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 (but not always), 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, as in 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

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.

August 25, 2026

When the map changes

A fish once thought to belong only to Norfolk turns up elsewhere; another appears here hundreds of kilometres beyond its known range. Cardinalfishes and a Sabre squirrelfish show how our maps of species distributions are revised.

August 24, 2026

Endemic, native, undescribed – what do these words actually mean?

A Norfolk chromis that turned out not to be Chromis fumea, a blenny found nowhere else, and a tiny goby that may not yet have a name. A look at what we mean by native, endemic, undescribed and newly described – and why scientific names sometimes change.

August 16, 2026

What’s grazing in the green stuff?

A proliferation of Bryopsis on the reef has brought some beautiful little underwater grazers with it. I follow the trail from sap-sucking sea slugs and egg masses to a bigger question – why is there so much Bryopsis around at the moment?

August 8, 2026

The clam beneath Emily Bay

I’ve seen the beautiful shells of Laciolina chloroleuca in Emily Bay for decades, but I’d never really stopped to wonder where the living animals were. It turns out they may be buried half a metre beneath the sand. A closer look at an easily overlooked clam, its extraordinary shell markings, and the hidden life beneath our lagoon.

August 1, 2026

Reflections of an accidental marine scientist

What I thought would be a steady plod towards retirement, with plenty of time spent nursing grandchildren, was abruptly upended exactly six years ago today. This is the story of how an environmental catastrophe changed the course of my life – and led me to reinvent myself. It is a reflection on how, and why, I became an accidental marine scientist.

July 24, 2026

From ridge to reef: unpicking the Waterway and Wetland Strategy

Norfolk Island’s draft Waterway and Wetland Strategy will guide work for the next ten years. I support its intent, but it needs to be clear, practical and measurable – joining the dots from ridge to reef so we know whether public money is improving water quality and helping the reef.

July 13, 2026

Reef reality in pictures

Let the pictures do the talking. This is the reality on Norfolk Island’s reef today, laid out in a two-minute video.