According to a new study conducted by ichthyologists Dr Ralf Britz of the Natural History Museum and Dr David Johnson of the Smithsonian National Museum of Natural History, the skeleton of the sharksucker fish’s disc is formed in development through the expansion of both the bases of the dorsal fin spines and the distal radial elements of the fin supports.
Sharksuckers, also called remoras, are a group of 8 species in the fish family Echeneidae. They are the only fish with a sucking disc. Their closest relatives are the cobia and dolphin fish.
Sharksuckers use the spines and suction of their sucking disc to attach themselves to large marine animals. They don’t seem to cause any harm, or benefit, to the animal they’re attached to, and they live off scraps of food, faeces or parasites from the larger animal. Some people are known to use sharksuckers to catch other fish, throwing them into the sea attached to a fishing line and pulling them in once they are attached to a larger sea animal.
The ichthyologists investigated how the sucking disc develops in larval fish of the genus Remora. They took snapshots of the developmental stages, staining the bones red to see changes more clearly.
To see whether the sucking disc is created from the existing dorsal fin that most other fish have, they looked at how the dorsal fin developed in the same early stages in another fish, of the genus Morone (temperate basses), which has a typical dorsal fin as an adult consisting of spinous and soft dorsal fin parts. They compared the two.
Such comparisons help to establish whether structures in different species have the same evolutionary origin despite looking and functioning differently. Up to a certain stage in the fishes development, the dorsal fin can be seen developing in the same way and looking very similar in both fishes. Then, over a series of small changes, the dorsal fin in the Remora begins to expand and shift towards the head.
By the time the Remora has reached around 30 mm in length, the dorsal fin has become a fully formed 2 mm 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. So, the sucking disc is formed by a massive expansion of the dorsal fin through small changes while the fish is developing. It is not the result of the evolution of a completely new structure.
“What keeps impressing me when I study the development of some of the weirdest structures in the fish world is that natura non facit saltus, ‘nature does not make jumps,’ and even the strangest anatomical modifications happen through small gradual changes in development,” said Dr Britz, who reported the findings in the Journal of Morphology.
Bibliographic information: Ralf Britz, G. David Johnson. 2012. Ontogeny and homology of the skeletal elements that form the sucking disc of remoras (Teleostei, Echeneoidei, Echeneidae). Journal of Morphology, vol. 273, no. 12; doi: 10.1002/jmor.20105