Odd Sucking Disc of Remora Fish Developed From Dorsal Fin

Posted on June 7, 2013

Sucking Disc on Remora Fish

Remora fish are 1 to 3 feet long and are known for having a very odd sucking disc on top of their heads. They sometimes attach themselves to the hulls of boats. They even try to attach themselves to divers. Remoras mainly use their sucking disc to attach themselves to large marine animals and feed off of scraps of food and parasites from the bigger animal.

Smithsonian scientists say that in ancient times when remora fish attached themselves to ships they were thought to be purposely trying to slow the boat down. Something else not well understood about the remora has been the origins of its odd sucking disc. Scientists at the Smithsonian Institution and London's Natural History Museum have finally solved the mystery. They say the sucking disc is actually a greatly modified dorsal fin. The research is published here in the Journal of Morphology.

Dave Johnson, zoologist at the Smithsonian's National Museum of Natural History and co-author of the research, said in a statement, "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 of elements of the dorsal fins of another fish, the white perch (Morone americana), which has the typical dorsal fin of most fishes."

The team identified three main fin elements that are radically modified and develop into the different elements of the remora's unusual disc. Up to a certain stage in the remora's development it has a developing dorsal fin that is similar to other 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 a regular dorsal fin - tiny fin spines, spine bases and supporting bones - but the spine bases have greatly expanded. This research confirms that the sucking disc is formed by a massive expansion of the dorsal fin through small changes while the fish is developing.

The scientists say the sucking disc of the remora fish is not the result of the evolution of a completely new structure.

Remora Fish

Photo: Dave Johnson/Smithsonian (top)/NOAA (bottom)

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