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How does the Remora develop its sucker?

Posted by John Jackson on Nov 30, 2012 9:54:14 AM

Ralf Britz and his Smithsonian colleague David Johnson have published a paper in the Journal of Morphology on the development of the sucking disc of remoras. Remoras are a group of marine fish that usually attach themselves to sharks or other large fish such as manta rays with their sucking disc.  This lifestyle appears not to harm the shark, nor does it bring any benefit: depending on the species of remora, they eat fragments of the larger fish's food that fall from its mouth;  faeces; or the larger fish's parasites.

 

Echeneis NaturalHistoryMuseum_PictureLibrary_009079_Comp.jpgEcheneis naucrates - watercolour painting by Sydney Parkinson made during Captain Cook's first voyage 1768-1771

 

Ralf's work on the sucker involved examination and comparison of fins of different species of fish to identify the homology of its components - homology is the term used to describe organs in two species that have the same evolutionary origin, despite sometimes different appearance and function (so the human arm and a bat's wing are homologous).  The remora's sucker is not found in other fish - is it a totally new organ, or is it a highly modified version of an organ found in other fish?

 

By studying the development of larval remoras ranging from 9.3 to 26.7 mm in length, they demonstrated that the skeleton of the sucking disc forms by enormous expansion of the dorsal fin supports and the bases of the associated fin spines. The evolution of a sucking disc from a regular spinous dorsal fin seems like a major step in evolution but is actually a gradual process involving small incremental changes of structures during development.


Britz, R. & G. D. Johnson. 2012. Ontogeny and homology of the skeletal elements that form the sucking disc of remoras (Teleostei, Echeneoidei, Echeneidae). Journal of Morphology, 273 (12) 1353-1366 , DOI: 10.1002/jmor.20063


Ralf has also published a paper with a Brazilian colleague, Mônica Toledo-Piza, analysing the egg surface structure of the poorly known and highly venomous freshwater toadfish Thalassophryne amazonica with the NHM's scanning electron microscopes (SEM). Eggs of this fish show a highly unusual and complex system of ridges and intermittent grooves that originate at the equator of the egg and run toward the animal egg pole and end in a spiraling pattern at the micropyle (the only opening for sperm to enter). This striking modification may help to increase the chances of eggs being fertilized.

Britz, R. & M. Toledo-Piza. 2012. Egg surface structure of the freshwater toadfish Thalassophryne amazonica (Teleostei: Batrachoididae) with information on its distribution and natural habitat. Neotropical Ichthyology, 10: 593-599.


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