Sex-changing fish quick to assert dominance

University of Otago – Ōtākou Whakaihu Waka scientists have discovered that it takes mere minutes for a species of sex-changing fish to develop dominant behaviour after a change in the pecking order.

The new study led by the Department of Anatomy and published on Proceedings of the Royal Society B, examines the New Zealand spotty, or paketi, a fish that can change from female to male during adulthood in response to a change in social hierarchy.

It found that the sex change process begins almost immediately when a dominant spotty is removed from a group.

Lead author Haylee Quertermous, a PhD Candidate in the Department of Anatomy, says although the full sex change process takes weeks, it only takes minutes for a second-ranked fish to take advantage of the power vacuum and assert dominant behaviours.

“The aggressive behaviours (called ‘rushes’) involved the dominant fish swimming rapidly towards subordinate individuals,” she says.

“Sometimes the dominant fish will make physical contact with the subordinates, including taking bites at them, usually around their tail and fins. These aggressive behaviors are usually accompanied by the subordinate quickly swimming away (‘escaping’) from the dominant fish.”

While she expected to be able to see behavior changes within an hour of removing the dominant fish, she was surprised by just how rapid the change could be.

“In many of the tanks, second-ranked fish increased their aggression within just a few minutes after removal of the dominant fish.”

She cautions the dominant behaviour that accompanies a female to male sex change in spotties does not indicate a change from typically ‘female’ to ‘male’ behaviour, as other sex-changing fish species such as clownfish for example, change from male to more dominant female fish.

The researchers observed that spotties form linear dominance hierarchies based on size, with larger individuals dominating smaller ones.

They sought to determine which fish in the hierarchy were more likely to change sex when the opportunity arose.

Results show dominant, larger fish are more likely to change sex, and when social hierarchies are disrupted, less dominant fish can quickly change their behavior to seize new opportunities.

The study also delved into the neural mechanisms underlying spotties’ social interactions, finding that the social decision-making network in the fish brain is highly involved in establishing dominance.

Fish that attained dominant positions showed significant differences in this network compared to fish of all other ranks.

Dr Kaj Kamstra, who led the neurobiological aspects of the research, says the findings provide valuable insights into the complex interplay between social behavior and neural processes in these fish.

“They also highlight the importance of social context in shaping individual behavior, shedding light on the evolution of social behavior and the flexibility of brain mechanisms in adapting to changing social environments.

“The research has broader implications for understanding social dynamics in other species, even humans.”

The findings can be applied to other species of sex-changing fish where social dominance appears to be the most common trigger for sex change, and could prove beneficial for aquaculture and open water fisheries, with many commercial valuable fisheries dependent on fishes that change sex, for example, New Zealand’s blue cod.