Nuptial tubercles

These tubercles are dermal structures present in 15 families of fish belonging to 4 orders of Actinopterygii – Salmoniformes, Gonorhynchiformes, Cypriniformes and Perciformes[1][2] are used in the courtship and reproduction process.

[4][5] They normally form on the heads of male fish, often covering the whole of the top part of the snout, but may also occur on fins, or anywhere else on the scaled integumentary system.

There are many theories about their function, which include: use to stimulate females during courtship, in aggressive rituals performed by males during mate selection, to maintain contact between individuals during reproduction,[3] or in defence of the nest from predators.

There is definite proof that they grow mainly on areas with greater mechanical stress,[4] which means that those species that head-butt females to stimulate them to release eggs will have more on the head, whereas those that dig nests in gravel substrates will develop them more on their fins.

Nuptial pad - a similar ornament of frogs Stephanie C. McMillan; Zhe T. Xu; Jing Zhang; Cathleen Teh; Vladimir Korzh; Vance L. Trudeau; Marie-Andrée Akimenko (2013).