Does reproductive success of corals decline with age?
It is easy to determine the age of a human. When a baby is born approximately nine months after conception, we start counting days, months, and years to tell the age of this new, genetically unique human being. For most corals, and especially for branching corals, this normally straight-forward concept of age is actually quite ambiguous because they also reproduce asexually through fragmentation. Branches of elkhorn or staghorn coral colonies break off, reattach somewhere else and grow into new colonies, which are genetic clones of their parent.
So, counting the days from the initiation of an independent colony is not a valid age estimate for the genotype of these colonies. “With clonal coral species, we would naturally consider a given small colony growing on a reef as young because of its size and the short time it has been growing. But the chances are reasonably high that it in fact represents an ancient genetic individual which has persisted for thousands of years since that original point of conception, when that fresh combination of genes began,” SECORE Research Director Dr. Margaret Miller says.
Large branching coral colonies like this staghorn coral growing on a reef in Curaçao might be thousands of years old. (Paul Selvaggio)
Field observation trigger new research project
Yet the genetic age of a coral colony might be an important parameter regarding its ability to reproduce sexually – just as it is for organisms such as dogs, cats or humans. “The reproductive fitness of most organisms goes down when they grow old,” Margaret Miller says. “For corals, because they do not have a set age span, we generally assume their reproduction is not affected by age.”
However, anecdotal observations from SECORE’s Curaçao field team and its partners at CARMABI Marine Research Station suggest the opposite. During last year’s spawning event young elkhorn corals, which were raised by SECORE from larvae and outplanted in 2012, produced many more gametes compared to larger (older) colonies nearby. On the other hand, fertilization using eggs and sperm from bigger, older colonies sometimes failed altogether, raising the question to what degree gametes of older colonies contribute to fertilization and eventually a new generation of corals. “Combined, these observations suggest that reproductive success does decline with age and may indeed contribute to reduced recruitment and recovery success of branching Caribbean corals,” Margaret Miller says.
Young versus old: In our new research project we will investigate, which type of elkhorn coral colony produces fitter offspring – genetically young colonies that were outplanted by SECORE and scientists from CARMABI Marine Research Station in 2012 (left) or bigger and older wild colonies (right). (Paul Selvaggio / Valerie Chamberland)
This summer, SECORE and its partners from the Pennsylvania State University and with support from NOAA, are going to investigate in Curaçao whether outplanted elkhorn coral genotypes that are less than 10 years old produce more and fitter offspring than neighboring wild colonies that are much older. “Our hypothesis is that as corals age, the number of replication mistakes occurring during cell-division increases. Such mistakes are called somatic mutations. They accumulate in a coral’s DNA and can contribute to reduced reproductive performance of older colonies,” says Dr. Iliana Baums, SECORE’s collaborator from Pennsylvania State University.
Using newly developed genetic tools, Dr. Baums’ team developed a method to score the number of somatic mutations accumulated in the genome of elkhorn coral which can be used to estimate the age of a coral regardless of its size. ”We estimated that some of the bigger wild elkhorn colonies growing on the Sea Aquarium Reef in Curaçao are almost 2000 years old,” explains Iliana Baums.
Planned scientific work
Funded by NOAA, SECORE’s field team on Curaçao together with members of Baums’ lab will collect spawn from outplanted colonies of known age and large wild colonies whose age has already been determined by Iliana Baums. Gametes from individual colonies in each age category will be counted and examined for differences in function (such as sperm motility) and quality (e.g. lipid content of eggs) traits. Then, gametes will be crossed amongst colonies within each age group, so that resulting coral larvae will have only young or old parents. “We will then compare the performance of both groups during their earliest life stages and look for example for differences in fertilization and settlement success and survivorship after outplanting them on the reef”, says Dr. Valerie Chamberland, SECORE’s Research Scientist and Co-Principal Investigator on the project.
This new joint research project is a collaboration between Dr. Iliana Baums from Pennsylvania State University (left) and SECORE, represented by Research Scientist and Co-Principal Dr. Valérie Chamberland (right) and Research Director Dr. Margaret Miller (middle). (private archive / Paul Selvaggio)
Old genotypes might be overrepresented in restoration
If the scientists’ hypothesis is confirmed and the offspring of younger colonies has indeed a higher fitness than offspring from older coral genotypes, it would have far-reaching implications for larval rearing and coral gardening, which are the main techniques used in present-day coral restoration efforts.
“If we are using parents that are extremely old for larval propagation, it is likely that we are raising babies that are not going to do that well due to the accumulation of somatic mutations in such colonies,” Margaret Miller says. “If increased age indeed impairs reproductive success in newly raised generations of elkhorn coral, we may be able to screen for and avoid this trait in the parents we collect gametes from, for example by explicitly targeting younger colonies that despite their smaller size are better suited to be used in restoration efforts.”
The same is true for coral gardening approaches where old genotypes are likely overrepresented in the fragment culture stocks as large, older colonies are often targeted during harvesting of fragments to stock nurseries. “If we can demonstrate that these older genotypes mean decreased reproductive output, it would be advantageous to target young genotypes that are more likely to be reproductively successful after they grew in nurseries and are outplanted on the reef,” Margaret Miller says.
Text: Sina Löschke / Margaret Miller