The paketi: NZ’s spotty fish species that can change its sex

Paketi can change sex within an hour, a study reveals. Photo / Sascha Schulz (CC BY-NC)

If you have ever dropped a line off a Northland wharf and reeled in a spotty wrasse / paketi, you may have caught a fish in the middle of a dramatic transformation.

Research has found that this common coastal species can switch sex within an hour in response to social change, such as a male being removed from its environment.

Researchers from the University of Otago, University of Waikato, Deakin University, Lincoln University and Macquarie University found there was increased activation in nearly all areas of the fish brain, especially social decision-making networks.

Professor Neil Gemmell, who was part of the research funded by a Marsden Fund grant, said he has been fascinated by the paketi since childhood, when he used to catch them and read about their unique biology.

He said once the sex change is complete, researchers believe it is irreversible.

Within a few hours behaviour alters. The female exhibits more aggressive, male-like behaviours. Then within days to weeks the female gonad is destroyed, a new testis formed, the brain rewires, and other structural changes occur

“Not quite the Rocky Horror show where ‘in just seven days I can make you a man’, but not far off.”

Researchers from Otago and Waikato University said one of the big questions they are trying to answer is what exactly happens in the brain during the transition.

“We do know that the cue for sex change is loss of the dominant male from the social group, and that it would appear that this is a predominantly visual cue - if the female can no longer see the male she will change sex,” they wrote in a joint statement.

“Exactly what is happening in the brain we are still working on but we know that it involves the same hormones that control stress, reproduction and social behaviours in humans.”

They said understanding sex change was important for managing species that do change sex in their lives.

Karen Middlemiss, Department of Conservation senior science adviser - marine, said social dynamics in fish influence broader marine biodiversity or ecosystem health.

“This kind of sexual plasticity is surprisingly common in marine fish and highlights the complex and adaptable nature of marine ecosystems.

“Structured social dynamics like this can help stabilise fish populations and contribute to overall ecosystem health.”

Middlemiss said knowing and understanding fish hierarchies helped in understanding what habitats and environmental conditions are important for them to thrive.

She said while paketi themselves are not a protected species and are common around our coastline, they serve as an excellent study species to better understand the complexity of marine life.

The paketi was voted Te Ika o te Tau | Fish of the Year last year.

Marine reserves, especially in regions like Northland, offer valuable opportunities to study fish populations that are relatively uninhibited by human activity. This gives scientists a clearer picture of natural behaviours and population dynamics, which can then be compared with fished areas to inform wider ecosystem management.