Color-changing fish turns black with rage when provoked

Dominant males of an Indonesian species of fish turn black during periods of heightened aggression, scientists have discovered. Their blackened markings, which quickly emerge at the start of a coNFLict, appear to serve as a signal of dominance to other fish. 

Celebes medaka (Oryzias celebensis) males with these distinctive black markings were more likely to attack and less likely to be attacked, researchers reported on Dec. 24, 2023 in the preprint repository BioRxiv. The markings appeared within a minute of the onset of a coNFLict.

To test the conditions under which males darkened in color, the researchers set up three tanks. Two tanks were covered with algae — one of which contained two males and a female, and the other three males. The third tank contained no algae and housed two males and a female. 

The fish attacked each other in both algae-covered tanks but not in the other tank, suggesting that some level of cover might be required for them to feel secure enough to launch an attack.

Males with dark markings concentrated their attacks on other dominant males with similar markings, but they also attacked females and males without dark markings. While males without dark markings rarely attacked males with dark markings, they did attack other males without dark markings and females. 

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Females mostly concentrated their attacks on males without dark markings. Males with dark markings were more likely to attack females.