An integrative approach to understanding the population structure & dispersal patterns of two commercial octopus species (Octopus maorum & Octopus pallidus)

The population structure and dispersal patterns of two commercially harvested octopus species, Octopus maorum and Octopus pallidus, were examined using a combination of natural elemental stylet signatures and genetic microsatellite markers. The early life history strategies of the two species are markedly different: O. pallidus produce large well-developed benthic hatchlings (holobenthic) and O. maorum produce small planktonic paralarvae (merobenthic). Such differences will influence the species’ dispersal potential and population structure and thus resilience to fishing pressure. Both species were collected from several sample sites in Tasmania (Australia), and O. maorum was also collected from South Australia (SA) and New Zealand (NZ). The spatial distribution of elements within octopus stylets (a small internal remnant ‘shell’), was investigated using the nuclear microprobe. Proton induced x-ray emission (PIXE) was conducted using the Dynamic Analysis method and GeoPIXE software package, which produced high resolution, quantitative elemental maps of whole O. pallidus stylet cross-sections. The analyses indicated that Ca was a suitable internal standard for laser ablation inductively-coupled plasma-mass spectrometry (LA ICPMS), due to its homogeneous distribution and consistent concentration between individuals. Elemental signatures representing the early life history region of the stylet were used to investigate connectivity and the common origins of adults. Using LA ICPMS stylets were analysed for 12 elements, several of which were excellent spatial discriminators. There was evidence of sub-structuring within the O. maorum population despite the species’ high dispersal potential. Individuals from an aggregation in south-east Tasmania were particularly distinct and appeared to share a local common viii origin. Octopus pallidus showed a relatively high level of population structure with all samples appearing distinct from each other, which is in accord with the species’ limited dispersal potential. The stylet signatures of O. pallidus hatchlings were also analysed from three locations (collected 6 – 10 months prior to the adults), to determine if they could classify adults back to their natal site. Although hatchling signatures showed significant spatial variation, they were unable to be used as markers of natal origin. Microsatellite markers and morphometrics were also employed to investigate the population structure of O. maorum, as they can provide information relevant to longer-term inter-generational structural patterns. Five polymorphic microsatellite markers were isolated from O. maorum DNA, and then used to investigate levels of gene flow and genetic structure. Overall, within-sample variability was very high (mean number alleles = 15, mean expected heterozygosity = 0.85). Multi-locus pairwise FST values revealed a significant level of structuring, which did not fit an isolation-by-distance model of population differentiation. Divergence was observed between most populations, except for SA and the southern Tasmanian populations which were genetically homogeneous, indicating a level of connectivity on a scale of 1,500 km. Morphometrics indicated divergence between Australian and NZ populations. The structural patterns identified can be explained largely in relation to the regional oceanographic features. This study presents valuable insights into the population structure and dispersal patterns of both a merobenthic and holobenthic octopus species, and will provide essential information for the sustainable management of O. maorum and O. pallidus. Additionally, this study shows that the targeted elemental analysis of stylets will be a beneficial new tool for examining octopus populations, and that the utilisation of both genetics and elemental signatures is a robust and powerful method to investigate population linkages in marine species