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. 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, and provides new unique insights into coral immunity¹.

Figure 1: Israeli scientists collected larvae in situ from the stony coral Stylophora pistillata at the gulf of Eilat, and found that their larvae have developed an intricate immune system (photograph: Dr. Keren-Or Amar).

Coral chimeras

Scientists from the Israel Oceanographic and Limnological research institute and Auburn University (Keren-Or Amar, Baruch Rinkevich and Nanette Chadwick) studied settlement, growth, development and survivorship of 544 larvae during a one-year study. They 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. 

Figure 2: Young Stylophora pistillata colonies between 0 and 4 months old, which settled and grew in close proximity, either formed chimeras or rejected one another. a: single genotype, 2 months old. b: bi- chimera; 2 fused colonies. c: 2 genotypes which rejected one another. d: tri-chimera, 1,5 months old. e: 3 rejecting genotypes. f: fused colony of 7 different genotypes; a so-called multi-partner entity. Interestingly, only a single genotype is rejected (far left). Asterisks indicate areas of rejection, dotted lines indicate borders between original colonies, arrows indicate primary polyps. White scale bars are 1 mm (Amar et al,  BMC Evol. Biol., 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. Colonies made up of 4 or more individuals were called multi-partner entities (or MPE's). 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 MPE’s in totality were larger. This was likely the result of the cost (energy expenditure) inflicted to the MPE members by their interactions, such as adverse reactions 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 chimeras?

If costs exist 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!

Development of immunity

During another study, scientists found that S. pistillata polyps developed their immunity during the first 4 months of life³. Polyps which were less than 4 months of age always fused with their counterparts, irregardless of genetic differences. When fusions took place within 2 to 4 months, this sometimes lead to permanent fusions, and sometimes rejections. Fusions which were started during the first 2 months of life were always permanent. Interestingly, polyps which were older than 4 months always rejected one another. These results provide unique insights into the development of coral immunology; already very early in life, corals are able to distinguish between self and non-self tissue. When two genetically different tissues fuse before the immune system is able to detect it, it becomes permanent. Interestingly, during the previously discussed study, some very young polyps did reject each other (figure 2c,e).

Figure 3: A schematical model of several distinct stages (A,B and C) of Stylophora pistillata immunity development. A: First stage; tissue contacts immediately after metamorphosis (t=0) up to 2 months of age. After fusion, a stable chimera is formed. B: Second stage; tissue contacts 2-4 months after metamorphosis. A transitory chimera is formed; at an age of 4 months, tissue rejection starts. C: third stage; tissue contacts after 4 months lead to rejection, what is also called a histoincompatibility response (Frank et al, Proc. Roy. Soc. Lond. B, 1997).

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^2,3. The fact that corals are able to distinguish between self and non-self tissue, shows that immunity has deep roots in evolution.

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

Itzchak Brickner, Uri Oren, Uri Frank and Yossi Loya, Energy integration between the solitary polyps of the clonal coral Lobophyllia corymbosa, Journal of Experimental Biology, 2006, 209, pp 1690-1695

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