Hammer coral time! — Norfolk Island's Reef

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 was looking through some of my photos of hammer corals the other day and found myself pausing for longer than expected. They are such eye-catching corals when you see them in the water, all those flowing, fleshy tentacles moving about, but close-up some of these photos tell a more interesting story.

In the image, above, the tentacles are partly withdrawn and the skeleton is showing, which is why I like this photo. It strips away the softness a bit. It becomes very obvious that the hammer coral is a hard coral, built on a solid limestone framework.

At certain times of year corals are full of activity that is completely hidden unless you happen to catch them at the right moment. Hammer corals reproduce both sexually through spawning and asexually through budding and head splitting – where polyps split or grow new heads at the base (budding) to form colonies. An example of sexual reproduction can be seen in the photo, below. This was taken just before spawning, and you can actually see the pink gametes quite clearly inside the tentacles. Amazing!

A hammer coral with developing gametes, photographed 9 January 2021. The next full moon was on 28 January.

For all their beauty, hammer corals can be quite combative. They may extend long sweeper tentacles to sting nearby corals and hold onto space on the reef. I look at these in the brain coral, Paragoniastrea australensis, in more depth here While you were sleeping ... and here War of the coral worlds!

Hammer corals are primarily photosynthetic, obtaining their energy from light via symbiotic zooxanthellae, although they will feed on plankton, particularly at night time. Those zooxanthellae give them their colour and they will expel these when they are stressed, resulting in bleaching (photo, right).

Then there is the question of whether all hammer corals are the same. Looking at my last two photos, below, for example, makes me wonder. The tips are quite different in shape, and I suspect I may be looking at different species, or at least different forms. Or maybe I’m not! Part of the difficulty is that ‘hammer coral’ is not one neat, single thing. These corals are now generally placed in the genus Fimbriaphyllia, and the name ‘hammer coral’ is most often used for Fimbriaphyllia ancora and Fimbriaphyllia paraancora. That may be what I am seeing in these last two photos, though it could also be natural variation. I would not want to identify them with certainty from tentacle shape alone. In this group, colony form and skeleton can be just as important as the fleshy parts we notice first.

I enjoy looking back through older images. A photo that once seemed to be just a nice coral shot starts to reveal many other things – structure, reproduction, variation, behaviour – especially when compared with other images in my database. Certainly, the reef does not give everything away at first glance.

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.

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.