Sunfish are the world's largest bony fish species - and yet scientists know little about their lives. This week, Our Changing World meets a sunfish researcher unravelling mola mysteries and dives into the weird world of sunfishes as a museum specimen is examined and prepared.

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

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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.

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.

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.

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?

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.

To learn more:

Check out Marianne's Ocean Sunfish Research Facebook page …

Go to this episode on rnz.co.nz for more details