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

October 28, 2026

When storms rearrange the reef

Sometimes the sea rearranges a reef in a matter of hours! In the third article of my six-part photographic series, I revisit corals broken, overturned or swept away by powerful swells and storm surges. Some disappeared altogether, while others found surprising ways to survive. Five photographic sequences reveal just how dramatically Norfolk Island's reef can change.

October 26, 2026

When the reef fights back – coral growth and recovery

Not every coral story ends badly! Among six years of underwater photographs are some wonderful examples of growth, survival and recovery. From corals that have grown spectacularly to others that have survived bleaching, breakage or even being turned upside down, this second article in my photographic series celebrates some of the reef's more encouraging stories.

October 24, 2026

The neighbours – six years of watching the same reef

When I began photographing Norfolk Island's reef in 2020, I simply recorded whatever caught my eye. Six years later, those photographs have become a record of individual corals and their changing fortunes – growth, competition, survival and loss. This is the first of six articles looking back at what I've seen, and sometimes what I wish I hadn't.

October 10, 2026

Pulling the rugosity from out of the reef

What do ‘rugosity’, structural complexity and channeling your inner fish have in common? Quite a lot, as it turns out. This week I’m looking at how all the lumps, bumps, branches, holes and ledges on a reef create habitat – with a brief detour into one of my long-standing tricks for navigating tertiary study: write down the jargon until it starts to make sense.

September 27, 2026

How does a sea slug smell?

It started with a close-up of a little Elysia and two rhinophores that looked like tiny rolled-up scrolls. Once I noticed them, I started going back through years of sea-slug photos and realised just how wildly different these sensory organs can be – ridged, rolled, curled, tubular and, in sea hares, distinctly rabbit-eared. So this week’s rabbit hole is all about how sea slugs ‘smell’ underwater, and why their noses come in so many different shapes.

September 19, 2026

The Elite Fleet sails in

Nearly six years ago, my very first blog post was about the strange blue and violet creatures that live at the ocean’s surface – a group I christened Norfolk Island’s ‘Elite Fleet’. Since then I’ve seen plenty of bluebottles and the occasional violet sea snail, but never a by-the-wind sailor. Until this morning. Velella velella is not a true jellyfish, but a colonial hydrozoan with a transparent sail that lets the wind help carry it across the ocean. So what exactly is this extraordinary little creature, how does it ‘sail’, and what connects it to the rest of the fleet?

September 18, 2026

In a nutshell – where I stand on the proposed Norfolk Island Marine Park

I support the proposed expansion of Norfolk Marine Park and its renaming as Norfolk Island Marine Park. There is much to welcome – strong offshore protection, no mining, and recognition of the importance of sustainable local fishing. But I also have some reservations. Why does Cascade need to be removed from the Marine Park at all? How will more than 430,000 km² actually be monitored and enforced? How do we make sure Norfolk people have a genuine role in management? And, above all, how do we ensure that the enormous new park does not draw attention away from the small, stressed reef immediately beside us?

September 13, 2026

No seagrass, but plenty of turtles

Norfolk Island does not have seagrass, although one of our most conspicuous green algae is often mistaken for it. Feather and grape Caulerpa can both form striking patches across the lagoon. I take a closer look at what Caulerpa actually is, why it is so easily confused with seagrass, and where our green turtles fit into the story.

September 6, 2026

Watching El Niño from Norfolk Island

El Niño is strengthening across the Pacific, but that does not tell us exactly what the coming months will bring to Norfolk Island. After a long wet spell, I am watching our rainfall and water supplies more closely again – and, underwater, keeping an eye on sea temperature, algae, coral recruits and the fish that turn up on the reef.

September 2, 2026

What the world’s reefs are telling Norfolk Island

Two major new coral reef reports carry a clear message for Norfolk Island: reefs can recover when pressures ease, but the time available for recovery is shrinking. Norfolk Islanders were raising concerns about water quality, runoff and the health of Emily and Slaughter Bays nearly forty years ago. Today we have much better science, a changing climate and a far clearer understanding of how closely the catchment and reef are connected. This article looks at what that means for water quality, long-term monitoring, dredging and how we manage Kingston from land to sea.