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.

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.

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.