Sea stars? Starfish? What’s the difference?

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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In recent years, marine scientists have been giving these underwater stars an image makeover. The starfish of our childhoods, for those of you who have a few beachside summers under your beach towel, is now more properly known as a sea star.

Starfish, sea stars, are not fish; they don’t have gills, fins or scales or any other fishy bits, but instead they have more in common with sea urchins and sand dollars, and, like them, are a member of the echinoderm family.

So now you know that they are called sea stars. No excuses!

As we have a few different varieties inside the reef, but not in huge numbers, I thought I’d put together a few cool facts about them:

The madreporite, or sieve plate, is a little disc on the central section of the body

Sea stars don’t have blood. Instead, they pump sea water in through a madreporite, or sieve plate, which looks like a little disc on the central section of the body. You’ve probably seen these but never really registered that it was anything special! I just thought it was an anomaly on the skin, like a little bit of scar tissue!

The water is pumped around the sea star through a web of channels to tube feet. Each tube foot consists of an ampulla and a podium. The ampullae are little pouches that control the water going into the podia of the sea star. When a sea star needs some suction, the ampulla draws the water up from its podium allowing it to secrete a ‘glue’ that lets them adhere to whatever surface they are on.

Detail of one of the sea star’s ‘eyes’ at the end of an arm

At the end of each arm of the sea star is a tiny red dot. This is its ‘eye’, except it can’t really see, instead it senses light and dark, like when a shadow passes across it.

Sensitive to smells in the water, they sniff out their prey’s chemical signature with their feet, and then it is their feet that carry them towards their prey. They have extra-long tube feet on each of their arms that take the lead when it comes to finding food. The sea star moves in the direction of whichever arm is pulling it the strongest.

A sea star has no brain, but it does have two stomachs. One is called a cardiac stomach, the other a pyloric one. The cardiac one is pushed out of its mouth that will engulf their food. There it will secrete powerful enzymes to start the break-down process, before pulling it back inside and passing its meal to the other stomach to finish off the process of digestion.

Sea stars can breed the ‘normal’ sexual way releasing sperm (males) and eggs (females) into the water column. These hopefully find each other and live as plankton floating to new areas for the sea star to colonise. When they are ready they settle to the bottom and become new baby sea stars.

But the really cool thing is they can also reproduce asexually by dividing into two and becoming two new sea stars. This is rather handy, because it also means they can regenerate a lost arm or two if needed.

Sea stars can live as long as 35 years. However, since 2014, many sea stars, particularly along the North American Pacific coast, are succumbing to the sea star wasting disease and with a world-wide massive die off some species are now being listed as critically endangered. In places a recovery has been recorded but it is uneven and the disease is persisting in most areas. You can read more, here.

This has been linked to warming ocean temperatures.

Genus Ophidiaster

Purple Velvet Star - Leiaster leachii

Indo-Pacific Comb Star - Astropecten polyacanthus

Eleven-armed Sea Star - Coscinasterias muricata

Regrowing tentacles

Possibly an example of asexual reproduction

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.

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.