A Blog by Carl Zimmer

What Good Is Half A Sucker?

The remora is so ridiculous that no one would try to make it up. The top of its head is a giant, flat suction cup. It uses the cup to lock onto the bodies of bigger animals, such as sharks, sea turtles, and whales. As the big animal swims for miles in search of a meal, the remora hangs on for the ride. When its host finds a victim, the remora detaches and feasts on the remains. It sometimes cleans its host’s body and mouth of parasites, and then clamps its head back on for another ride.

The remora’s ridiculousness makes it a fascinating evolutionary puzzle just waiting for the solving. Other species clamp themselves onto other animals–whale barnacles, for example, grow prongs from their shells that anchor them to whale skin–but among fish, remoras are exceptional. Their closest relatives include Mahi-Mahi and amberjacks, neither of which has anything on their head that even faintly resembles the remora’s sucker. Only after the ancestors of remoras and these ordinary fish split apart some 50 million years ago, the remoras evolved a remarkably new piece of anatomy.

A pair of free-swimming remoras, displaying their suction disks. Heather Perry/National Geographic
A pair of free-swimming remoras, displaying their suction disks. Heather Perry/National Geographic

When you look closely at the remora’s suction disk, its remarkableness only grows. It looks like a spiked Venetian blind. Pairs of slat-like bones called lamellae form a series of rows running down the length of its head, and muscles running from the remora’s skull to those bones pivot them, creating spaces between the rows.

That negative pressure pulls the remora towards its host’s body. Each lamella also has a comb-like set of pins that help make its clamp even more secure. The whole structure is surrounding by a loose fleshy lip, ensuring that no water slips in, keeping the seal tight.

As a result, remoras can create a vacuum that’s not just strong enough to attach them to an animal, but to stay attached as water rushes past them. They can even hold tight as their hosts try to scrape them off on rocks. But a remora can instantly release itself when it’s time to eat, with just a flick of its muscles.

The head of a young remora from the side, above, and the front. Photo courtesy of Ralf Britz
The head of a young remora from the side, above, and the front. Photo courtesy of Ralf Britz

As impressive as the remora’s suction disk may be, however, it’s not actually all that new. As is so often the case in nature, it’s actually just evolutionary tinkering with old parts.

Scientists have gotten some clues to the remora’s origins by looking at how they grow up. When remoras hatch, they don’t yet have a suction disk. Last year Ralf Britz of the Natural History Museum in London and David Johnson of the Smithsonian made a careful study of young remoras to document their development.  They found that the bones and muscles of the remora’s sucker start out much like the bones and muscles in a fin found on the back of other fish, known as the dorsal fin. They develop in the same location and have the same structure. But later, the bones and muscles move forward to the head.

They also change shape along the way. The fin spines spread out into lamellae that sprout a comb of spikes. In ordinary fish, the fin spine sits atop a small round bone. In remoras, that small bone widens out into another set of lamellae.

While the development of embryos doesn’t recapitulate evolution, it can offer some hints about how new things evolved from old ones. Britz and Johnson’s research indicates that the remora suction disk started out, improbably enough, as a dorsal fin. The fin stretched out into a complex vacuum device and moved up to the head. The underlying similarity between sucking disks and fins only becomes clear when you see how they both develop along the same path at first before diverging.

If all this were true, you might be able to test it by looking at the fossils of early relatives of remoras. Perhaps they captured the early stages of the transition.

That is precisely what Matt Friedman at the University of Oxford and his colleagues have done, as they report today in the Proceedings of the Royal Society.

Frieman is an expert on the larger group of fish to which the remora belongs, known as spiny rayed fishes. The group has evolved into some spectacularly weird forms, including sea horses and flatfishes. In 2008, I blogged about how Friedman found an intermediate flatfish, with one eye moving towards the other side. Remoras, with their own brand of weirdness, seemed to Friedman another spiny rayed fish worth spending some time on.

The only problem was that most of the fossils of remoras belonged to living lineages. Their suction disks were pretty much like what you’d find on a remora today. At least, that’s what most paleontologists who study remoras have thought.

Friedman decided to take a closer look at one of those remora fossils, called Opisthomyzon. The 30-million-year-old specimen was the first remora fossil ever found, in 1886, and it has sat in a museum in Switzerland ever since.

Ophithsomyzon, a 30-million-year-old remora. Photo courtesy of Matt Friedman
Opisthomyzon, a 30-million-year-old remora. White box shows suction disk.  Photo courtesy of Matt Friedman.

