Volume 11, Number 3: 16 January 2008
In an "As We See It" piece in the journal Marine Ecology Progress Series, Baird et al. (2007) give new hope that the world's corals will indeed be able to successfully cope with the possibility of further global warming, be it anthropogenic-induced or natural. They begin, however, with a more somber perspective, noting that a number of marine researchers have "presented convincing data which suggest that few coral symbioses host more than one type of symbiont," and that "this lack of flexibility in present-day coral-algae symbioses has led some to argue that changing symbionts to cope with climate change [or symbiont shuffling] is, therefore, not an option for most coral species."
The four Australian scientists discount this argument, however, because, "as they see it," it is the sub-clade that must be considered within this context, for they cite studies that indicate "there are both heat tolerant and heat susceptible sub-clades within both clades C and D Symbiodinium." Hence, the more relevant question becomes: How many coral species can host more than one sub-clade? The answer, of course, is that most if not all of them likely do; for they note that "biogeographical data suggest that when species need to respond to novel environments, they have the flexibility to do so."
So how and when might such sub-clade changes occur? Although most prior research in this area has been on adult colonies switching symbionts in response to warming-induced bleaching episodes, Baird et al. suggest that "change is more likely to occur between generations," for initial coral infection typically occurs in larvae or early juveniles, which are much more flexible than adults. In this regard, for example, they note that "juveniles of Acropora tenuis regularly harbor mixed assemblages of symbionts, whereas adults of the species almost invariably host a single clade," and they indicate that larvae of Fungia scutaria ingest symbionts from multiple hosts, although they generally harbor but one symbiont as adults.
Because of these facts, the Australian researchers say there is no need for an acute disturbance, such as bleaching, to induce clade or sub-clade change. Instead, if ocean temperatures rise to new heights in the future, they foresee juveniles naturally hosting more heat-tolerant sub-clades and maintaining them into adulthood.
Finally, as an alternative means of surviving gradually rising water temperatures, Baird et al. remind us that both partners in the coral/symbiont association "have the capacity to evolve," that they can do so in many different ways (Coles and Brown, 2003), and that "it is increasingly apparent that evolution by natural selection can occur very rapidly in some situations (Carroll et al., 2007)."
In light of these several observations from marine biologists at James Cook University's School of Molecular Science and the ARC Centre of Excellence for Coral Reef Studies at that university and the University of Queensland, we feel confident earth's corals will be able to successfully cope with the possibility of further increases in water temperatures, be they anthropogenic-induced or natural. Corals have survived such warmth -- and worse -- many times in the past, including the Medieval Warm Period, Roman Warm Period, Holocene Optimum, as well as throughout numerous similar periods during a number of prior interglacial periods; and there is no reason to believe they cannot do it again, if the need arises.
Much more threatening are a number of direct adverse impacts of humanity, including rising nutrient levels caused by runoff from agricultural activity on land, outbreaks of the coral-devouring crown-of-thorns starfish, the barbed hooks and scything nets used in fishing, tourists and the developers who build resorts and marinas for them, increased sediment levels, the nets of prawn trawlers stirring up growing loads of sediments, the 6-10 tons of wasted "bycatch" associated with each ton of prawns that are caught, sea life depleted to the point of exhaustion by over-fishing, huge catamarans and dive boats that daily take thousands of visitors to places such as Australia's Great Barrier Reef and dump their sewage in the sea on the way back, the live reef-fish trade, fishermen using cyanide and dynamite, as well as a wide assortment of pollutants.
If we're really worried about the planet's corals, these are the things on which we need to focus our attention.
Sherwood, Keith and Craig Idso
References
Baird, A.H., Cumbo, V.R., Leggat, W. and Rodriguez-Lanetty, M. 2007. Fidelity and flexibility in coral symbioses. Marine Ecology Progress Series 347: 307-309.
Carroll, S.P., Hendry, A.P., Reznick, D.N. and Fox, C.W. 2007. Evolution on ecological time-scales. Functional Ecology 21: 387-393.
Coles, S.L. and Brown, B.E. 2003. Coral bleaching: capacity for acclimatization and adaption. Advances in Marine Biology 46: 183-223.