Theory regarding origins of remora's sucking disc confirmed


Scientists have confirmed a theory regarding the origins of the remora's sucking disc.

Remora have been the subject of legend, with ancient stories claiming the fish attached themselves to the hulls of boats to purposely slow the vessel down.
Although that is a misunderstanding, scientists have wondered how the sucking disc on the head of the remora develops.
Now, scientists at the Smithsonian Institution and London's Natural History Museum have solved the mystery, proving the disc is a greatly modified dorsal fin.
There are eight species of remoras, ranging from one to three feet, and they are generally found in tropical open-ocean waters.
Their sucking disc is used to attach themselves to large marine animals and they feed on scraps of food and parasites coming from the host.
Though scientists have now proved the origins of the sucking disc, the idea was not new and dates back to the 1800s.
Dave Johnson, zoologist at the Smithsonian's National Museum of Natural History and co-author of the research, said: "One reason I think this hasn't been done before is due to the difficulty in finding early stage remora larvae.
"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 or elements in the dorsal finds of another fish, the white perch, which has the typical dorsal fin of most fishes."
This enabled the team to identify which part of the fin was modified and developed into the unusual disc.
Up to a certain stage in development, the dorsal fin of both fish developed in the same way. Then, through a series of slight changes, the remora's dorsal fin began to move towards the head.
When the remora reached around 30mm in length, the dorsal fin had become a fully formed 2mm sucking disc. The disc still had the components found in the dorsal fin, fin spines, spine bases and supporting bones, however the spine bases had greatly expanded.
The research confirmed that the sucking disc is formed during the fish's development and is not the result of evolution of a completely new structure.
Scientists now have other ideas about the remora's larval stage. Johnson said: "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.
"Fodder for future research."