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.

This blog is rated in the Top 20 Coral Reef Blogs in the world.

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)

Dec 8, 2025

Signs of bleaching – 8 December 2025

I took these photographs this morning, Monday, 8 December 2025. A few warm days of settled weather, little cloud cover and low tides in the hottest part of the day have led to some early bleaching on our reef. Bleaching doesn’t always mean death for our corals, but it is concerning to have this so early in the summer season. Fingers crossed the conditions don’t last and the reef can recover.

Dec 3, 2025

Nature is my teacher

This is a thank-you note. Five years after my first Out on a swim post – written with zero marine science quals and a head full of questions – I’m still in the water, now as a PhD candidate, because an extraordinary mix of locals, volunteers, researchers and public servants decided to share what they knew. This is the story of how nature – and a very patient community – became my teachers.

Nov 30, 2025

Reef grief: what dredging has done to other reefs

From Miami to Fiji, from Dubai to tiny village harbours on atolls, dredging near coral reefs has left a long trail of scars – even on ‘small’ projects. This follow-up to last week’s Kingston post walks through real examples of what happened elsewhere, and what that should make us think about before we dig up our own reef.

Nov 20, 2025

To dredge or not to dredge? The Kingston Pier channel project

How much risk are we really taking with the planned dredging at Kingston Pier – and how much protection do our corals actually have on paper? This piece walks through what the federal approval does and doesn’t guarantee, explains why sediment and light matter so much to the reef, and asks the hard questions we need answered before we trade a deeper channel for a shallower future.

Nov 8, 2025

A coral reef out of balance

After the long dry spell, the lagoon was crystal clear and full of life. But with the return of the rains, something else has returned too – the brown, filamentous mats of Lyngbya. It’s not seaweed, it’s a cyanobacterium, and when it takes hold it smothers coral and rubble alike. The reef’s way of showing us that every drop of water, from tank to tide, is connected.

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.