The awesome, giant, black-mouthed tun snail

Out on A Swim

‘Out on a swim’ is a coral reef blog that tells the stories of the characters who live under the waves and what has caught my eye when ‘out on a swim’ in the lagoons of Norfolk Island. It is also a record of the difficulties Norfolk Island’s reef faces, like many others around the world, as a result of the poor water quality that has been allowed to flow onto it.

This page shows the most recent blog posts. For the complete catalogue, visit the ‘Out on a swim index’ page.

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Day 3 – March focus on Norfolk Island’s reef

The more usual view of a tun snail during daylight hours

Today, on day 3 of my March focus, I wanted to feature one of my favourite animals inside our lagoon – the giant (the size of a small beach ball) mollusc that goes by the scientific name Tonna melanostoma, or the black-mouthed tun. When I took the first photographs of this live animal and posted them on iNaturalist, I had no idea that there were no other – as in none, zip, nada – images of it in the public domain; just a couple of photographs of a broken shell housed in the Auckland Museum.

I'll just repeat that because it is important: there were no other photographs of the live animal anywhere in the world available in the public domain.

Back in the late 1990s I used to see these guys quite regularly, or at least the tops of the shells poking out of the sand during daylight hours (they are generally nocturnal). These days it is quite an exciting event to see one. Just over a year ago, we went through a period when we (as in, other snorkellers and myself) saw a few dead, empty shells. Now, although I can’t be certain, judging by the photographs I have taken over time, I think we are down to just a couple of live individuals of the black-mouthed tun.

There are two other species of Tonna – the tankervillii and perdix. Again, these are rarely seen in our lagoons today.

I have written a couple of blog posts about the black-mouthed tun, including a co-written a scientific paper published in the Royal Belgian Conchology Society's publication Gloria Maris. You can find these, here:

Sep 7, 2025

Celebrating Biodiversity Month on Norfolk Island

September is Biodiversity Month – the perfect time to celebrate the astonishing variety of life on Norfolk Island’s reef. From new fish sightings to coral mosaics, every observation is a reminder of how much there is still to learn and protect.

Read more about why biodiversity matters, globally and right here in our lagoon.

Aug 24, 2025

The fate of a coral colony when it succumbs to white syndrome – four years on

I’ve tracked one plating Acropora coral from 2021 to 2025. In just a few weeks, white syndrome wiped it out. Nearly four years years on, it’s still smothered in algae and sea squirts, with only the tiniest hint of new growth. It’s a stark reminder: without tackling the root cause, we’re just watching the same sad story repeat itself.

Aug 17, 2025

The Candy-Striped Cleaner Keeping the Reef Healthy

Candy-cane stripes, long white feelers, and a reef spa on offer – the banded coral shrimp waves its antennae to advertise cleaning services to passing fish.

Aug 10, 2025

Biomimicry: How a Boxfish Caught Mercedes Benz’s Eye

Meet Mr Lemonhead – our lagoon’s teeny yellow boxfish with a big design legacy. He inspired a Mercedes Benz concept car, proving how nature is full of surprises. And he shares the lagoon with other critters whose tricks have also shaped real-world inventions.

Aug 3, 2025

Patchwork Corals: How Colonies Fuse to Form Living Mosaics

Some corals wear more than one colour for a reason. When Paragoniastrea australensis colonies fuse early in life, they form living mosaics. A beautiful reminder of coral cooperation on Norfolk Island’s reef.

Jul 28, 2025

Reef relief

Today, 28 July, is World Nature Conservation Day. After the dry 2024, Norfolk Island’s reef is looking healthier – a brief reprieve as less water - laden with nutrients - flowed into the lagoon. These photos show what’s possible. It’s a reminder that recovery is within reach – though renewed runoff could quickly undo the gains.

Jul 20, 2025

Emily Bay's big 'brain' coral

In Emily Bay, Norfolk Island, a single coral bommie – Paragoniastrea australensis – has stood for decades as a micro-reef, harbouring diverse marine life and local memories. Once photographed in 1988 and still thriving today, it remains a keystone of reef biodiversity and a living link between past and present.

Jul 14, 2025

Biodiversity matters

Over five and a half years of snorkelling Norfolk’s lagoon, we’ve documented 23 fish species not previously recorded in this area. Some are local ghosts, others climate migrants. These observations help us understand and protect what makes our reef so special.

Jun 17, 2025

Poop power

Not all poop on a reef is bad poop. In fact some kinds of poop can be a reef’s most important invisible engine. Fish poop, bird poop – even poop that gets eaten again by other fish – all of it keeps the ecosystem ticking over in a way that’s nothing short of extraordinary.

Jun 13, 2025

Glimpses of recovery: what the reef could be if we let it

Day 6 of this photo series from Norfolk Island coincides with the final day of the UN Ocean Conference in Nice. After a week of documenting decline, today’s post offers a different view – what reef recovery can look like when conditions improve. Drought in 2024 gave the reef a break, and the results were unmistakable: healthier corals, lower disease, and more fish. This is what’s possible if we act.