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How the sharksucker got its suction disc

The views expressed are those of the author and are not necessarily those of Scientific American.


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remora-sharksucker

The head of a 26.7 mm remora, or sharksucker, with its strange suction disc. Credit: Dave Johnson

“The lazy remora’s inhaling lips | Hung on the keel, retard the struggling ships” – Erasmus Darwin, 1830

Remoras, or sharksuckers, are a family of eight species of tropical fish that for more than a millennium have inspired a mythology that is almost stranger than those odd little hats they’ve got on. And now scientists have figured out where the sharksucker’s sucking discs actually came from.

Remoras use their sucking discs to latch onto pretty much anything for a free ride, whether it’s other fish, turtles, divers or ships, and so they were named accordingly – in Latin, remora means “delay”, while the scientific family name Echeneidae comes from Echeneis, which is made up of two Greek words, echein (“to hold”) and naus (“a ship”). Once the meaning ‘‘holding onto ships’’ shifted to “holding back ships’’ over time, a myth was born that was so influential, it lived in the consciousness of nearly every sailor on nearly every sea voyage in the Mediterranean and Mid-Atlantic Oceans up until a few hundred years ago.

Nurse_shark_with_remoras

Remoras stuck to a nurse shark in the Bahamas. The suction is not seen to hurt either party. Credit: Duncan Wright, Wikimedia

The myth of ship-holding sharksuckers can be traced as far back as 350 BC, when Aristotle wrote a 10-volume encyclopaedia called History of Animals, in which he very briefly mentions, “a tiny [fish], which some call the Echeneis, or ‘ship-holder’”. While Aristotle describes the remora as being somewhat of a good-luck charm – “…which is by some people used as a charm to bring luck in affairs of law and love” – by the first century AD, it had become more than a nuisance for sailors, and for one historian, an omen that even the most powerful men in the world would do well to heed.

ship-holder

Six ship-holders holding a ship. From the first Strassburg edition of the Hortus Sanitatis, 1497. Credit: E. W. Gudger

In 77–79 AD, Pliny published his enormous Naturalis Historia encyclopedia, one of the largest single texts we have left from the Roman Empire. In it, he describes the phenomenon of the ‘ship-holder’ – an eel-sized fish with a suction disc that could bring warships to a halt despite the force of violent seas, whirlwinds, and storms. And not just any warships: Pliny blamed remoras for the defeat of Mark Antony at the Battle of Actium in 31 BC and hints that they were indirectly responsible for the assassination of Gaius Caligula.

Upon returning to Rome from Gaul in 41 AD, Pliny says, Caligula’s ship stopped dead, while the rest of his fleet breezed past unfettered. Unable to make any progress despite the best efforts of his four hundred rowers, Caligula sent his men down to discover the cause:

“They found this fish sticking to the rudder and showed it to Gaius, who was furious that it had been such a thing that was keeping him back and vetoing the obedience to himself of four hundred rowers. It was agreed that what astonished him in particular was how the fish had stopped him by sticking to the outside, yet when inside the ship it had not the same power. Those who saw the fish then or afterwards say that it is like a large slug.”

This was, Pliny opined, a wonderful example of “Nature surpassing herself”.

“Bales may blow and storms may rage; this fish rules their fury, restrains their mighty strength, and brings vessels to a stop, a thing no cables can do, nor yet anchors of unmanageable weight that have been cast. It cheeks their attacks and tames the madness of the Universe with no toil of its own, not by resistance, or in any way except by adhesion. This little creature suffices in the face of all these forces to prevent vessels from moving. But armoured fleets bear aloft on their decks a rampart of towers, so that fighting may like place even at sea as from the walls of a fortress. How futile a creature is man, seeing that those rams, armed for striking with bronze and iron, can be checked and held fast by a little fish six inches long!”

The remora removed, Pliny goes on to say, Caligula was free to return home, but the little fish proved ominous, because the Emperor was soon met by the murderous blades of his own men. If only he’d gotten home sooner, perhaps?