The specimen was in bad shape, Friedman found. It had a suction disk, for example, but it wasn’t clear if the whole disk had been pushed away from its original location. Friedman wondered if there might be other Opisthomyzon fossils hidden in other museums. Sometimes paleontologists can’t quite figure out what they’ve found, and they file away fossils without describing them.

At the National History Museum of London, Friedman found not one hidden Opisthomyzon fossil, but two. Mark Graham, a preparator at the museum, painstakingly chipped away at the underside of one of the new fossils, until all that was left was a paper-thin slab of rock. Friedman and his colleagues then compared the anatomy of Opisthomyzon to living remoras, as well as to extinct and living relatives, such as Mahi-Mahi.

Opisthomyzon proved to be exactly what Friedman was looking for: an extinct species that branched off before the origin of the living lineages of remoras. And when he and his colleagues examined its anatomy, they found exactly the kind of fish you’d expect to see from developing remoras: a fish with a suction disk still evolving from a fin.

This figure below sums up the story. The top fish is a conventional relative of remoras, with its dorsal fin bones shown to the right. In the middle is Opisthomyzon, with its corresponding suction bones. And at the bottom is a living remora.

Top: A relative of remoras without a suction disk. On the right are two bones that make up a dorsal fin spine. MIddle: Opisthomyzon, with an intermediate suction disk. Bottom: Living remora. Drawing by Matt Friedman
Top: A relative of remoras with bones from its dorsal fin shown on right. Middle: Opisthomyzon. Bottom: Living remora. Drawing by Matt Friedman

You can see that the suction disk on Opisthomyzon is smaller than that of living remoras and does not sit over its whole head as it does today. The lamellae themselves bear more of a resemblance to the spines of dorsal fins. Opisthomyzon’s lamellae lacked a comb of spikes, for example, still retaining a single spine at the center.

Friedman’s research now gives us a richer hypothesis for how the remora got its sucker. Some of the remora’s closest living relatives, like cobia, tag along with bigger fish to scavenge on their scraps. The ancestors of remoras may have lived a similar life.

It’s not rare for spiny rayed fishes to grow extra dorsal fin spines. In the ancestors of remoras, such an anatomical fluke may have allowed them to latch their dorsal fin into the skin of a host fish, if only briefly. Even if they could spend a little time hitch-hiking this way, they would save energy that they’d otherwise have to spend on swimming for themselves.

Gradually, the remora’s dorsal fin became better adapted to latching onto other animals. As it moved towards the remora’s head, for example, it reduced drag. And as the fin bones spread outward, they attached the remora more strongly.

Sometimes, when we look at an adaptation in living animals, it seems to exquisitely well-suited to the animal’s life that we can’t imagine how a more primitive version of it could have provided any benefit. What good is half a wing, for example? What good is half a sucker? Fossils can give our limited powers of imagination a boost, by showing us that these intermediate forms did indeed exist. Opisthomyzon probably could ride on other animals, although it may have been more prone to get peeled off along the way. Remoras are so good at clamping onto their hosts that they’ve lost some of the traits that other fishes have. Their tails are weak, and they need a strong current of water passing over them in order to breathe through their gills. It’s probably no coincidence that Opisthomyzon had a much stronger tail than living remoras. It was only part-way down the road to ridiculousness.

10 thoughts on “What Good Is Half A Sucker?

  1. I’ve been noticing something, a possible ARGUMENT AGAINST I.D. and IN FAVOR OF NATURAL SELECTION. The evolution of specialized body parts is the exception in nature rather than the rule. For instance remora has many scavenging cousin species who lack the specialized parts to allow them to hitch a ride with large predators. Other examples include the fact that only a few kinds of mammals developed large brains: the cetacea, elephants, and apes/humans, not all mammals developed large brains. And though many beetles naturally produce the chemicals needed in order to produce a hot liquid, and though they also have chambers in the rear of their bodies to store such chemicals, most did not evolve the ability to shoot hot liquids. And among species of beetles that CAN produce hot liquids most cannot direct where the liquid goes once it is shot out of their rear ends, but it simply winds up on their own backs. Only the Bombardier Beetle has evolved a means to point such a hot squirt of liquid at its foes with accuracy, a firing turret of sorts. Same with the one species of bedbug known to traumatically inseminate other males of their own species in order to try and get the second male’s sperm into the female that the first male has mounted. Most bedbug species have evolved traumatic insemination of the female by males (to get past the mating plug in females), but male-on-male traumatic insemination is rare among bedbug species, since it is a highly specialized behavior. Same with the fact that the majority of bee species (and wasps too I think), did not evolve to live in colonies with all of the concomitant complex social behaviors, they don’t form hives, most species of bees live their lives solo. At least that’s what I’ve gathered from my own readings in biology.

