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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Dec 8, 2025

Signs of bleaching – 8 December 2025

I took these photographs this morning, Monday, 8 December 2025. A few warm days of settled weather, little cloud cover and low tides in the hottest part of the day have led to some early bleaching on our reef. Bleaching doesn’t always mean death for our corals, but it is concerning to have this so early in the summer season. Fingers crossed the conditions don’t last and the reef can recover.

Dec 3, 2025

Nature is my teacher

This is a thank-you note. Five years after my first Out on a swim post – written with zero marine science quals and a head full of questions – I’m still in the water, now as a PhD candidate, because an extraordinary mix of locals, volunteers, researchers and public servants decided to share what they knew. This is the story of how nature – and a very patient community – became my teachers.

Nov 30, 2025

Reef grief: what dredging has done to other reefs

From Miami to Fiji, from Dubai to tiny village harbours on atolls, dredging near coral reefs has left a long trail of scars – even on ‘small’ projects. This follow-up to last week’s Kingston post walks through real examples of what happened elsewhere, and what that should make us think about before we dig up our own reef.

Nov 20, 2025

To dredge or not to dredge? The Kingston Pier channel project

How much risk are we really taking with the planned dredging at Kingston Pier – and how much protection do our corals actually have on paper? This piece walks through what the federal approval does and doesn’t guarantee, explains why sediment and light matter so much to the reef, and asks the hard questions we need answered before we trade a deeper channel for a shallower future.

Nov 8, 2025

A coral reef out of balance

After the long dry spell, the lagoon was crystal clear and full of life. But with the return of the rains, something else has returned too – the brown, filamentous mats of Lyngbya. It’s not seaweed, it’s a cyanobacterium, and when it takes hold it smothers coral and rubble alike. The reef’s way of showing us that every drop of water, from tank to tide, is connected.

Oct 25, 2025

Aglow among the spines

Ever seen a sea urchin that seems to glow blue from the shadows? That’s Diadema savignyi showing off its reef shimmer. Beautiful, a little spiky, and definitely not to be messed with.

Oct 15, 2025

The funky seventies sea slug – Halgerda willeyi

If ever a sea slug was channeling the 1970s, it’s Halgerda willeyi. With its groovy orange lines and chocolate-brown bumps, it looks straight out of a vintage lounge suite – the kind with shag pile carpet and bold floral cushions. Proof that nature was nailing retro design long before humans caught on.

Oct 5, 2025

Haddon's barometer

This Haddon’s anemone has been quietly living in the middle of Norfolk Island’s Emily Bay for years, bleaching and recovering with the seasons. Like corals, sea anemones host microscopic algae that provide most of their food. When stressed by heat or rainfall changes, they lose colour – and tell a story about seasonal changes to the weather.

Sep 30, 2025

Honoured to be featured

I left school in the UK nearly 50 years ago, so it was a pleasant surprise to be invited to share some images and take part in an interview for an article about my work, to be published in the annual glossy magazine the school now produces. Here is the end product.

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.