Casting your offspring to the currents – sea star reproduction

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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On 6 May 2023, the day after the full moon and at low tide, I witnessed an enormous Leach’s sea star, Leiaster leachi, as it broadcast its gametes into the water column. These star fish have been recorded as large as 73 cm in diameter. This one was definitely right up there.

Sea stars are either female or male. Just by looking at this one it is impossible to tell which it is. But females will be broadcasting eggs from their gonads, and the males, sperm. The gonads are located in each arm. Hopefully, nearby, another Leach’s sea star will be doing exactly the same thing at the same time, and the eggs and sperms will meet and form larvae.

A baby sea star’s parents will never meet. For reproduction to occur, it’s important that sea stars of the same species are synchronised. The usual triggers for spawning are factors such as the tides and water temperature. To help boy meet girl, specific proteins are released by both the eggs and sperm so they know they are hitching themselves to the right species. In other words, they follow a chemical trail to their target.

While what I was witnessing here was sexual reproduction, sea stars can also reproduce asexually, too, by regenerating an entire body. They can also regrow lost limbs.

When I first came across this sea star I honestly thought it was in distress, then I wondered if it was dead and was breaking down, but usually that attracts a host of fish, and that definitely wasn’t happening here. THEN I twigged what was happening. What an amazing WOW moment! And what a privilege to witness such a rare event.

Here are some photos, below. You can click on each one to enlarge. In each one, I am looking directly down from the surface at the sea star, and it is clinging to the side of a coral bommie. You can see that as it releases its gametes it stands proud of the reef, twisting and turning slightly to enable their release.

At the bottom of this post is a short video clip.

You can read more about sea stars in my blog post Sea stars? Starfish? What’s the difference?

Further reading

The weird and wonderful world of breeding sea stars (used as a source for this post)

Oct 25, 2025

Aglow among the spines

Ever seen a sea urchin that seems to glow blue from the shadows? That’s Diadema savignyi showing off its reef shimmer. Beautiful, a little spiky, and definitely not to be messed with.

Oct 15, 2025

The funky seventies sea slug – Halgerda willeyi

If ever a sea slug was channeling the 1970s, it’s Halgerda willeyi. With its groovy orange lines and chocolate-brown bumps, it looks straight out of a vintage lounge suite – the kind with shag pile carpet and bold floral cushions. Proof that nature was nailing retro design long before humans caught on.

Oct 5, 2025

Haddon's barometer

This Haddon’s anemone has been quietly living in the middle of Norfolk Island’s Emily Bay for years, bleaching and recovering with the seasons. Like corals, sea anemones host microscopic algae that provide most of their food. When stressed by heat or rainfall changes, they lose colour – and tell a story about seasonal changes to the weather.

Sep 30, 2025

Honoured to be featured

I left school in the UK nearly 50 years ago, so it was a pleasant surprise to be invited to share some images and take part in an interview for an article about my work, to be published in the annual glossy magazine the school now produces. Here is the end product.

Sep 7, 2025

Celebrating Biodiversity Month on Norfolk Island

September is Biodiversity Month – the perfect time to celebrate the astonishing variety of life on Norfolk Island’s reef. From new fish sightings to coral mosaics, every observation is a reminder of how much there is still to learn and protect.

Read more about why biodiversity matters, globally and right here in our lagoon.

Aug 24, 2025

The fate of a coral colony when it succumbs to white syndrome – four years on

I’ve tracked one plating Acropora coral from 2021 to 2025. In just a few weeks, white syndrome wiped it out. Nearly four years years on, it’s still smothered in algae and sea squirts, with only the tiniest hint of new growth. It’s a stark reminder: without tackling the root cause, we’re just watching the same sad story repeat itself.

Aug 17, 2025

The Candy-Striped Cleaner Keeping the Reef Healthy

Candy-cane stripes, long white feelers, and a reef spa on offer – the banded coral shrimp waves its antennae to advertise cleaning services to passing fish.

Aug 10, 2025

Biomimicry: How a Boxfish Caught Mercedes Benz’s Eye

Meet Mr Lemonhead – our lagoon’s teeny yellow boxfish with a big design legacy. He inspired a Mercedes Benz concept car, proving how nature is full of surprises. And he shares the lagoon with other critters whose tricks have also shaped real-world inventions.

Aug 3, 2025

Patchwork Corals: How Colonies Fuse to Form Living Mosaics

Some corals wear more than one colour for a reason. When Paragoniastrea australensis colonies fuse early in life, they form living mosaics. A beautiful reminder of coral cooperation on Norfolk Island’s reef.

Jul 28, 2025

Reef relief

Today, 28 July, is World Nature Conservation Day. After the dry 2024, Norfolk Island’s reef is looking healthier – a brief reprieve as less water - laden with nutrients - flowed into the lagoon. These photos show what’s possible. It’s a reminder that recovery is within reach – though renewed runoff could quickly undo the gains.