17 Nov 2022

Sunfish secrets

From Our Changing World, 5:00 am on 17 November 2022

When you first see a sunfish – also known as a mola – your first question might be: how on earth can that thing swim? 

Dr Marianne Nyegaard diving with a large bump-head sunfish.

Dr Marianne Nyegaard diving with a bump-head sunfish (Mola alexandrini). Photo: Supplied / Auckland Museum

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In fact, sunfish are pretty good swimmers, according to Dr Marianne Nyegaard, research associate at the Auckland Museum. They’re also impressive in stature and are the world’s largest bony fish, weighing up to 2.7 tonnes and measuring more than 3m long. 

Yet, these hefty ocean-dwellers often end up washed up on beaches around the world, with no signs of illness or injury. 

That’s exactly what happened to a young sunfish I found in May this year, it was dead and stranded on a tidal mudflat about an hour and a half north of Auckland. 

A sunfish, about the size of a steering wheel, lying on gravel.

The sunfish specimen found washed up north of Auckland. Photo: RNZ / Ellen Rykers

The specimen was fresh (no fishy smells here) and its good condition meant the Auckland War Memorial Museum were keen to add it to their collection. 

I joined Marianne in a lab at the museum to inspect the juvenile sunfish and try to uncover its scientific secrets. 

Marianne is smiling and wearing a navy blue lab coat in front of a fume hood in a lab. She is holding a small wrinkly sunfish

Dr Marianne Nyegaard holds a small Mola alexandrini specimen in the lab at the Auckland Museum. Photo: RNZ / Ellen Rykers

Until recently, there were two known Mola species: Mola mola, the common ocean sunfish, and Mola alexandrini, the bump-head or giant sunfish.  

But during Marianne’s PhD, she decided to follow up on genetic clues that indicated a mysterious third species might be lurking in the waters around Australia and New Zealand. 

Her worldwide detective hunt led her to Birdlings Flat near Christchurch, where a sunfish with a peculiar-looking backside had washed up, and to the foyer of the Otago Museum, where a huge sunfish specimen has been on display for the last 50-odd years. 

Marianne’s efforts resulted in the identification of a new species, Mola tecta, or the hoodwinker sunfish. It had been hiding in plain sight for 125 years. 

Marianne Nyegaard is kneeling next to a large dead sunfish on a dark-sand beach. She is smiling and holding a machete which she has used to make an incision in the sunfish's belly.

Marianne dissecting the hoodwinker sunfish (Mola tecta). Photo: Supplied

But there are plenty more sunfish secrets to unravel – such as, how do they grow from teeny larva that could fit on your thumbnail, into multitonne giants the size of a car? What do they eat? And how can you tell the three species apart when they’re young? 

Marianne is wearing a lab coat, gloves and glasses as she leans over the sunfish specimen on the lab bench and pries open an incision with callipers.

Marianne inspecting the gonad (sex organ) of the sunfish specimen. Photo: RNZ / Ellen Rykers

That’s where the sunfish I found comes in – as a juvenile Mola alexandrini, this specimen is one puzzle piece in the ongoing Mola mystery. 

Listen to the episode to dive into the weird world of sunfish and hear what Marianne’s investigations of the mini-mola revealed. 

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