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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November 1, 2026

When algae move in – the changing reef neighbourhood

Algae are a natural part of Norfolk Island's reef, changing with the underwater seasons. But sometimes their beautiful green growth conceals a rather sad story. In the fifth article of my six-part photographic series, I revisit familiar places to see how algae have changed over the years – and what might be hiding underneath.

October 30, 2026

The slow disappearance – disease on the reef

What happens after coral disease has run its course? In the fourth article of my six-part photographic series, I revisit colonies I've watched lose living tissue, sometimes alarmingly quickly. From a potentially century-old brain coral that lost most of its living tissue in less than 100 days to the algae-covered skeletons left years later, these photographs reveal the longer-term consequences of coral disease on Norfolk Island's reef.

October 28, 2026

When storms rearrange the reef

Sometimes the sea rearranges a reef in a matter of hours! In the third article of my six-part photographic series, I revisit corals broken, overturned or swept away by powerful swells and storm surges. Some disappeared altogether, while others found surprising ways to survive. Five photographic sequences reveal just how dramatically Norfolk Island's reef can change.

October 26, 2026

When the reef fights back – coral growth and recovery

Not every coral story ends badly! Among six years of underwater photographs are some wonderful examples of growth, survival and recovery. From corals that have grown spectacularly to others that have survived bleaching, breakage or even being turned upside down, this second article in my photographic series celebrates some of the reef's more encouraging stories.

October 24, 2026

The neighbours – six years of watching the same reef

When I began photographing Norfolk Island's reef in 2020, I simply recorded whatever caught my eye. Six years later, those photographs have become a record of individual corals and their changing fortunes – growth, competition, survival and loss. This is the first of six articles looking back at what I've seen, and sometimes what I wish I hadn't.

October 10, 2026

Pulling the rugosity from out of the reef

What do ‘rugosity’, structural complexity and channeling your inner fish have in common? Quite a lot, as it turns out. This week I’m looking at how all the lumps, bumps, branches, holes and ledges on a reef create habitat – with a brief detour into one of my long-standing tricks for navigating tertiary study: write down the jargon until it starts to make sense.

September 27, 2026

How does a sea slug smell?

It started with a close-up of a little Elysia and two rhinophores that looked like tiny rolled-up scrolls. Once I noticed them, I started going back through years of sea-slug photos and realised just how wildly different these sensory organs can be – ridged, rolled, curled, tubular and, in sea hares, distinctly rabbit-eared. So this week’s rabbit hole is all about how sea slugs ‘smell’ underwater, and why their noses come in so many different shapes.

September 19, 2026

The Elite Fleet sails in

Nearly six years ago, my very first blog post was about the strange blue and violet creatures that live at the ocean’s surface – a group I christened Norfolk Island’s ‘Elite Fleet’. Since then I’ve seen plenty of bluebottles and the occasional violet sea snail, but never a by-the-wind sailor. Until this morning. Velella velella is not a true jellyfish, but a colonial hydrozoan with a transparent sail that lets the wind help carry it across the ocean. So what exactly is this extraordinary little creature, how does it ‘sail’, and what connects it to the rest of the fleet?

September 18, 2026

In a nutshell – where I stand on the proposed Norfolk Island Marine Park

I support the proposed expansion of Norfolk Marine Park and its renaming as Norfolk Island Marine Park. There is much to welcome – strong offshore protection, no mining, and recognition of the importance of sustainable local fishing. But I also have some reservations. Why does Cascade need to be removed from the Marine Park at all? How will more than 430,000 km² actually be monitored and enforced? How do we make sure Norfolk people have a genuine role in management? And, above all, how do we ensure that the enormous new park does not draw attention away from the small, stressed reef immediately beside us?

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.