Reef real estate – a bubble-tip’s six-year stand-off

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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Is there such a thing as a peaceful reef life? No, not by a long chalk. Let me explain.

In an earlier post – War of the coral worlds! – wrote about corals jostling for space. This is the same story, but with different combatants: bubble-tip sea anemones (Entacmaea quadricolor) and Montipora corals.

Both belong to the phylum Cnidaria, and both come armed. Corals are genuinely sessile – they grow where they’ve settled. Bubble-tips are usually anchored in one spot too, but they can move if conditions turn against them. When they don’t move, it’s because they’re holding a patch worth keeping.

Reef real estate is finite, which makes the best nooks and ledges valuable. Neither anemones nor corals are passive tenants. Their ‘right’ to a spot is negotiated at the boundary – day after day, year after year.

Below are three photographs taken at different times of the same anemone–coral formation.

16 August 2020: A bubble-tip anemone sits in a crater surrounded by *Montipora*. On a reef, space is currency – and both sides treat the border as a live edge, not a backdrop.

7 November 2020: Same anemone, same coral, same standoff. The anemone holds the centre by making contact costly (stinging cells and venom); the coral holds the perimeter by building a raised rim – growth and calcification as a slow-motion barricade.

11 January 2026: Six years on, the line still holds, so this is a straight cnidarian-versus-cnidarian boundary: anemone chemistry and reach versus coral growth and persistence.

Bubble-tips defend themselves with nematocysts and venom – not just for feeding, but for deterring anything that presses too close. Corals have their own ways of fighting back, which vary by species and circumstance. I can’t tell you exactly which mechanisms are in play here, but the story is visible: the anemone holds the crater, and the surrounding coral maintains a thickened rim – a slow-motion wall that limits where the anemone can spread.

I’ll keep returning to this spot. It’s easy to miss what’s happening underwater when it happens in coral time as this border holds, shifts, and is continually tested.

Below are a few more examples of this underwater tension at play.

June 28, 2026

The red seaweed behind low-methane beef

A small red seaweed on Norfolk’s reef has become part of a much bigger story. Asparagopsis taxiformis can look like a delicate red feather duster or, at another stage of its life cycle, like a tiny cottony pom-pom. It is beautiful, easily overlooked, and now being used in the cattle industry to help reduce methane emissions. This post looks at the reef oddity behind the low-methane beef story – and why repeated local observation can be more useful than it first appears.

June 20, 2026

From coral scar to aatuti farm

Aatuti are bold little algae farmers, but how does one of their farms begin? Over the past year, I have been following several coral patches as small white scars became algal footholds, then larger defended patches. I still cannot say what caused the first wounds, but the photo sequences show something fascinating: on a reef where algae is already gaining ground, even tiny changes on the coral surface can become part of a much bigger story.

June 15, 2026

Norfolk’s water quality – when action is reported as outcome

A recent Australian Government media release presents investment, monitoring and catchment works as progress on Norfolk Island’s water quality. Some of that work is useful, and some of it was badly needed. But activity is not the same as proven improvement. This post looks at Kingston sewerage, wetlands, cattle, acid sulfate soils, groundwater and reef health, and asks whether Emily Bay and Slaughter Bay are actually being better protected.

June 14, 2026

How surgeonfishes got their name

Surgeonfish are named for the sharp little scalpels near their tails, but on Norfolk’s reef their more useful work happens at the other end. Pencil surgeonfish, bluespine unicornfish and their relatives help browse algae across the reef – a small daily job that becomes very valuable on an algae-rich lagoon reef like ours.

May 28, 2026

A shrimp storm

While setting my research cams last week, I swam into what looked like an underwater snowstorm. It appeared to be the aftermath of a mass moulting event, with large numbers of tiny, translucent shrimp-like exoskeletons drifting together near the surface.

Kingston dredging: what happens when a reef does not fit the framework

This correspondence with DCCEEW is about more than one dredging proposal. It is about what happens when an ecologically distinctive place is assessed through standard tools that do not always make its most important values easy to see. I am publishing it here because that is something we need to be aware of, both on Norfolk Island and more broadly in Australia.

May 24, 2026

Kingston dredging: the project advances, the questions remain

Kingston dredging is edging closer, and the paper trail is growing. This post brings together earlier correspondence with the Department and the latest media release so readers can see what has been asked, what has been answered, and what still remains unclear about the project, its rationale, and the protections proposed for the reef.

May 17, 2026

The lime-green coral in Slaughter Bay – a 40-year paper trail

Green Mountain – the name I give this coral in my database – is a coral I’ve photographed for years as I swim past. Then I found its backstory in the Norfolk Island National Parks archives: a rough map, reused paper, a note in the margin – ‘still thriving’. That’s how baselines begin.

April 5, 2026

What Norfolk Island’s reef tells us about environmental blind spots

The Kingston dredging proposal on Norfolk Island raises a bigger question than dredging alone: how well do standard environmental assessment tools capture the real significance of a remote and unusual reef system like Norfolk Island’s?

March 30, 2026

Hammer coral time!

Hammer corals have unique tentacles that are large, fleshy, and tubular; these terminate in a ‘T’-shaped, hammer-head or anchor. Beneath all these softly waving tentacles is an extraordinary skeleton structure, which helps define them as a large polyp stony coral.