Reversible phenomena are associated with spawning and appear in the form of mating attire.

A temporary change in colouration, and sometimes shape, which has a sexual value and is used to attract the opposite sex. Mating apparel appears before mating and is more noticeable in males. For example, the male crested newt (Triton cristatus) has a serrated skin fold on its back at this time. Mating display is most commonly observed in fish. For example, the male Labrus ibex is covered with red and blue stripes during mating, which are interspersed and quite wide. The gorchak (Rhateus amarus), which is usually greyish green with silvery sides and belly, becomes brightly coloured during spawning: the body surface is tinted with all the colours of the rainbow, with steel grey and purple being particularly prominent, and the emerald green stripe on the sides is sharply defined; the abdomen is orange-yellow, and the dorsal and ventral fins are red with a black edge.

Mating apparel is a change in the appearance of fish during the spawning period, which occurs mainly in males at the time of breeding and is manifested by a change in colour (gobies, salmonids, sticklebacks, etc.), a change in the shape of the skull, skeleton (salmonids), with the appearance of white, bright horn tubercles on the head (sturgeons, salmonids) and scales (carpids). Mating colouration indicates the maturity of the gonads and the readiness of the fish to spawn.