The myth of the ship-holder survived for centuries after Pliny’s death, the first English version printed in a 1494/1495 edition of an encyclopaedia by French scholar Batholomeus Anglicus. According to American historian, E. W. Gudger, who wrote a paper in Isis in 1930, the remora’s ship-holding power was believed by fishermen of the Adriatic Sea up until around 1778. He’s collected a handful of visual representations, which were particularly popular during the Renaissance period of the 15th and 16th Century.

ship-holding remoras

The remora, having laid hold of the rudder of this ship, slows its progress even though its three large sails are distended by the gale. Camerarius, 1654. Credit: E. W. Gudger

Even more recently, when their boats began to slow inexplicably, some fishermen weren’t above pointing the finger at an unwitting remora or two. “They sure will hold a boat. I have seen ten or twelve under a boat at one time,” a fisherman from Key West in Florida told Gudger in the late 1920s. “My brother and me had boats just like each other in size and build but his was a little better sailor than mine. The first day he beat me, sailing before the wind, but the second day I beat him. He said, ‘No wonder I am losing, too many suckers hanging on her bottom’. All Key West fishermen know that suckers will sure hold a boat.”

Even if in reality, the remora’s suction force is no where near enough to anchor even the smallest of boats, its suction disc is no less remarkable to its admirers now than it was thousands of years ago. So in December last year, ichthyologists Ralf Britz from the Department of Zoology at the Natural History Museum in London and David Johnson from the Division of Fishes at the National Museum of Natural History in Washington published a paper in the December 2012 issue of the Journal of Morphology explaining just how the remora got its suction disc.

remora suction disc

The remora's suction disc, with its numerous lamellae. Credit: PacificKlaus at Flickr

The remora’s suction disc is a ribbed and elongated structure that sits on the fish’s flattened head, a varying number of thin plate-like pieces of skin called lamellae (like on the sticky toes of geckos) flanking either side of a central vane. The disc is supported by a complex series of muscles and bones that create suction by erecting and depressing the lamellae. Once attached, the remora can slide backwards to increase the pressure of the suction, or forwards to release its grip. The oval suction disc extends from just past the nostrils to beyond the pectoral fins, which are the first fins as you run your eyes along the fish head to tail.

Since the early days of comparative vertebrate anatomy in the 1820s and 1830s, scientists have hypothesised that the remora’s sucking disc was a highly modified dorsal fin – the fin that sits on the top of a fish’s body (think: shark fin). But more recently, over the last ten years or so, scientists have been disputing this. So Britz and Johnson injected red dye into the bones of larval remora and other fish from the Morone genus, which includes variations of bass and white perch, so they could watch them grow, and took snapshots as they developed through their juvenile and adult stages. This way, the researchers could see if at any point these bones behaved differently to produce a sucking disc in one fish, and a perfectly formed dorsal fin in the other.

sharksucker-dorsal

A remora with its bones stained red so the early development of the dorsal fin (A, above) is visible. Below is a close-up of the developing dorsal fin. At this stage, the remora's development is about the same as that of other fish, but later it will change to form the suction disc. Credit: Ralf Britz

Up to a certain point in the fishes’ development, the dorsal fin and supporting skeleton appeared to be developing in very much the same way in both fishes. Then, gradually over time, a series of small changes saw the dorsal fin bones expand and shift forwards towards the remora’s head, and by the time the juvenile remora had grown to 30 mm long, it had a 2 mm-long, perfectly formed sucking disc. What Britz and Johnson also found was that this suction disc was made up of all the same components as the Morone fish’s dorsal fin, including the tiny, delicate fin spines, spine base and supporting bones, but the remora’s spine base, which now acts as the base of the disc, had greatly expanded.

“The sucking disc of the sharksuckers of the family Echeneidae is one of the most remarkable and most highly modified skeletal structures among vertebrates,” the researchers wrote, having solved a problem of which Gudger had said in 1926, ‘There is no embryological problem in all the realm of ichthyology today which is so unique and offers so much of interest as the mode of formation of the sucking disc in any form belonging to the family Echeneididae [sic].’’

And oh yes, you can get your remora Pokémon and Yu-Gi-Oh cards here and here.

Order my new book, Zombie Tits, Astronaut Fish and Other Weird Animals, here.

Bec Crew About the Author: Bec Crew is a Sydney-based science writer and award-winning blogger. She is the author of 'Zombie Tits, Astronaut Fish and Other Weird Animals' (NewSouth Press). Follow on Twitter @BecCrew.

The views expressed are those of the author and are not necessarily those of Scientific American.





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