds of creatures, from sea turtles to manta rays to whales. But here’s where things get interesting: How should scientists define this relationship? Are remoras parasitic, commensal (one party benefits and the other suffers no harm), or mutualistic (both parties benefit)? It turns out this relationship is far more complicated than it seems.

First, though, that sucker. It’s actually a modified dorsal fin—that is, the kind of fin you see sharks sticking out of the water. But a baby remora isn’t born with a fully formed sucker. It has to wait for the thing to develop as it grows up. In fact, when the remora is a youngster, the developing sucker starts off where you’d expect to find a dorsal fin, then migrates forward to the top of the fish’s head during development. (Puberty, amirite?)

An animation showing the anatomy of the head of the remora and its sucker. Not a real one of course. This is an animation, as I mentioned.

Take a look at the animation above. Your typical dorsal fin has vertical spines that give it support, but in the remora’s modified sucker, those spines have flattened. Each one has split in two, one going to the left of the base and the other to the right. Combined, all the spines form a disk, which is edged with a rim of flesh to form a suction cup. As a bonus, little offshoots come off of the base of each spine. “And those are presumably the things that provide extra friction when the remora attaches,” says Dave Johnson, an ichthyologist at the Smithsonian. Thus the remora can get a grip on uneven surfaces, like the body of another living creature or the ship of a soon-to-be-assassinated Roman emperor.

(Interestingly, an unrelated species whose name does not disappoint, the clingfish, uses modified pectoral and pelvic fins to form a suction cup on its belly. This allows it to hold fast to rocks on battered shores. Two unrelated organisms arriving at a similar adaptation like this is known as convergent evolution.)

Now, there are two groups of remoras: a reef variety is less picky about what it latches onto, sharks and fish and such, while an open ocean or pelagic variety tends to specialize, sometimes hitchhiking on only one species. “The general pattern is there’s a group that will hitchhike on anything,” says Christopher Kenaley, a biologist at Boston College, “and then there’s a group that’s way offshore and sticks to bigger things and only a few things.”

Behold the remora’s sucker, the most adhesive hat on Earth other than that time a British dude super-glued a tiny hat to his head.

In either case, the remoras are getting a free ride, thus saving them valuable energy, not to mention providing them protection. But what are they feeding on? That also seems to vary between the reef and open ocean types. According to Kenaley, stomach contents from the latter show they’re mainly eating the parasitic copepods (small crustaceans) that also attach to their hosts. This would suggest that far from just mooching a ride, the remora is doing its host a service by hoovering up parasites.

But not so fast, says Kenaley. “The idea of a remora crawling up on the side of a fish and removing a parasite seems a little far-fetched. It’s probably the case that these parasites are falling off,” and the remora gobbles them up when they do. In contrast, around the reefs, remoras likely collect more scraps that their hosts don’t manage to eat—think of the cloud of flesh a shark produces when it tears its prey apart. Also, sorry to make you think of the cloud of flesh a shark produces when it tears its prey apart.

What’s a mystery, though, is what remoras feed on when they’re larvae—remember that they aren’t born with a fully realized sucker with which to latch onto other creatures. Scientists aren’t yet sure, but Johnson has a hypothesis. “Maybe they actually have an association with hosts at a very early stage,” he says, “and maybe they’re sitting, for example, inside the gill cavities of other fishes.”

A top-down view of the gnarly teeth of the larval remora (its lower jaw juts forward), which may help it cling to the gills of other fish. Boy, that remora. Always clinging to something or other.

They’ve got huge, hooked teeth as larvae, which is quite a different set of chompers than they have as adults. Perhaps they’re using them to latch onto other fishes’ gills, sitting there and picking off bits of food as their host feeds, instead of swimming around looking for their own meals. “It’s just hard to imagine what those big hook-like teeth would be there for in a free-living larva.” As of yet, though, scientists haven’t found any remora larvae lodged in fish gills, so the idea remains speculation.

As for the adult remora, whether or not it’s parasitic, commensal, or mutualistic is a tricky subject. The host may be benefiting from cleaning services—that is, the remora is plucking off parasites—though as Kenaley mentioned, it’s more likely the parasites are falling off on their own, which would mean the remora isn’t doing a lick of work. Additionally, remoras can rub their hosts raw, potentially opening them up to infection—not exactly the civilized behavior required of commensalism or mutualism.

Kenaley has also run models that show remoras significantly slow down their hosts. Not a bringing-a-ship-to-a-halt kind of slowdown, but definitely contributing some measure of drag. This would mean the host would need to put more energy into locomotion. That may not seem like a big deal, but wasting energy is a big no-no in the animal kingdom. Sharks with attached remoras would have to eat that much more to gain back the lost energy. And scientists have spotted whales with a dozen of these things attached. You can only imagine the drag involved there.

Read more at Wired Science