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.

This page shows the most recent blog posts. For the complete catalogue, visit the ‘Out on a swim index’ page.

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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

May 28, 2026

A shrimp storm

While setting my research cams last week, I swam into what looked like an underwater snowstorm. It appeared to be the aftermath of a mass moulting event, with large numbers of tiny, translucent shrimp-like exoskeletons drifting together near the surface.

Kingston dredging: what happens when a reef does not fit the framework

This correspondence with DCCEEW is about more than one dredging proposal. It is about what happens when an ecologically distinctive place is assessed through standard tools that do not always make its most important values easy to see. I am publishing it here because that is something we need to be aware of, both on Norfolk Island and more broadly in Australia.

May 24, 2026

Kingston dredging: the project advances, the questions remain

Kingston dredging is edging closer, and the paper trail is growing. This post brings together earlier correspondence with the Department and the latest media release so readers can see what has been asked, what has been answered, and what still remains unclear about the project, its rationale, and the protections proposed for the reef.

May 17, 2026

The lime-green coral in Slaughter Bay – a 40-year paper trail

Green Mountain – the name I give this coral in my database – is a coral I’ve photographed for years as I swim past. Then I found its backstory in the Norfolk Island National Parks archives: a rough map, reused paper, a note in the margin – ‘still thriving’. That’s how baselines begin.

April 5, 2026

What Norfolk Island’s reef tells us about environmental blind spots

The Kingston dredging proposal on Norfolk Island raises a bigger question than dredging alone: how well do standard environmental assessment tools capture the real significance of a remote and unusual reef system like Norfolk Island’s?

March 30, 2026

Hammer coral time!

Hammer corals have unique tentacles that are large, fleshy, and tubular; these terminate in a ‘T’-shaped, hammer-head or anchor. Beneath all these softly waving tentacles is an extraordinary skeleton structure, which helps define them as a large polyp stony coral.

March 24, 2026

Norfolk Island’s fishes: drifters, residents and the ones still missing

Norfolk Island’s fish fauna reflects both connection and isolation. Some species may arrive from elsewhere as drifting larvae, some populations appear to persist locally, and some fishes known from islands on either side of Norfolk have still not been recorded here. This post looks at what old survey work, regional checklists and genetic studies suggest about that more complicated picture.

March 7, 2026

Alveopora or flowerpot coral – how to tell the difference

They look alike at first glance, but Alveopora and flowerpot corals are not the same. The easiest way to tell them apart is to count the tentacles.

February 27, 2026

Norfolk’s lagoonal reef – the 2025 report, in plain English

We now have the 2025 Norfolk Island reef health report, so I’m taking the opportunity to translate it into plain English here. Sadly, it’s more of the same story in Emily and Slaughter Bays – a reef that can cope with some stress, but is being asked to cope with too much, too often.

February 20, 2026

Halimeda’s night shift – why this reef algae changes colour

Halimeda is a calcareous green reef alga that forms new segments overnight, shifts from white to bright green by dawn, then pales again as calcification begins. A quick look at one of the reef’s smartest algae.