Bailing out of trouble: can polyp bailout save the day?

Written by Gus Fordyce

The fight or flight response underpins how everyone and everything responds to stress. As natural selection is a product of disturbance, it wouldn’t be too much of a stretch to think of evolution as being driven by a series of decisions to stand our ground or retreat to safety. Now corals aren’t exactly the best runners – even those weird fungi who move stealthily around the reef are pretty slow; and while extruding your stomach on your neighbor is an impressive attack when competing for space, it’s fairly useless against the existential threat of climate change. So what can corals do? One solution, so simple yet seemingly overlooked, is for corals to run away.

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Fig. 1 The odd, mobile fungid corals are unlikely to win any races. Photo by Charlotte Page

Polyp bailout as an area of research has been almost willfully ignored since the term was coined by Paul Sammarco in 1982. I’d heard the term in learning about the wonderful ways in which corals reproduce, and happily accepted it without further thought or question. That was, until I saw the bizarre process myself. I watched polyps withdraw unto themselves and pop off their limestone home as if leaping from a burning building. If this was polyp bailout, it seemed like a pretty extreme solution to the challenge of reproduction.

But as it transpires, the reproductive strategy I had learnt about was actually polyp expulsion – a process described in a 1997 Nature article where individual polyps are squeezed off of a healthy parent colony. This might seem like semantics; but the distinction is key. Expulsion is voluntary, a seemingly lazy way of trying to propagate without losing too much of yourself. Bailout is something else entirely. It is a panicked, sacrificial response that breaks up a lifelong colony in a final gasp for survival.

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Fig. 2 Polyps leaping from the wreckage of their now bare skeleton. Photo by Emma Camp

Aside from the dramatics of this behaviour, or rather my description, the existence of this escape response has not received the scientific attention it merits. As put eloquently by Hagit Kvitt in 2015, corals are quite literally changing their phenotype between hard and soft-bodied forms in response to stress. Polyps kill off their connective tissue through apoptosis, preserving themselves intact until conditions ameliorate; and if they find themselves in a happier place, they can re-settle and grow once again.

This requires a precise level of cellular control, the ability to reverse developmental processes and a fine-tuned sensitivity to the external environment. It makes polyps seem positively sentient. It speaks to their evolutionary history: offering an explanation for gaps in their fossil record, and suggesting new relatedness to soft corals that seem to share this behavior. If corals have switched between soft- and hard-bodied forms in the past, might this be an outcome of climate change?

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Fig. 3 Free-living polyps after bailout, desperately seeking a place to settle down. Photo by Alexander Fordyce

Unfortunately, this research has slipped under the radar of many a coral scientist. Consequently, we are not only missing out on opportunities to learn more about coral evolution, but also the ecological relevance of this behavior. This latter is exacerbated by a trend amongst previous articles to subject corals to extreme conditions way beyond the realms of today’s, or the near future’s, climate.

Many have placed corals in aquaria without water exchange, leading to hypoxia and overall poor water quality. Others have put them in conditions of salinity or acidity akin to brine or freshwater. In the instances where temperature was used to induce bailout, the conditions have been realistic, albeit harsh. What this tells us is that this response is generalized across the small number of species in which it has been recorded. A whole host of different stresses can trigger this attempt to escape. But does this happen in nature?

As we consider the reality of this behavior, and how widespread it may be, we shine a light on innumerable questions about polyp bailout. Will this protect diversity during bleaching events? How many species are capable of this behavior? How long can the polyps survive? Can they adapt to deeper water where they might seek refuge? In answering these questions, polyp bailout may soon step out into the spotlight after 50 years back stage.

Further Reading

Fordyce AJ, Camp EF and Ainsworth TD. Polyp bailout in Pocillopora damicornis following thermal stress. F1000Research 2017, 6:687 (doi: 10.12688/f1000research.11522.2)

Kramarsky-Winter E, Fine M, Loya Y: Coral polyp expulsion. Nature. 1997; 387: 137.

Kvitt H, Kramarsky-Winter E, Maor-Landaw K, et al.: Breakdown of coral colonial form under reduced pH conditions is initiated in polyps and mediated through apoptosis. Proc Natl Acad Sci U S A. 2015; 112(7): 2082–2086.

Sammarco PW: Polyp bail-out: an escape response to environmental stress and a new means of reproduction in corals. Mar Ecol Prog Ser. 1982; 10: 57–65.

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