The black-mouthed tun snail – diary of an egg mass

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.

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I’ve written about the black-mouthed tun snail – a giant sea snail – found in the lagoons on Norfolk Island before on this blog, in an article titled Citizen science in action on Norfolk Island. In that article I republish a paper co-authored with tun-snail expert Chris Vos, originally published in the Royal Belgian Society for Conchology called Gloria Maris on 31 December 2021.

I’ve been following these tuns in our lagoons with interest. They are a rare sight and I can only confidently identify two individuals of this species; however, that doesn’t mean to say there aren’t more, but this is all I’ve seen. You can find additional photos of this amazing animal on the Everything Else page of this website – keep scrolling down until you find them.

Tun snail eggs tucked under a small rock in Emily Bay

On 13 October 2022, I witnessed a mature black-mouthed tun snail in Emily Bay at around 6.30 am. About 7 or 8 metres away from the snail I found a newly laid egg mass tucked beneath a small rock (see photograph, right). As these tun snails are a rare, I think I am safe in saying that this one is the likely culprit responsible.

I decided to follow the eggs and record them as they developed. In the early photographs you can see the neat rows of capsules. You can follow the sequence as they gradually deteriorate due of wave action and algal growth. After six weeks of carefully watching (with some very poor visibility on some days, hence the gaps in photographs) we experienced some wild weather with huge surf and swells. The following day the capsules had detached and disappeared.

I’ve looked for research papers on this snail’s reproductive cycle without any luck apart from one paper on a relative, Tonna galea. The development of its eggs were recorded in captivity. These became free-swimming veligers* in the water column after 34 days. Sadly, I don’t know if any of my tun snail’s embryos were released into the water column before the mass dislodged. And I don’t even know how long they would take to get to that stage because so little research has been done.

I emailed Chris Vos to tell him of the sighting. To say we were both like excited expectant parents is an understatement, so it’s a shame the sequence came to an inconclusive end.

I don’t believe this has been recorded in this species before, and certainly not in the wild.

* A veliger is the final planktonic larval stage of some molluscs.

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September 13, 2026

No seagrass, but plenty of turtles

Norfolk Island does not have seagrass, although one of our most conspicuous green algae is often mistaken for it. Feather and grape Caulerpa can both form striking patches across the lagoon. I take a closer look at what Caulerpa actually is, why it is so easily confused with seagrass, and where our green turtles fit into the story.

September 6, 2026

Watching El Niño from Norfolk Island

El Niño is strengthening across the Pacific, but that does not tell us exactly what the coming months will bring to Norfolk Island. After a long wet spell, I am watching our rainfall and water supplies more closely again – and, underwater, keeping an eye on sea temperature, algae, coral recruits and the fish that turn up on the reef.

September 2, 2026

What the world’s reefs are telling Norfolk Island

Two major new coral reef reports carry a clear message for Norfolk Island: reefs can recover when pressures ease, but the time available for recovery is shrinking. Norfolk Islanders were raising concerns about water quality, runoff and the health of Emily and Slaughter Bays nearly forty years ago. Today we have much better science, a changing climate and a far clearer understanding of how closely the catchment and reef are connected. This article looks at what that means for water quality, long-term monitoring, dredging and how we manage Kingston from land to sea.

August 25, 2026

When the map changes

A fish once thought to belong only to Norfolk turns up elsewhere; another appears here hundreds of kilometres beyond its known range. Cardinalfishes and a Sabre squirrelfish show how our maps of species distributions are revised.

August 24, 2026

Endemic, native, undescribed – what do these words actually mean?

A Norfolk chromis that turned out not to be Chromis fumea, a blenny found nowhere else, and a tiny goby that may not yet have a name. A look at what we mean by native, endemic, undescribed and newly described – and why scientific names sometimes change.

August 16, 2026

What’s grazing in the green stuff?

A proliferation of Bryopsis on the reef has brought some beautiful little underwater grazers with it. I follow the trail from sap-sucking sea slugs and egg masses to a bigger question – why is there so much Bryopsis around at the moment?

August 8, 2026

The clam beneath Emily Bay

I’ve seen the beautiful shells of Laciolina chloroleuca in Emily Bay for decades, but I’d never really stopped to wonder where the living animals were. It turns out they may be buried half a metre beneath the sand. A closer look at an easily overlooked clam, its extraordinary shell markings, and the hidden life beneath our lagoon.

August 1, 2026

Reflections of an accidental marine scientist

What I thought would be a steady plod towards retirement, with plenty of time spent nursing grandchildren, was abruptly upended exactly six years ago today. This is the story of how an environmental catastrophe changed the course of my life – and led me to reinvent myself. It is a reflection on how, and why, I became an accidental marine scientist.

July 24, 2026

From ridge to reef: unpicking the Waterway and Wetland Strategy

Norfolk Island’s draft Waterway and Wetland Strategy will guide work for the next ten years. I support its intent, but it needs to be clear, practical and measurable – joining the dots from ridge to reef so we know whether public money is improving water quality and helping the reef.

July 13, 2026

Reef reality in pictures

Let the pictures do the talking. This is the reality on Norfolk Island’s reef today, laid out in a two-minute video.