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.

This blog is rated in the Top 20 Coral Reef Blogs in the world.

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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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.

Jun 12, 2025

Warning signs: quiet and unnoticed collapse of two coral colonies

Day 5 of my blog series for the UN Ocean Conference: two long-lived coral colonies in Norfolk’s lagoon died quietly from disease. No drama – just slow collapse and overgrowth by algae. A reminder that not all reef losses are loud, but they are happening.

Jun 11, 2025

Warning signs: what Norfolk Island’s reef is telling us

Day 4 of a week-long photo series from Norfolk Island, shared during the UN Ocean Conference in Nice. Today’s post spotlights a Hydnophora pilosa colony where white syndrome appeared suddenly and spread quickly, taking out around a quarter of the coral. In the months that followed, algae quietly filled the gap – a subtle but telling shift from coral to algae that’s happening across the reef.

Jun 10, 2025

Warning signs: coral disease takes hold

In Day 3 of this blog post series, published while leaders gather at the UN Ocean Conference in Nice, we see Norfolk Island’s coral reef lagoon quietly delivering a stark warning: recurrent land-based pollution, coral disease, and delayed decisions are dismantling this ecosystem in real time.

Jun 9, 2025

Warning signs: coral growth anomalies – the slow cancers of the reef

Day 2’s post coinciding with the UN Ocean Conference looks at coral growth anomalies – sometimes called coral ‘cancers’. These slow-moving diseases quietly weaken coral colonies, making them far more vulnerable to storm damage and algal takeover. On Norfolk Island’s reef, I’ve watched this exact process play out over several years. This is how chronic stress silently dismantles coral ecosystems.

Jun 8, 2025

Warning signs: shifting baselines on Norfolk Island’s reef

Today is World Ocean Day — a timely moment to launch my week-long blog series on Norfolk Island’s reef. Each day this week, I’ll be sharing photo essays that document the slow but steady pressures reshaping this fragile reef. Today: how shifting baselines make us blind to what we’ve already lost.

Apr 29, 2025

The Governance–Government Vacuum: Norfolk Island’s Forgotten Ecology

A personal reflection on Norfolk Island’s coral reef environment, political denial, and what John Wyndham’s The Kraken Wakes can still teach us about slow-moving disasters — and why this election matters more than ever.

Feb 20, 2025

Cute as buttons – Astrea curta

Astrea curta corals are ‘small, moderately plocoid [flattened], distinct, and almost circular’ . Normally grey-green in colour, you can see from the images here, ours are often beautiful rich gold, although they do vary. They have a neat growth habit and button-like corallites, which can grow in columns, spherically or flattened. Large colonies of these can form gorgeous undulating bumps.

Jan 26, 2025

From 'Watch' to 'Warning'

Last week, the chance of coral bleaching in Norfolk Island’s inshore lagoons was raised from ‘Watch’ to ‘Warning’ and will more than likely rise to Alert levels one and two in coming weeks. So why do I worry about water quality all the time when bleaching seems inevitable these days and so the reef is probably doomed anyway? Read on to find out.

Jan 4, 2025

From little things – watching them grow

Small numbers of different fish species is not an unusual phenomenon on Norfolk Island’s reef, but it does demonstrate what a tiny, precious, coral reef ecosystem we have, when we can count individuals on one hand and watch each of them grow, like these little blackeye thicklips, a member of the wrasse family.