The form, function and phylogenetic significance of the vomero-palatine organ in cyprinid fishes

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Abstract
The vomero-palatine organ is a peculiar fleshy structure which occurs on the buccopharyngeal roof of 15-20% of cyprinid fish species. Researchers have advanced 3 main hypotheses for the function of the organ: respiratory, sensory and trophic. From a study of comparative biology a trophic function is most likely, but there are points in favor of the 2 remaining ideas. The organ is probably primarily concerned in bolus-formation by the active co-mixing of precipitating mucous secretions with ingested particles (food and sediment). Comparative studies also show the organ to be of taxonomic use. It is a shared specialization (synapomorphy) which unites the labeine group it characterizes. The vomero-palatine organ is found in a regressed state in rheophilic algae-scraping species of Labeo and Garra. Using standard cladistic arguments and ecological data on feeding habits, this regression is probably a derived condition (autapomorphy) which has evolved independently in these 2 genera. The fact that the vomero-palatine organ is restricted to a large monophyletic group of tropical-Old World cyprinids is of biogeographic significance.