Did you know that corals are able to form chimeras? This means that genetically distinct colonies are able to join together and form "super-colonies", which increases their survival!

When corals such as Stylophora pistillata reproduce sexually, they release already developed planula larvae into the water column, as they are brooders. The larvae, or spat, are able to attach themselves onto the reef substrate and form new colonies. All these colonies are genetically distinct (just like e.g. human siblings), although they can be similar (human siblings have distinct but similar DNA, because they have the same parents). These corals are called 'genets', being genetically different.

When these colonies start growing next to each other on the reef, they will eventually start touching. It is known that many corals display aggressive behaviour when being in close proximity to one another, as they all compete for living space. This is different for corals from the same species. Their behaviour is difficult to predict. A very recent study sheds more light on this matter.

Coral chimaeras

Scientists from the Israel Oceanographic and Limnological research institute and Auburn University (Keren-Or Amar, Baruch Rinkevich and Nanette Chadwick) collected planula larvae in situ from 10 Stylophora pistillata colonies on the coral reef in Eilat, Israel and shipped them to the laboratory within two days after collection. Larvae settlement began within several hours after collection and continued for up to three weeks. 

A so-called MPE (multi-partner entity) composed of different Stylophora pistillata genets (coral colonies which are genetically distinct). The dotted black lines indicate the borders between original colonies. The asterisks, *, indicate rejection of the far left colony, causing tissue separation. The separation resulted in a layer of calcified skeleton without tissue. This shows that this coral species has evolved an immune system. Interestingly, all of the other genets accepted one another. Scale bar is 1 mm or 0.04 inches, arrows denote founder polyps (adapted from Amar, K-O et al, BMC Evolutionary Biology, 2008). 

After the larvae had settled, they started to metamorphose into primary polyps. They found that genetically distinct Stylophora pistillata polyps were able to form aggregates, or chimeras. A chimaera is an organism with tissues of different genetic origin. They discovered that these chimaeras can consist of either two, three, or multiple fused colonies. Although a partnership between organisms can be beneficial, it often comes with costs as well. They also found that a connection between two colonies sometimes resulted in aggressive behaviour. This led to several outcomes; rejection, bleaching and even death.

It was also found that when regarding the average size of MPE members, each was found to be smaller than the solitary colonies, although the total MPE’s were larger. This was likely the result of the cost (energy expenditure) inflicted to the MPE members by their interactions (adverse reactions such as rejection of tissue). The fact that the MPE itself was larger showed improved survivorship under field conditions. What we learn from this is that although a partnership between organisms can be beneficial, it often comes with costs as well.

Why do corals form chimaeras?

If there are costs such as death and growth inhibition, why do corals form these chimaera “super-colonies”? Survival on the reef is all about claiming space, and MPE’s were found to grow faster, and were therefore able to occupy a section of the reef within a short time. This allowed them to increase their chances of survival, by quickly claiming their space for light and nutrient uptake. Other scientists also recently found that larvae of the species Acropora millepora are able to form chimeras, both in ex situ as in the natural environment². During an experiment, 47% of A. millepora larvae settled in aggregates of 2 or more individuals. When they sampled the DNA of adult specimens on the Great Barrier Reef, they found that 3-6% of all adult colonies were in fact chimeras!

These results show that corals are really unique animals. As we learn more about them, we find more intriguing ways in which coral polyps cooperate. Not only do most coral species clone themselves and form large colonies, they also fuse with genetically different individuals of the same species to increase their survival. This feature, natural chimerism, has also been observed in many other sessile marine invertebrates. Examples are sponges, soft corals, tunicates and even scleractinian corals. The fact that corals are able to distinguish between self and non-self tissue, shows that immunity has deep roots in evolution.

We regularly publish news and results from the CORALZOO project.

This research was funded by the EU (European Union). CORALZOO is an international collaboration between universities, zoos and aquaria, dedicated to optimizing coral aquaculture by scientific research. The project aims to make zoos self-supporting and coral aquaculture sustainable. Its partners are Wageningen University (coordinator), the Israel Oceanographic and Limnological Research Institute, Technical University of Dresden, the Italian consortium for Marine Sciences, Burgers’ Zoo, Rotterdam Zoo, Running Deep, London Zoo, Acquario di Genova, Oceanario de Lisboa, Nausicaa, Oceanopolis, Schoenbrunner Tiergarten, Tierpark Hagenbeck, Red Sea Corals Ltd. and the European Association for Zoos and Aquaria (EAZA).

References:

Keren-Or Amar, Nanette E Chadwick and Baruch Rinkevich, Coral kin aggregations exhibit mixed allogeneic reactions and enhanced fitness during early ontogeny, BMC Evolutionary Biology, april 2008, 30;8:126

Puill-Stephan E, Willes B, van Oppen M and van Herwerden L, Chimera Formation During The Early Life History Of Acropora Millepora And Its Persistence Through Time, 11^th ICRS, Fort Lauderdale, Florida, USA, 2008