Furry sea hares as eco-warriors — Norfolk Island's Reef

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.

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Last week I noticed lots of sea hares, Stylocheilus striatus – commonly called the lined sea hare, blue ring sea hare or furry sea hare – in the shallows of Emily Bay. I’ve only seen this species twice before in the lagoons here: once on 4 January 2021, and once on 6 November 2021 – both times I observed lone individuals. That doesn’t mean they haven’t been there before, but I certainly haven’t noticed them, so I was astonished by the numbers that seemingly appeared almost overnight.

These little sea hares are incredibly well disguised in their algal habitat, but in the last few days, I can say, once you’ve seen one, you will suddenly see lots. And lots!

This week, I’ve come home from my swims armed with a bunch of photos and lots of questions. So I decided to do some research on these critters.

The facts

Sea hares are herbivores that feed on alga, with most species (as adults) not being too fussy about which species of alga they consume. However, Stylocheilus striatus are a particularly specialised type of feeder, consuming the cyanobacteria (blue-green) species of alga, especially one that is commonly known as mermaid’s hair, stinging limu or fireweed, Lyngbya majuscula[i]. This is a toxic filamentous blue-green alga that grows on sand or rocks.
Lyngbya majuscula grows in a nutrient rich environment. It can cause a range of medical conditions in humans, especially skin and eye irritations (swimmers itch), but also irritations to the gastrointestinal and respiratory systems.[ii]
Lyngbya majuscula contains chemicals that have been shown to be a deterrent to reef fish being able to feed; however, research suggests Stylocheilus striatus may cleverly sequester these toxic chemicals to use as a defensive mechanism.[iii]
Lyngbya majuscule can prevent the settlement of coral larvae, precipitating a phase shift in a lagoon habitat from a coral- to an algal-dominated reef.[iv]
Lyngbya majuscula, the blue-green alga, occurs naturally, but is not something we really want a great deal of in our reef environment.
Stylocheilus striatus have been reported to be found in dense feeding aggregations in association with blooms in cyanobacteria, or blue-green alga (Lyngbya majuscula).[v]
By eating the toxic blue-green alga, Lyngbya majuscula, Stylocheilus striatus play an important ecological role in controlling this algal growth.[vi]

Conclusion

These little sea hares are great to have around as they consume the toxic blue-green alga that fish and other herbivores don’t or can’t eat or tolerate.

The fact we have quite a few in Emily Bay begs the question as to why. Presumably, it means the conditions are right, asin there is plenty of food for them to eat, i.e. toxic blue-green alga.

I’m not a scientist, nor a researcher, so I am not qualified to draw any conclusions, but I did find it fascinating to learn how our eco-systems are balanced.

Well disguised among the weed

Multiple sea hares, mating

Lyngbya majuscula

Blue-green alga, cyanobacteria

Fun facts

Interestingly, studies have shown that fewer Stylocheilus striatus eggs develop when they are subject to noise pollution from, for example, traffic noise from motorboats![vii]

Stylocheilus striatus develops blue dots, or ocelli, which become more prominent and numerous as it ages.[viii] You can see some of these if you click on the photos, above.

[i] The Feeding Ecology of Tropical Intertidal Herbivores, Thesis by Cathryn L. Clarke, James Cook University, March 2004, https://researchonline.jcu.edu.au/1083/2/02whole.pdf

[ii] Lyngbya (blue-green algae), WorkSafe Queensland, https://www.worksafe.qld.gov.au/safety-and-prevention/hazards/hazardous-exposures/biological-hazards/lyngbya

[iii] Ibid, p. 21

[iv] Horwitz R, Jackson MD, Mills SC. 2017. The embryonic life history of the tropical sea hare Stylocheilus striatus (Gastropoda: Opisthobranchia) under ambient and elevated ocean temperatures. PeerJ 5:e2956 https://doi.org/10.7717/peerj.2956

[v]The Feeding Ecology of Tropical Intertidal Herbivores, Thesis by Cathryn L. Clarke, James Cook University, March 2004, https://researchonline.jcu.edu.au/1083/2/02whole.pdf

[vi] https://en.wikipedia.org/wiki/Stylocheilus_striatus

[vii] Boat noise impacts development, survival of sea hares, Science Daily, 31 July 2014, https://www.sciencedaily.com/releases/2014/07/140731095013.htm

[viii] Sea Slugs of Hawai’i, by Cory Pittman and Pauline Fiene, http://seaslugsofhawaii.com/species/Stylocheilus-striatus-a.html

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.

Jul 20, 2025

Emily Bay's big 'brain' coral

In Emily Bay, Norfolk Island, a single coral bommie – Paragoniastrea australensis – has stood for decades as a micro-reef, harbouring diverse marine life and local memories. Once photographed in 1988 and still thriving today, it remains a keystone of reef biodiversity and a living link between past and present.

Jul 14, 2025

Biodiversity matters

Over five and a half years of snorkelling Norfolk’s lagoon, we’ve documented 23 fish species not previously recorded in this area. Some are local ghosts, others climate migrants. These observations help us understand and protect what makes our reef so special.

Jun 17, 2025

Poop power

Not all poop on a reef is bad poop. In fact some kinds of poop can be a reef’s most important invisible engine. Fish poop, bird poop – even poop that gets eaten again by other fish – all of it keeps the ecosystem ticking over in a way that’s nothing short of extraordinary.

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.