breeding

Tubercles

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Breeding tubercles (photo credit: G. Engman and P. Cooney).

Tubercles are skin nodules made of keratin, the same materials as hair, hooves, and fingernails.  They are present on species representing at least 15 families of fishes, including even pet goldfish.  In many species, tubercles are found only on males during the breeding season and are shed shortly there after.  They are often called breeding tubercles for that reason.  The main functions for tubercles include species recognition, fighting and defense of spawning territory or nests, and stimulation of breeding females.

For more information, please see “Breeding tubercles and contact organs in fishes: their occurrence, structure, and significance” (Wiley and Collette 1970) and the related Fisheries Blog post: “Top 10 Weirdest Things Found on a Fish’s Head.”

Panmictic

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Panmictic refers to a random mating strategy, frequently employed by fish, where breeding is just as likely to occur between any two individuals in a population as between any two others.  Mating in this way is not influenced by any any environmental (e.g., geographic proximity), hereditary (e.g., timing of spawning), or social interaction (e.g., polygamous mating systems).

American Eels have a panmictic population.

In a population genetics context, if a species is a panmictic population, there is no genetic evidence of population structure throughout its range.  For example, American Eels (Anguilla rostrata) are catadromous fish found from the Caribbean to Greenland.  However, they all migrate to breed in the same location in the Sargasso Sea so genetic samples from throughout their North American distribution range show a complete lack of genetic differentiation, or complete panmixia.