Coral Evolution: From Socialists to Soloists

Last week’s blog post on the ancestry of the malarial plasmid attracted several insightful comments by Psi Wavefunction. One of the issues discussed was when exactly the malarial ancestor changed his lifestyle from being a coral symbiont to a coral parasite. This week I came across a paper in PNAS that shows that corals themselves also have an interesting evolutionary story to tell about switching lifestyles!

Two lifestyles
Corals come in all kind of sizes, shapes and (fluorescent) colours. But almost all of them can broadly be divided into two separate lifestyles. First up are the loners. These corals, like the sun coral below, prefer a nocturnal lifestyle . They capture prey with the tentacles that extend from the polyps. The word “Prey” means different things for different coral species, as it can range from feeding on zooplankton to digesting entire shrimps, or even fish!

The solitary (and appropriately nicknamed) sun coral Tubastrea Faulkneri.

The second walk of life is the ‘social’ one. These corals are social in more than one way. Firstly, they don’t seem to mind crowds since many of them form colonies. When a young larval coral of this type has found a nice place to settle, it spawns other polyps via asexual budding. The polyps then integrate into a single colony by removing the skeletal barriers between them. The other social aspect of these corals is that they live in symbiosis with zooxanthella, which function as little energy factories for the corals by trapping light from the sun and converting it into energy via photosynthesis.

Because of this symbiotic relationship with zooxanthella, symbiotic corals have to live in shallow waters. The corals receive huge benefits from the symbiosis, receiving orders of magnitude more energy than their heterotrophic nephews. However, the heterotrophic solitary corals can be found in much wider distributions and at much greater depths than their symbiotic peers, because they’re not restricted to the photic zone.

Coral Clades
The differences between the two different lifestyle are pretty dramatic. Consider the confusion when scientists discovered that several different clades of closely related corals contained both solitary and colonial members! No matter how you’re going to spin it, this either implies that coloniality evolved multiple times, or that it was lost multiple times during coral evolution.

If you’ve got a progressive view of evolution (which you shouldn’t) you might think that corals have evolved from the solitary single polyps, into the ‘complexer’ colonial forms that share nutrients and have colony-wide regenerative responses. A recent publication in PNAS shows that different scenario is more likely and (I think) way more interesting.

To find a consensus phylogenetic tree, Barbeitos and colleagues used Bayesian inference methods. They also used Bayesian analyses to reconstruct the possible lifestyles of ancestral corals and to estimate all possible transition rates (e.g. how likely it is that a symbiotic coral becomes solitary).

The consensus coral phylogeny obtained by Barbeitos et al. Open/closed squares stand for asymbiotic/symbiotic corals, open/closed circles stand for solitary/colonial corals.

Coral evolution
Their results are pretty conclusive: their models rarely showed solitary corals gaining coloniality. Since many of the reconstructed ancestors are colonial and symbiotic, the authors find that coloniality was lost at least 4 times in different coral clades, while symbiosis was lost at least 1 time. From these results it’s hard to say whether coloniality or symbiosis was preferentially lost first. Since some facultative symbiotic corals exist, it seems relatively easy for corals to lose the symbiosis. For losing coloniality, the researchers also provide an interesting explanation: heterochrony, of which neoteny is a special form.

Heterochrony seems to be one of the main drivers of phenotypic evolution and is pretty easy to understand if you realise that developmental changes underlie phenotypic changes most of the time. A small change in the embryonal stage can have big effects for the mature organism. However, evolution is not a magical inventor that makes new structures or life stages spring into existence. What evolution can do is tinker or interfere with what’s there already. By delaying maturation, organisms can become sexually mature even in their larval/juvenile forms! This could similarly have worked the ancestor of solitary corals, where the asexual expansion phase is terminated and an earlier sexual maturation of the corals. The observation that many solitary corals still have the capability (even if they don’t do so in the wild) to reproduce asexually seems to support this hypothesis!

Neotenous to the extreme. The mental and physical qualities of mature chihuahuas seem to match those of unborn wolves. Seriously, what

So where does that leave us with the evolutionary history of corals? If we put together the facts that a) the distribution of symbiotic corals is limited to depths where light can penetrate and b) deep-water solitary corals evolved from symbiotic and colonial corals, it’s safe to conclude that the shallow waters and reefs of the past were the evolutionary cradles from which all extant corals evolved. The robust loners can trace back their ancestry to the more social, but also more fragile, symbiotic corals. Nowadays, coral reefs still sustain some of the richest biodiversity on this planet. Their fragility and current rapid destruction should worry us, as the authors gloomily conclude:

The potential disappearance of tropical reefs in the next century could have evolutionary consequences for Scleractinia and other taxa that may span eons to come.

The Ancestry of the Malarial Plasmid Revealed
Where the wild things glow
Green eggs power solar salamanders