Origins Of Remora Fish Sucking Disc Confirmed By New Research

Those that have seen remora fish before, have likely wondered — how exactly did such a strange feature as the sucking disc on the top of their head come to be? And now, new research from the Smithsonian Institution and London’s Natural History Museum may have finally shed some light on this — the disc is simply a very modified dorsal fin.

Image Credit: Dave Johnson
Image Credit: Dave Johnson

The eight species of remoras currently in the world utilize the “sucking disc” to attach themselves to large marine animals, and also boats. They do so in order to feed off the scraps left behind by their host animal, as well as eating the larger animals parasites. Thy typically are between 1-3 feet in length.

It’s been theorized for some time that the sucker disc is a modified form of the dorsal fin — put forward at least as far back the early 1800s. But there hasn’t been much research done to verify or refute the theory, until now. The new research was done by observing the formation of the sucking disc as the species develops — from a larvae into a full-grown adult.

“One reason I think this hasn’t been done before is due to the difficulty in finding early stage remora larvae” said Dave Johnson, zoologist at the Smithsonian’s National Museum of Natural History and co-author of the research. “In our study we closely tracked the development of the sucking disc beginning with tiny remora larvae, through to juvenile and adult remoras. We followed the earliest stages of the disc’s development by matching the first vestiges of elements of the sucking disc with the first vestiges of elements of the dorsal fins of another fish, the white perch (Morone americana), which has the typical dorsal fin of most fishes.”

This was done so that the researchers would be “able to identify which of three main fin elements — the distal radials, the proximal-middle radials, and the fin spines — are radically modified and develop into the different elements of the remora’s unusual disc.”

“Up to a certain stage in the fish’s development, the dorsal fin can be seen developing in the same way and looking very similar in both fishes. Then, through a series of small changes, the remora’s dorsal fin begins to expand and shift towards the head. By the time the remora has reached about 30mm in length, the dorsal fin has become a fully formed 2mm sucking disc. It still has the components found in the dorsal fin, the tiny fin spines, spine bases and supporting bones, but the spine bases have greatly expanded.”

Thus confirming origin of the sucking disc as being a massively modified and expanded form of the dorsal fin — not a structure which evolved entirely from scratch.

Very interestingly, before the sucking disc develops, remora larvae possess rather strange and very distinctive hooked teeth, which protrude from their lower jaw. “Because remora larvae at this stage are relatively rare in plankton collections, I have often wondered, although we don’t have any evidence for it, if maybe remora larvae are not free living in the plankton layer but go into the gill cavities of fishes and use their hooked teeth to hang on until they develop a disc,” said Johnson. “Fodder for future research.”

The new research was recently published in the Journal of Morphology.

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