Following on from a previous article at Climate Shifts, a recent article published in PLoS One shows that corals are proving to be even more non-conformist than previously thought. Zoe Richards and co-authors from the ARC Centre of Excellence for Coral Reef Studies found that ‘rare’ species of branching corals are able to cross breed with other branching corals to create hybrids, therefore avoiding probable extinction:
“Coral reefs worldwide face a variety of marine and land-based threats and hundreds of corals are now on the red list of threatened species. It is often assumed that rare coral species face higher risks of extinction than common species because they have very small effective population sizes, which implies that they may have limited genetic diversity and high levels of inbreeding and therefore be unable to adapt to changing conditions.
When we studied some particularly rare species of Acropora (staghorn corals), which you might expect to be highly vulnerable to extinction, we found some of them were actually hybrids – in other words they had cross-bred with other Acropora species. This breaks all the traditional rules about what a species is. By hybridising with other species, these rare corals draw on genetic variation in other species, increasing their own potential to adapt to changing conditions.
When we looked at the genetic history of rare corals, we found that they exhibited unexpected patterns of genetic diversity. This suggests that, rather than being the dying remnants of once-common species, they may actually be coral pioneers pushing into new environments and developing new traits by virtue of the interbreeding that has enabled them to survive there.
This is good news, to the extent that it suggests that corals may have evolved genetic strategies for survival in unusual niches – and may prove tougher to exterminate than many people feared. With such tricks up their sleeve, it is even possible that the rare corals of today could become the common corals of the future.” (Link)
Crown of thorns starfish (COTS – Acanthaster planci) are notorious throughout the Indo-Pacific region. COTS are voracious coralivores, and in outbreak proportions can eat vast areas of reef by exuding their stomachs and digesting coral polyps (read more). Having been diving in oceans around the world over the past few decades, i’ve often pondered the differences in colourations of COTS between reef regions, and whether they represented a single species. A recent paper published in Biology Letters by Catherine Vogler from Göttingen University and colleagues at the Smithsonian and University of California confirms that COTS aren’t a single taxonomic entity, and in fact represent a ‘species complex’ of upto four seperate species.
Different appearances of the Crown of Thorns starfish across locations, clockwise from Top Left: Madagascar (Image credit: Mila Zinkova), Thailand (Image credit: Jon Hanson) Okinawa, Japan (Image credit: Gary Hughes), Fiji (Image credit: Matt Wright)
Using a genetic approach, the researchers analysed DNA from over 237 starfish collected from reefs around the world. Their results strongly suggest that their are in fact four species of COTS, located in the Pacific Ocean, Red Sea, Southern Indian Ocean and Northern Indian Ocean).
Geographical distribution of the different species of crown of thorns (each colour represents a different species where sampled, piecharts indicte the frequency of each species per location)
It’s fascinating to think that the divergence of these species occured between the Pliocene (3.65 million years ago) and early Pliestocene (1.95 million years ago). More importantly though, this discovery may have fairly interesting implications for conservation biology. The researchers point out that whilst outbreaks of COTS are well-researched phenomena on the GBR and Indo-Pacific reefs since the early 1960’s, outbreaks in the Indian Ocean and Red Sea are much less severe. It seems that a better understanding of the genetic structure of COTS populations and identifying species boundaries may go a long way to explaining the intensity and magnitude of COTS outbreaks in different regions.
Reference: Vogler et al (2008) A threat to coral reefs multiplied? Four species of crown of thorns starfish. Biology Letters doi:10.1098/rsbl.2008.0454 (Link)
An article published in PLoS One has huge implications for almost everything we do in our research on corals. In summary, using an array of genetic markers, a highly respected group of leading scientists including Fukami, Chen, Knowlton and others have shown that whilst Scleractinia (the stony corals) have a single origin in evolution, to date we have lumped many species and genera into families incorrectly, at least partly due to the traditional system of classification . This finding has the interesting implication that morphological features (at the heart of coral taxonomy) may have been much more plastic in time than we have appreciated. Such findings make sense given how variable the skeletal structure of corals is in response to the environmental circumstances the coral is growing in.
Fukami et al (2008) Mitochondrial and Nuclear Genes Suggest that Stony Corals Are Monophyletic but Most Families of Stony Corals Are Not (Order Scleractinia, Class Anthozoa, Phylum Cnidaria). PLoS ONE 3(9): doi:10.1371/journal.pone.0003222
Modern hard corals (Class Hexacorallia; Order Scleractinia) are widely studied because of their fundamental role in reef building and their superb fossil record extending back to the Triassic. Nevertheless, interpretations of their evolutionary relationships have been in flux for over a decade. Recent analyses undermine the legitimacy of traditional suborders, families and genera, and suggest that a non-skeletal sister clade (Order Corallimorpharia) might be imbedded within the stony corals. However, these studies either sampled a relatively limited array of taxa or assembled trees from heterogeneous data sets. Here we provide a more comprehensive analysis of Scleractinia (127 species, 75 genera, 17 families) and various outgroups, based on two mitochondrial genes (cytochrome oxidase I, cytochrome b), with analyses of nuclear genes (ß-tubulin, ribosomal DNA) of a subset of taxa to test unexpected relationships. Eleven of 16 families were found to be polyphyletic. Strikingly, over one third of all families as conventionally defined contain representatives from the highly divergent “robust” and “complex” clades. However, the recent suggestion that corallimorpharians are true corals that have lost their skeletons was not upheld. Relationships were supported not only by mitochondrial and nuclear genes, but also often by morphological characters which had been ignored or never noted previously. The concordance of molecular characters and more carefully examined morphological characters suggests a future of greater taxonomic stability, as well as the potential to trace the evolutionary history of this ecologically important group using fossils.