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 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)

Jun 13, 2025

Glimpses of recovery: what the reef could be if we let it

Day 6 of this photo series from Norfolk Island coincides with the final day of the UN Ocean Conference in Nice. After a week of documenting decline, today’s post offers a different view – what reef recovery can look like when conditions improve. Drought in 2024 gave the reef a break, and the results were unmistakable: healthier corals, lower disease, and more fish. This is what’s possible if we act.

Jun 12, 2025

Warning signs: quiet and unnoticed collapse of two coral colonies

Day 5 of my blog series for the UN Ocean Conference: two long-lived coral colonies in Norfolk’s lagoon died quietly from disease. No drama – just slow collapse and overgrowth by algae. A reminder that not all reef losses are loud, but they are happening.

Jun 11, 2025

Warning signs: what Norfolk Island’s reef is telling us

Day 4 of a week-long photo series from Norfolk Island, shared during the UN Ocean Conference in Nice. Today’s post spotlights a Hydnophora pilosa colony where white syndrome appeared suddenly and spread quickly, taking out around a quarter of the coral. In the months that followed, algae quietly filled the gap – a subtle but telling shift from coral to algae that’s happening across the reef.

Jun 10, 2025

Warning signs: coral disease takes hold

In Day 3 of this blog post series, published while leaders gather at the UN Ocean Conference in Nice, we see Norfolk Island’s coral reef lagoon quietly delivering a stark warning: recurrent land-based pollution, coral disease, and delayed decisions are dismantling this ecosystem in real time.

Jun 9, 2025

Warning signs: coral growth anomalies – the slow cancers of the reef

Day 2’s post coinciding with the UN Ocean Conference looks at coral growth anomalies – sometimes called coral ‘cancers’. These slow-moving diseases quietly weaken coral colonies, making them far more vulnerable to storm damage and algal takeover. On Norfolk Island’s reef, I’ve watched this exact process play out over several years. This is how chronic stress silently dismantles coral ecosystems.

Jun 8, 2025

Warning signs: shifting baselines on Norfolk Island’s reef

Today is World Ocean Day — a timely moment to launch my week-long blog series on Norfolk Island’s reef. Each day this week, I’ll be sharing photo essays that document the slow but steady pressures reshaping this fragile reef. Today: how shifting baselines make us blind to what we’ve already lost.

Apr 29, 2025

The Governance–Government Vacuum: Norfolk Island’s Forgotten Ecology

A personal reflection on Norfolk Island’s coral reef environment, political denial, and what John Wyndham’s The Kraken Wakes can still teach us about slow-moving disasters — and why this election matters more than ever.

Feb 20, 2025

Cute as buttons – Astrea curta

Astrea curta corals are ‘small, moderately plocoid [flattened], distinct, and almost circular’ . Normally grey-green in colour, you can see from the images here, ours are often beautiful rich gold, although they do vary. They have a neat growth habit and button-like corallites, which can grow in columns, spherically or flattened. Large colonies of these can form gorgeous undulating bumps.

Jan 26, 2025

From 'Watch' to 'Warning'

Last week, the chance of coral bleaching in Norfolk Island’s inshore lagoons was raised from ‘Watch’ to ‘Warning’ and will more than likely rise to Alert levels one and two in coming weeks. So why do I worry about water quality all the time when bleaching seems inevitable these days and so the reef is probably doomed anyway? Read on to find out.

Jan 4, 2025

From little things – watching them grow

Small numbers of different fish species is not an unusual phenomenon on Norfolk Island’s reef, but it does demonstrate what a tiny, precious, coral reef ecosystem we have, when we can count individuals on one hand and watch each of them grow, like these little blackeye thicklips, a member of the wrasse family.