Daphne Cortese, PhD student at PSL Université Paris, Moorea, French Polynesia, explains the detrimental effects of anemone bleaching on the fish that depend on them in her recently accepted Functional Ecology paper, “Physiological and behavioural effects of anemone bleaching on symbiont anemonefish in the wild.”

What’s your paper about?

Our paper is about the cascading effect of bleaching on the physiology and behaviour of clownfish – or anemonefish. As a result of increased water temperature caused by global change, corals and anemones on coral reefs lose their symbiotic algae and bleach (i.e. lose their colour and look white). Bleached animals may survive if temperatures come back to normal, but need months to recover. In this study we explored the effects of several months of anemone bleaching on anemonefish, who live in close association with anemones for food, shelter and habitat. Specifically, we assessed the impact on juvenile fish behaviour and physiology after one and two months of exposure to a bleached anemone host and measured the impact on survival over nine months of bleaching. We found that when juvenile anemonefish live in a bleached anemone for more than a month they start to decrease their metabolism, grow less and change their behaviour, becoming less active.

What is the background behind your paper?

Anemone bleaching, together with coral bleaching, is widespread and is affecting a large proportion of reefs worldwide. While the direct impact of bleaching on animals that bleach (i.e. corals) has received a lot of attention, the indirect impacts of bleaching on the fish community that live in association with animals that bleach is less known. Previous studies have found that bleaching can affect fish reproduction impacting their fecundity and hormone production in nature (Beldade et al., 2017) but also that a short-term bleaching event (2 weeks) can increase fish metabolism (Norin et al., 2018). These studies suggest that anemonefish respond to such a stressor by activating their stress response. We were therefore interested to understand what were the effects over a longer time exposure (months) on fish physiology and behaviour, and whether fish could have the capability to acclimate to such a stressor.

How did you come up with the idea for it?

Sadly over the last 5 years, we witnessed four large-scale bleaching events in Polynesia and also in the Great Barrier Reef. In addition, the length of these bleaching events seemed to be getting longer lasting for 3 months at first to over 8 months. Therefore, we realised the urgency to understand the cascading effects of bleaching onto reef fish.

What are the key messages of your article?

In this study we observed a progressive decrease in fish metabolic rate and lower growth in fish living in bleached anemones, which may be explained by a reduced food intake. If fish can’t get enough food from their environment they will have to adjust their behaviour in order to feed more. Activity and movement are important for feeding as well as territorial defense in anemonefish, but also require a lot of energy. In food-limited environments, it can be advantageous to down-regulate metabolism, but there is a limit to how much it can be decreased and we didn’t observe any stabilization over time. This finding, together with lower growth and modified behavior, suggests that fish in bleached anemones are at an energetic disadvantage and that there is an increasingly negative impact of anemone bleaching on anemonefish over time rather than an acclimation to the new condition. All organismal traits are inter-connected and if energy, the fuel, is missing, this can have life-long consequences. For example, size is often crucial for competition and reproduction and a reduced size-at-age might compromise fitness at later stages. If adult fitness is impacted then there could be consequences at the population level. Furthermore, if other reef fish respond in a similar way then these long-term bleaching events could impact up to 12% of fish on the reef that associate with organisms that beach.

How is your paper new or different from other work in this area?

This is a study carried out on a natural population in the wild and it therefore provides a realistic picture of what happens in nature when anemone bleach. So far scientists have mainly focused their attention on either laboratory studies or the direct effects of bleaching on corals and on the effects of bleaching-induced coral mortality on the coral reef community. Our results suggest that even if corals survive and recover from a bleaching event, this doesn’t occur without costs for their associated communities that structure coral reef ecosystems.

Does this article raise any new research questions?

Yes, we’d like to know whether any offspring produced during this period of bleaching are better able to withstand future bleaching events compared to offspring produced outside of bleaching events. In other words, can mothers modify the offspring they produce so they can survive better in future bleaching conditions?

What is the broader impact of your paper?

Coral reefs are one of the most diverse ecosystems hosting almost a quarter of all marine fish species. Bleaching is a widespread phenomenon and the effects we observed in our study may be applicable to other species of fish that live in association not only with anemones but also with corals at least during some part of their life cycle. The impact may therefore extend to a large part of the community living in those ecosystems with potential socio-economic repercussions on human populations.

What are the big questions still to answer?

Although we didn’t observe signs of acclimation to bleaching we still don’t know if fish that survive one bleaching event can cope better in response to repeated bleaching events. In addition, it would be interesting to observe how the effect of bleaching can act over the long-term life of fish, i.e. whether juveniles are able to develop compensatory growth to compensate for the negative effects that arise from a bleaching event. Finally, bleaching usually follows a period of increased temperature. Although corals and anemones can take time, i.e. weeks to months, to recover even when temperature come back to normal, temperature is also known to affect fish physiology and it will therefore be interesting to see the synergetic effects.

About The Author

How did you get involved in ecology?

I would say that the origin came from my childhood. I grew up in a little house in the middle of the woods among Tuscany hills. I quickly started to be fascinated by the natural world that was surrounding me and by the ability of animals to cope with their continuously changing environment over the seasons for example, cold and snowy winters and hot and dry summers. As I grew up and discovered more about the natural world, my curiosity was piqued by underwater ecosystems, completely new and unknown to me who grew up surrounded by land, mountain, hills and woods. Whilst studying biology it was a natural next step to deepen my understanding of the relationships among living organisms and how they interact with their surrounding environment. I quickly became fascinated by all processes in ecology and the work of an ecologist, from experimental design, data collection, especially carrying out fieldwork, and understanding the results.

What is the best thing about being an ecologist?

To be able to observe the natural world with a different eye, recognizing the richness of the processes involved and being aware of all what is still unknown, trying to find answers that may help to understand and protect it. Being an ecologist also implies never getting bored as every day is different from the previous one and it often means that your office changes everyday.

Read the article in full here

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References:

Norin, T., Mills, S. C., Crespel, A., Cortese, D., Killen, S. S., & Beldade, R. (2018). Anemone bleaching increases the metabolic demands of symbiont anemonefish. Proceedings of the Royal Society B: Biological Sciences, 285(1876), 20180282.

Beldade, R., Blandin, A., O’Donnell, R., & Mills, S. C. (2017). Cascading effects of thermally-induced anemone bleaching on associated anemonefish hormonal stress response and reproduction. Nature communications, 8(1), 1-9.