Scientists have discovered that the dominant males of Indonesian fish species turn black during periods of increased aggression. Their black spots, which appear quickly at the beginning of a conflict, appear to be a signal of dominance to other fish.

Researchers published a post on the preprint repository BioRxiv on December 24, 2023, of male Celebes medaka (Oryzias celebensis) those with these distinctive black markings are more likely to attack and less likely to be attacked. The signs appeared within a minute of the start of the conflict.

To test the conditions under which the men fell into darkness, the researchers set up three tanks. Two tanks were covered in algae; one had two males and a female, the other had three males. In the third aquarium there was no algae, two males and one female lived in it.

The fish attacked each other in both algae-covered tanks but not in the other tank; This suggests that some protection may be necessary for them to feel safe enough to attack.

Dark-marked males focused their attacks on other dominant males with similar markings, but they also attacked females and males without dark markings. They did attack males and females without dark markings, although males without dark markings rarely attacked males with dark markings.

Females focused their attacks mostly on males without blackheads. Males with dark markings attacked females more often. Attacks on females suggest that dominance displays may be related to access to territory and resources rather than mating rights.

“I don’t think there’s any reason to expect signals based on color change to be any more sex-specific than other forms of communication, which then raises the question of whether aggression itself, and therefore the communication of aggression in general, is sex-specific,” biologist Robert Heathcote of the University of Oxford told LiveScience. Heathcote, who was not involved in the research, studied color signals in fish, he told .com.

The authors suggest that these dark spots may indicate conformity, meaning that unmarked men and women are less likely to engage in protracted arguments where they are less likely to win. Because of this warning signal, dominant males also have to spend less energy defending their resources.

Although the researchers have not analyzed the mechanisms behind the color change, similar changes in other fish species are mediated by neurotransmitters acting on cells containing pigment granules. The color becomes more intense when the pigment concentrates in the cells. As it disperses, its color becomes lighter. In the case of medaka, the cells are likely to be melanophores containing dark-colored granules known as melanosomes.

This color change phenomenon is also seen in other fish species. In Trinidadian guppies (Poecilia reticulata), the eyes of dominant fish turn black as they prepare to attack rivals.

“We know that adrenaline often stimulates melanophores, but not always in the direction of dominant animals becoming darker,” Heathcote said. “In fact, the opposite is true for most fish; frightened individuals often darken greatly, especially when hiding, which may be due to camouflage against subsequent attacks.”

In oscars (Astronotus ocellatus), the appearance of black stripes indicates submission, not dominance. Similarly, in juvenile Atlantic salmon (Salmo salar), the whites of the eyes of subdominant specimens darken under experimental conditions.

“The rate of color change is controlled by many factors, but perhaps the most important is whether it is controlled by direct neural stimulation, hormonally or by the actual production of new pigments,” Heathcote said. “In fish and cephalopods, cells often have direct nerve innervation,” which means their color can change within seconds.