The effects of fiord hydrography and environment on the population genetic structures of the sea urchin Evechinus chloroticus and the sea star Coscinasterias muricata in New Zealand

Abstract

New Zealand's fourteen deep water fiords possess complex physical hydrographic features and strong environmental gradients that may serve to isolate the marine organisms that occupy them. In order to investigate the population genetic structure of some of the marine fauna that occupy the fiords and to investigate the mechanisms that may contribute towards this pattern, we analysed two echinoderm species, the sea urchin Evechinus chloroticus and the sea star Coscinasterias muricata using neutral genetic markers. Two populations per fiord were genotyped for E. chloroticus across six microsatellite loci and one population per fiord of C. muricata was analysed using 366- base pairs of mitochondrial DNA D-loop. For each species we compared fiord populations to several others collected from around New Zealand.

At a macro-geographic scale, restricted gene flow between North and South Island was observed for both species. This was likely a result of the complex water circulation regimes around New Zealand. At a meso-geographic scale, significant population structure was found at short distances within fiords, among fiords and between fiords and open coast for both species. For E. chloroticus, two groups of populations were revealed: one group included samples from outer sites of the fiords, and the second group comprised samples from inner sites. Low genetic heterogeneity was observed within each of these groups. For C. muricata, population genetic differentiation presented a north to south cline amongst fiordic sites.

In order to assess whether population differentiation has resulted primarily from restricted larval dispersal or from natural· selection and/or local adaptation, we correlated genetic variation of each species with physical and biological characteristics of the fiords. Several isolating mechanisms were identified, common or specific to each species. From our results, I suggest that, for both species, historical colonisation events and subsequent restriction of larval dispersal due to fiordic hydrography have caused significant genetic differentiation among populations. For E. chloroticus, the differentiation between the two groups, corresponding to two habitat types, was most likely the result of local adaptation due to strong gradients in a number of specific environmental variables between outer and inner sites of the fiords. For C. muricata, the pattern of population genetic structure among the fiords likely indicates a secondary contact between a northern population and a southern one, separated by a contact or mixing zone. In addition, isolation by distance has likely caused the genetic structure revealed among the northern fiord samples.

Comparison of the two species have allowed me to better understand the interaction between physical and environmental factors as well as historical events and life history on the genetic structure of moderately motile species in the marine environment.