Estimates of Population Connectivity in the New Zealand Endemic Sea Urchin (Evechinus chloroticus)

Abstract

Marine organisms with a planktonic larval stage have the potential to be transported substantial distances, with the distance travelled depending on factors such as pelagic larval duration (PLD) and physical factors such as ocean currents and geographical barriers. This study attempts to quantify the degree of gene flow and connectivity in populations of the New Zealand endemic sea urchin Evechinus chloroticus, which has a relatively long PLD of approximately 30 days, and may be expected to show strong connectivity among populations. In this thesis I test if there is significant genetic differentiation among populations of E. chloroticus in New Zealand over both a geographically broadscale (throughout the North and South Islands) and a fine scale (within the Hauraki Gulf). More specifically, I also test if a phylogeographic barrier south of Cook Strait,proposed to exist for some other New Zealand marine coastal invertebrates, also affects population connectivity in E. chloroticus.This study used a combination of six nuclear polymorphic microsatellite loci and mitochondrial Cytochrome Oxidase subunit I (COI) sequences. Spines from adult Evechinus chloroticus were collected from eleven sampling sites: seven populations throughout the North Island, with focused sampling in the Hauraki Gulf, and four populations from the South Island, including two on Stewart Island and two populations around Cook Strait, which has previously been shown to be a phylogeographic barrier for marine invertebrates. The analysis of mitochondrial COI sequences (approximately 620bp) from 254 individuals revealed 36 different haplotypes. As a majority of individuals (73%) were one of three haplotypes, pooling of the large number of singleton haplotypes increased the power of the tests of differentiation. Significant population structure was observed consistent with a northern region and southern region division to the south of the Cook Strait (among group genetic variance FST= 0.09647 p<0.01). The analysis of the microsatellite data also supported the North/South divide (FST = 0.0120 p<0.01). Fine-scale population differentiation was not evident among the three Hauraki Gulf populations; however, significant population differentiation was observed among other North Island populations and those in the Gulf. Both marker types indicated a significant isolation by distance pattern present in E. chloroticus, although this was largely driven by the divergent Stewart Island samples. This indicates that across approximately 2,000km north to south, distance begins to restrict gene flow resulting in population differentiation. While there were few regions with strongly restricted gene flow, there did appear to be lower gene flow occurring and between northern and southern regions, and around East Cape. An east/west coast divide was not evident in E. chloroticus, unlike that reported for some other species. Thus, population connectivity was relatively high between neighboring populations. Where gene flow was restricted and population differentiation was observed, large geographic features and complex ocean currents were also observed. The information presented in this thesis has implications for advancing suitable management and conservation strategies, and can be used to help design effective marine protected areas and networks.