Sex changing fish waste no time claiming top spot in hierarchy

Power is not just about strength. Sometimes, it’s about who dares to act fast. In New Zealand’s coastal waters, the spotty wrasse (Notolabrus celidotus) demonstrates this with an almost cinematic twist. This small fish, capable of rapid sex change, has become the center of a remarkable scientific discovery.

New research from Ōtākou Whakaihu Waka has revealed that when social order among these fish collapses, biology and behavior shift almost instantly.

The study, published in Proceedings of the Royal Society B, examines what happens when the top fish in the hierarchy disappears. What unfolds is a fascinating sequence of behavioral dominance and brain transformation that begins within minutes.

The findings offer fresh insight into how living creatures manage power, identity, and sex. And in the case of the spotty wrasse, these things are deeply connected.

Fish size drives sex change

The spotty wrasse is a protogynous species. This means that younger individuals are female and they become males as they grow larger.

The study tracked how this transition is linked to changes in dominance rank. Larger fish quickly climbed to the top and began behaving more aggressively.

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

The study confirmed a key prediction of the size-advantage model. Size and dominance are tightly linked. Bigger fish showed more aggressive behaviors and had higher dominance scores. Smaller ones showed submissive behaviors and avoided confrontations.

Second-in-line fish take charge

The researchers formed 11 groups of five female-looking initial phase (IP) fish. They ranked them by watching behaviors like “rushing” (aggression) and “escaping” (submission).

After 11 days, the experts removed the top-ranked fish from some groups and observed the fallout. The second-in-line often stepped up immediately. Their aggression spiked, while submission dropped.

This shift took less than 30 minutes. The removed fish in control groups were reintroduced right away. In those tanks, behavior remained mostly stable, except for slight agitation in the dominant fish.

Fish sex change starts internally

In seven of the 11 experimental tanks, the researchers found that the dominant fish had already begun the internal process of changing sex from female to male.

Although these individuals still appeared female on the outside, closer examination through histological analysis revealed that their gonads were undergoing masculinization.

This internal shift in reproductive tissue occurred even before any visible signs of sex change had emerged.

The findings strongly suggest that the fish’s behavior and brain activity begin transforming well before the body outwardly reflects the transition.

In other words, the social and neurological signals that initiate sex change are set in motion early, preceding the physical transformation that follows.

The brain reacts fast

The researchers studied brain activity using pS6 immunostaining. This technique highlights recent neural activity.

The experts found that 9 out of 10 brain regions involved in social behavior were more active in the newly dominant fish (rank 2) compared to lower-ranked ones.

These brain areas included the preoptic area (POA), periaqueductal gray (PAG), and parts of the telencephalon.

The only region that showed reduced activity in dominant fish was the anterior tuberal nucleus (aTn). This could reflect reduced defensive behavior.

Brain signals and power shift

Principal component analysis showed a strong link between behavioral changes and neural activity.

Aggression increased in tandem with higher brain activation, especially in the social behavior network. The aTn, in contrast, had strong negative correlations with most other brain areas.

Rank 2 fish formed a distinct group in brain-behavior plots. This cluster separated them from both lower-ranked fish and their former selves before dominance.

The researchers suggest this rapid shift in brain activity helps kick-start the complex process of sex change.

Why fish sex change matters

This study offers rare insights into how social disruption influences sex and power. Quertermous and her colleagues demonstrate that the brain’s decision-making circuits adapt almost instantly to social change.

“They also highlight the importance of social context in shaping individual behavior… even humans,” noted Dr. Kaj Kamstra.

These insights may help manage fisheries with sex-changing species. In New Zealand, for instance, the blue cod fishery could benefit from understanding how fish social structures respond to environmental changes.

Spotty wrasse do not just switch sex over weeks. They start rewiring their brains and shifting behavior within minutes.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences.

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