If evolution is true, one should be able to trace the origin of novel structures from their predecessors, with every step of this evolution being adaptive. If creationism were true, one would never or rarely be able to trace the evolutionary stages of such novelties. Well, we know that evolution is true (if you don’t accept that, go over to your own site, Answers in Genesis), but it’s exciting to see traits whose evolution wasn’t fully understood get this kind of clarification.
One of these traits is the sucker of the remora, and I’ve shamelessly stolen much of this post from Carl Zimmer’s explication over at the Loom, cleverly called “What good is half a sucker?“
Remoras, or suckerfishes (there are several species in the family Echeneidae), are known for a disk-shaped sucker on top of their heads, which they use to fasten themselves to larger sea creatures like sharks, tuna, manta rays, and sea turtles. (Fisherman also use this as a way to pull in sea turtles: they toss a remora on a rope into the water, wait for it to attach to the shell, and reel in the “bait” and the reptile.) In this interspecies association, the host is either unharmed or perhaps slightly slowed by the affixed remora) while the remora benefits by getting a free ride and some scraps from the host’s meals.
Here’s the weird head sucker, with the photo taken from a new paper by Ralph Britz and G. David Johnson in J. Morphology (well, actually December, 2012; only abstract is free online):
As Zimmer notes, this is a remarkable adaptation:
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.
But where did this thing come from? Earlier biologists suggested that it evolved from the dorsal fin, and now this suggestion is pretty much confirmed by the studies of Britz and Johnson, who did detailed morphological and developmental studies of the sucker and found that several parts of it are “homologous” (that is, have the same genetic and developmental origin) as the dorsal fin of its relatives. Here’s what they say in their last paragraph:
The sucking disc of remoras is one of the most unusual skeletal formations among vertebrates. Even though highly derived in its adult anatomical structure, homology of its constituent parts can be established in a clear and straightforward way by studying the development of the disc. The earliest stages of disc formation allow for the unambiguous recognition of a one to one relationship between interneural spine, intercalary bone, and pectinated lamella, with the proximal-middle radial, distal radial and basally expanded fin spine of other acanthomorphs. Our study highlights again the unique power of ontogenetic investigations, as previously demonstrated in a series of papers that have resolved long-standing issues of homology (Britz and Johnson, 2002; Johnson and Britz, 2005; Hoffmann and Britz, 2006).
One can then hypothesize that an earlier ancestor had a dorsal fin that was could be used to snag a larger fish, and then, over time, this fin would become more sucker-like. Such an ancestor (or one of its relatives) could exist in the fossil record, although of course the patchiness of that record doesn’t guarantee we’d find it. But someone did: Matt Friedman and a group of colleagues, and their report, as Zimmer notes, has just been published in the Proceedings of the Royal Society of London (B). Friedman et al. studied three fossil remoras (about 30 million years old) in the genus Opisthomyzon, and found clear homology between elements of their early suckers and bones of the dorsal fin. The sucker had not yet moved all the way up to the head, and the bones were intermediate between the bones of a fossil relative without a sucker, and the modern remora with a full sucker on the head. Here are two pictures of one fossil showing the intermediate location and shape of the fossil sucker:
A close-up of the adhesion disk (area highlighted in white box above):
Zimmer draws out this scenario:
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.
Lest I steal too much of Carl’s post, I’ll let you go over to his piece to see the intermediate condition of the bones in this fossil remora, again supporting their origin from a dorsal fin.
So what we have here is a mystery, a hypothesis, a proposed solution based on morphology and embryology, and then verification of that hypothesis based on a fossil find. What better evidence for evolution could we have—though we hardly need one more pebble of evidence on top of the Everest that already exists!
And, my poem in honor of these discoveries:
There are suckers born quite frequently
In evolution’s battle;
Some evolve upon the heads of fish,
And others in Seattle.
Britz, R., and G. D. Johnson. 2012. Ontogeny and homology of the skeletal elements that form the sucking disc of remoras (Teleostei, Echeneoidei, Echeneidae). J. Morphology 273:1353-1366.
Friedman, M., Z. Johanson, R. C. Harrington, T. J. Near, and M. R. Graham. 2013. An early fossil remora (Echinoidea) reveals the early fossil assembly of the adhesion disc. Proc. Roy. Soc. B, vol. 280 published 17 July 2013 doi: 10.1098/rspb.2013.1200.