Origins of hitchhiking fish discovered

18 July 2013

A team from Oxford University and the Natural History Museum has found the origins of the sucker in a specialised bony fish known as the Remora, based on a 30-million-year-old fossil in the Museum’s collection.

Modern Remoras use a sucker on the top of their head to hitch a ride on marine creatures such as whales, turtles and sharks, earning them the nickname sharksuckers. The fossilised Remora, Opisthomyzon, from Switzerland, reveals how the sucker evolved.

Sharksucker fish on a green turtle

Sharksucker fish on a green turtle. © Terry Dormer/ NHM

Previous evidence, such as the segmented structure of the sucker and how it develops in a similar way to fins in normal fish, led scientists to believe it must be a modified dorsal fin – the fin on the back. But how that fin evolved into a sucker was a mystery.

The sucker on the fossil Remora is a more primitive version and reveals the evolutionary path. The disc-shaped structure has more fin features than the modern sucker. For example it’s in more of a fin-like position, behind the skull, and has spines more like those on a dorsal fin. It has fewer segments than a modern sucker, more similar to the number of spines in a fin.

This led them to conclude that the fully-functioning sucker on modern Remoras evolved from a spiny dorsal fin.

It was only later in the evolutionary history of remoras that the number of segments increased in the sucker and it migrated from the back of the body to the top of the head, where it is found in all Remoras alive today.

The findings of the research have been published in the journal Proceedings of the Royal Society B.

Sharksucker fish (genus Remora)

Remora and the unusual sucking disc that it uses to attach itself to large marine animals. © Dave Johnson

Explaining the evolution of the sucker, lead author Matt Friedman, from the Department of Earth Sciences at Oxford University, says, ‘As strange as it may seem, it evolved from a spiny fin. In this fossil the fin is clearly modified as a disc but is found on the back of the fish. It enables us to say that first fin spines on the back broadened to form wide segments of a suction disc.

‘After the disc evolved, it migrated to the skull, and it was there that individual segments became divided in two, the number of segments increased, and a row of spines developed.’

‘It’s exciting that fossil fish from the Natural History Museum played such an important role in this story, and shows the importance of our collections for scientific research,’ says Zerina Johanson, a palaeontologist at the Museum. ‘Following painstaking preparation by our fossil preparator, Mark Graham, we were able to clearly see the important features of the disc in the fossil.’

Friedman adds, ‘One of the remarkable things we've learned about modern fishes is that some creatures that look very different, for example puffer fishes and anglerfishes, are actually very closely related.

‘It’s through fossils like this one, which preserve body plans and structures that have been pruned from the evolutionary tree by extinction, that we can unravel how they diverged from one another to assume the very different forms we see today.’

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