The Research Problem

We are trying to understand how coral colonies within a reef are genetically related to each other and how this might influence coral reef health and persistence. Coral reefs can be a single, genetically homogonous collection (i.e. one individual repeated many times) even though they reproduce both sexually and asexually, and can consist of many different colonies. How genetically diverse a coral reef is can have a significant influence on its ability to withstand environmental stress such as disease.

Methods

We are looking at the genetic relatedness of single coral reefs both in the Main Hawaiian Islands and Northwestern Hawaiian Islands. We have mapped and sampled all colonies of Pocillopora damicornis (Lace coral) and Porites lobata (Lobe coral) on a patch reef at French Frigate Shoals and all Pocillopora damicornis (Lace coral) colonies on a reef at Pearl and Hermes Atoll and in Kāne‘ohe Bay. We genetically identify (the genotype) each colony using nuclear DNA markers capable of providing an individual-specific DNA fingerprint.

Findings

Surprisingly, of the 1,623 colonies of Pocillopora damicornis (Lace coral) from Pearl and Hermes Atoll, only 129 different genotypes were found. Eight of these genotypes account for 53% of all colonies on the reef. That is to say, although there are thousands of colonies on the reef there are primarily only eight individuals repeated many times.

The density of Pocillopora damicornis (Lace coral) at French Frigate Shoals is much lower than Pearl and Hermes with only 156 colonies on the reef. Of the 125 genotyped colonies, 53 were unique; this is a larger percent of the total number than was seen at Pearl and Hermes Atoll, similarly, a small fraction of these (~9%) account for over half of the individuals on the reef. Taken together, Pocillopora damicornis (Lace coral) is highly clonal and, in spite of the large number of individual colonies, there are very few genotypes.

Importance

In general, a small portion of the reef contains most of the genetic variation present. The reef at Pearl and Hermes Atoll is connected to the one at French Frigate Shoals over evolutionary time (i.e., same alleles at the same frequencies are found at both reefs) but not over ecological time (i.e., no genotypes were shared between reefs). The small number of unique genotypes at these reefs may put them at risk due to lack of genetic variation. The fact that these genotypes are very different, however, is a positive sign that sufficient genetic variation may already exist, allowing resistance to disease or adaptation.

Other Research

We are also examining temperature stability by monitoring the temperature of individual reefs in Kāne‘ohe Bay, O‘ahu and Pearl and Hermes Atoll and French Frigate Shoals in the Northwestern Hawaiian Islands. The research thus far indicates that there is a considerable amount of temporally stable temperature variation across a reef. Locations 4-meters apart can experience different thermal environments. We have identified several locations that are hot or cold spots for 60% of the time over a three year period.