Remora Sucking Disc Actually Modified Dorsal Fin
June 7, 2013

Remora’s Sucking Disc Descended From Dorsal Fin

Brett Smith for — Your Universe Online

Remora fish have fascinated men since ancient times, when they would use the sucking disc on top of their heads to attach themselves to the hulls of boats. Our ancestors mistakenly believed the fish´s intent was to maliciously slow the boat down.

Our knowledge of these strange-looking fish has come a long way, as scientists at the Smithsonian Institution and London's Natural History Museum have just reported that the remora´s signature disc is actually a greatly modified dorsal fin, according to their research published in the Journal of Morphology.

The eight known species of remoras use their sucking disc to attach to large marine animals, such as sharks or turtles, and take in scraps of food and parasites that drift off the larger animal.

Biologists have theorized that the sucker disc is developed from the dorsal fin since the early 1800s, but didn´t have a method for conclusively proving it. In the latest study, US and UK marine biologists decided to look at the disc´s development from the fish´s earliest larval stages.

"One reason I think this hasn't been done before is due to the difficulty in finding early stage remora larvae" said study co-author Dave Johnson, zoologist at the Smithsonian's National Museum of Natural History.

"In our study we closely tracked the development of the sucking disc beginning with tiny remora larvae, through to juvenile and adult remoras,” Johnson said. “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."

In comparing the two species, the team was able to identify three fin elements -- the distal radials, the proximal-middle radials, and the fin spines -- are radically modified in developing remoras and eventually become the different components of the fish´s odd sucker disc.

At first, the remora´s ℠dorsal fin´ can be seen developing in the same way that it does in the white perch. After a series of small changes during development, the remora's dorsal fin begins to grow and reallocate towards the head. When the remora has reached about 1.2 inches in length, it has a fully formed two-millimeter-long sucking disc. The disc still retains the components found in an unmodified dorsal fin, but the spine bases have expanded significantly.

According to the scientists, remora larvae have very unique hooked teeth protruding from the lower jaw before developing their signature disc.

"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,” he noted.

Typically found in the open ocean, some remora species can grow up to three feet long. The relationship between a remora and its host is called commensalism, since the host appears to lose nothing by being involved in the relationship.