  2. See this is where pure evolution models don’t add up to me. Where are all the links between this suction cup fish and its precursors? If evolutionary mutations occur in small steps, there should be a gradient of fish with no suction cups and more and more suction cup like aspects leading up to the remora. Instead we see three versions – no sucker, half sucker and full sucker. Where is the 1/4 sucker and the 3/4 sucker?

    Just a thought.

    [CZ: Most species are extinct. So we can’t see the living transitional forms in the ocean today. Instead, we have to wait for paleontologists to find them. In this post, I just described how some fossils went for years unrecognized in a museum drawer. Many species have yet to be discovered in the fossil record. Many others never left an intact fossil. Thus, the absence of a complete fossil record for remoras is not a good reason to reject the hypothesis Friedman has put forward to explain all the evidence we do have.]

  3. @dan. And when we find the1/4 sucker and the 3/4 sucker fossils, what about the 1/8 sucker, the 3/8 sucker, the 5/8 sucker and the 7/8 sucker? Where will those fossils be? etc. etc. ad infinitum until you realise how odd it is to demand every link be present before you are satisfied. How about using your intellect to “fill in the gaps” and realise that the best fit for these fossils is evolution by natural selection?

  4. Dan: your argument is one that is lampooned on the TV show Futurama, and when you think about it you’ll see why it is worth lampooning.

    Prior to this specimens we just had the outgroups (close relatives) of remoras lacking suction cups and full-on remoras with big suction cups. Now we have a stem-remora with a partial suction cup.

    In reality what we have seen is reduction in a morphological gap, and a good indication of the process by which the cup evolved. Instead, you choose to see this as going from 1 “missing link” to 2 “missing link”.

    But let’s take this further: let’s say that we have additional discoveries of the 1/4 and 3/4 cups. That reduces any morphological gap further. But you could say: look, we know have 4 gaps (0-1/4; 1/4-1/2; 1/2-3/4; 3/4-1).

    And what if we had every “0.1 cup” step represented? Would you claim that there were now 10 gaps? Or would you recognize that the sequence of transformation was even better understood?

  5. Speaking of “cups,” the concept of missing links is an archetype from the “great chain of being” that the great paleontologist Baron Georges Cuvier soundly defeated. Michael Shermer describes the error of this thinking as follows: “Creationists demand just one transitional fossil. When you give it to them, they then claim there is a gap between these two fossils and ask you to present a transitional form between these two. If you do, there are now two more gaps in the fossil record, and so on ad infinitum. Simply pointing this out refutes the argument. You can do it with cups on a table, showing how each time the gap is filled with a cup it creates two gaps, which when each is filled with a cup creates four gaps, and so on. The absurdity of the argument is visually striking.”

  6. Did something slip out in an edit? I’m guessing that flow over the structure creates negative pressure but there’s no object for “that” in, “That negative pressure pulls the remora towards its host’s body.”

  7. @BJ Nicholls
    The negative pressure is formed by the fish flexing its muscles, which pivots the bones on the suction disk, creating space between them.
    The negative pressure was explained in the previous paragraph, though not very well.

  8. The only problem I have with slow evolution (and we all know it would have had to have been slow–like millions of years) leading to the development of these changes is that for a characteristic to be passed on to the next generation, it ought to have afforded some survival benefit. I fail to see the survival benefit of a non-functioning tiny speck of the beginning of a sucker. And for how many millions of years did it just sit there, unused, not being helpful at all, but somehow miraculously being passed on to the next generation in such a way that “Remoras with bits of suckers” were allowed to breed with more gusto (were naturally selected for) over those without bits of suckers? It is somewhat easier with finches on the Galapagos islands. Longer bills get to the bugs in the crevices in trees with thicker bark, ipso facto, only longer-billed birds make it and Viola!, you have within one breeding cycle selected birds that blew out there in a hurricane to survive only if they had longer bills.
    You have to admit, from either an I.D. standpoint or a natural selection standpoint, no one has the answer to this one. I can just see the “oh, how sexy you look with that little hat!” comments to follow this one.

  9. I am rather ashamed to belong to the same species as that of several of the persons who have replied here.

    Here we have an interesting look at a unique piece of morphology, and in the comments, there’s a debate about whether or not the scientific explanation is correct, or the goddunit explanation is correct.

    Nonetheless, interesting article.

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