There is a lot of grazing going on around the reef at the moment, and I’m not talking about cattle. This is underwater grazing, carried out by some particularly beautiful little sap-sucking sea slugs.
I have been seeing an extraordinary amount of Bryopsis on the reef recently. That’s the green fluffy algae in the photograph, above. I can’t yet put a species name to it, so for the moment Bryopsis is as far as I am prepared to go. It is made up of masses of fine, feathery filaments and, in some places at the moment, particularly off the Salt House in the channel area, there is a lot of it.
Living amongst it I am finding Elysia marginata and the reticulated sap-sucking slug, Elysia rufescens.
Once I started noticing the combination, an obvious question followed. Are there more of the slugs because there is more of their food?
These little animals are sacoglossans – often called sap-sucking sea slugs because they pierce algal cells and suck out their contents. So when I talk about them grazing, I mean quite literally that they are feeding their way through the algae.
For Elysia rufescens, the connection with Bryopsis is particularly well established. It feeds on Bryopsis and even acquires defensive chemicals through its diet. Research by Becerro and colleagues found that both the alga and the slug were chemically protected against fish predation (Becerro et al. 2001). Finding E. rufescens sitting in a luxuriant bed of Bryopsis, then, is hardly surprising.
Bryopsis in among other algae, channel area of Emily Bay, Norfolk Island
A tightly coiled white sacoglossan egg mass among the algae, Norfolk Island
One needs to be very cautious about drawing cause-and-effect conclusions, but it does give me another way of looking at what I am seeing. If Bryopsis has become much more abundant, there is suddenly considerably more food and habitat available for those animals that are capable of exploiting it. That does not necessarily mean the population of Elysia has increased. I may simply be finding more because they are concentrated in conspicuous patches of their food. To say their numbers have genuinely increased I would need something rather better than my impression from repeated swims. Having said that, in six and a half years of photographing the lagoon, I did not photograph my first Elysia rufescens until July 2025 – and that was a single animal.
There is another intriguing clue, though. I am also finding what appear to be tightly coiled white sacoglossan egg masses amongst the algae. I cannot identify the species responsible from the egg mass alone, so I am not going to attach a name to them. But adults feeding amongst Bryopsis, together with egg masses in the same habitat, certainly makes the association worth watching.
Which leads to the bigger question. Why is there so much Bryopsis?
It would be very tempting to look at all that green growth and simply say ‘nutrients’. But if I’ve learned one thing on this journey from casual observer, to citizen scientist to PhD candidate, it is that reefs do not work quite that neatly.
Macroalgal abundance can be affected by nutrient availability, herbivory, light, temperature, season, water movement, disturbance and the amount of suitable substrate available for algae to colonise, and several of those factors may be operating at once.
Nor should freshwater and nutrients be treated as though they are the same thing. A strong reduction in salinity can stress marine organisms. But freshwater entering the reef after heavy rain can also carry dissolved nutrients, sediments, organic material and pollutants from the land. Here on Norfolk Island, we already know that happens. Page et al. (2023) investigated terrestrial pollution entering the nearshore reefs at Norfolk Island and detected terrestrially derived nutrient inputs. Their work linked rainfall and creek discharge with the movement of dissolved inorganic nitrogen and other land-derived material into the lagoonal reef environment.
So increased rainfall does not simply mean ‘more freshwater’. It can mean repeated pulses of material from the catchment reaching the reef.
There is also some interesting evidence specifically involving Bryopsis. On reef flats at La Réunion in the Indian Ocean, Zubia et al. (2018) examined macroalgal communities in relation to water chemistry and identified Bryopsis pennata amongst algae associated with nitrogen-enriched sites. That does not mean the unidentified Bryopsis growing here is responding in exactly the same way. But it gives us a plausible ecological mechanism to consider.
The chain I am wondering about therefore looks something like this:
more land-derived nutrient input → conditions favouring Bryopsis growth → more food for underwater grazers such as Elysia.
But this is just a hypothesis.
To test the first part properly I would need to know far more about when and where the Bryopsis is increasing, how that relates to rainfall and water quality, whether the pattern repeats seasonally, and what is happening with other algae and the animals that graze them at the same time.
Likewise, proving that the slugs are responding to an increase in Bryopsis would require more systematic observations of their abundance rather than my decidedly opportunistic habit of spotting interesting things while swimming past. Still, this is one of the pleasures, and real benefits, of watching the same reef repeatedly.
I started with a lot of vivid green algae. Then I noticed the slugs grazing through it. Now I am seeing egg masses. And then, because it is a habit of mine, I start wondering about the links between rainfall, nutrients, algae, food supply and the small animals able to take advantage of whatever changes are occurring around them.
Reticulated sap sucking slug, Elysia rufescens
References
Becerro, M. A., Goetz, G., Paul, V. J., & Scheuer, P. J. (2001). Chemical defenses of the sacoglossan mollusk Elysia rufescens and its host alga Bryopsis sp. Journal of Chemical Ecology, 27, 2287–2299.
Page, C. E., Ainsworth, T. D., Leggat, W., Egan, S., Sen Gupta, A., Raoult, V., & Gaston, T. F. (2023). Localising terrestrially derived pollution inputs to threatened near-shore coral reefs through stable isotope, water quality and oceanographic analysis. Marine Pollution Bulletin, 193, 115193.
Zubia, M., Depetris, M., Flores, O., Turquet, J., & Cuet, P. (2018). Macroalgae as a tool for assessing the ecological status of coral reefs under the Water Framework Directive: A case study on the reef flats of La Réunion (Indian Ocean). Marine Pollution Bulletin, 137, 339–351.