Using Genetic Parentage Analysis to Measure Connectivity in a Pelagic Spawning Coral Reef Fish

Simon R. Thorrold, Biology
Michael Berumen, Biology

Grant Funded 2008

Population connectivity – the degree to which spatially discrete subpopulations supply, receive, or exchange larvae – is a fundamental feature of the dynamics of marine organisms. Connectivity  rates determine colonization patterns of new habitats, the resiliency of populations to disturbances or harvesting, and should be used to inform the design of marine protected areas (MPAs). Quantifying rates of exchange in marine organisms, is, unfortunately, extremely difficult because the natal origin of most adults is almost always unknown. This lack of knowledge is primarily due to the reproductive strategy employed by most marine species. Typically, this involves the production of large numbers of very small pelagic larvae that suffer high initially mortality rates, precluding traditional mark-recapture approaches. Although various studies have attempted to resolve questions of larval connectivity using models and population genetics, very little empirical evidence is available to test the scenarios generated by these studies. Managers are thus forced to make decisions about MPA designs without the critical knowledge of whether or not these designs will work effectively.

We have recently used innovative multi-disciplinary techniques to directly measure population connectivity in two fish species in Kimbe Bay, Papua New Guinea, where an extensive network of MPAs have been established. Using a chemical tagging method, we found that 60% of baby fish of both study species on our reef were offspring of parents on that same reef (known as selfrecruitment). This provided, for the first time, a direct measurement of connectivity in coral reef fish and demonstrated that it is possible to obtain this much-needed empirical evidence.

The second technique we employed to assess connectivity is genetic parentage analysis. While conducting our tagging study, we took genetic samples from individuals of both our study. We have now sufficiently refined genetic analyses to match individual offspring to the exact father and mother in our populations, providing us with high-resolution information about the patterns of reproductive output and larval dispersal. This opens the door for us to ask new questions: Do some parents produce more successful offspring than others? Can we identify areas of reefs that are more productive than others? For how many breeding seasons are individuals contributing to future generations? These questions, while of great significance, have been previously unanswerable due to the constraints described above as well as a lack of resolution in genetic methods.

To date, we have conducted the chemical tagging analyses for both of our study species, the orange clownfish (Amphiprion percula, of Finding Nemo fame) and the vagabond butterflyfish (Chaetodon vagabundus). The genetic parentage analysis has subsequently been completed for the clownfish, and it is important to note that there was 100% agreement between the two techniques. However, clownfish and butterflyfish have a major difference in their reproductive behavior. Clownfish lay their eggs on the actual surface of the reef. After hatching, clownfish larvae are planktonic for about 10 days before they settle back to a reef and begin the juvenile stage of their lives. About 75% of reef fish species (including butterflyfish and all commercially important species) do not lay their eggs on the substrate, but rather release their eggs and sperm into the water column, where the eggs will drift with the currents for several days before hatching.  Once these pelagically spawned eggs hatch, they typically spend 4-6 weeks (5 weeks for butterflyfish) in their pelagic stage before settling onto a reef. As this presents the potential for pelagically spawned larvae to disperse over great distances, it is significant that we found identical levels of self-recruitment in both species.

The present proposal seeks funds to complete the genetic parentage analyses on the samples from the vagabond butterflyfish that we have already collected as a part of our study. The potential to demonstrate that parentage analysis can be applied as a tool for determining connectivity in pelagic-spawning reef fish represents an opportunity to answer important fundamental questions of reef fish dynamics with clear management implications.