Population structure and dispersal of the spiny lobster (Panulirus ornatus) across the Indo-West Pacific

Abstract

Panulirus ornatus is a highly-valued tropical spiny lobster species with an extensive Indo- West Pacific distribution, ranging from the east coast of Africa through to the central Pacific Islands. The species is intensively fished throughout its range to supply a rapidly growing Asian seafood market, where colourful large lobsters are highly prized as a luxury seafood. Post-larval and juvenile lobsters are also intensively harvested as seed to supply a burgeoning lobster aquaculture industry that began in Vietnam and is quickly spreading, especially in Southeast Asia. With intense fishing pressure on wild stocks of P. ornatus, and the prospect of new technology for hatchery-raising this species, there is a need to better understand the genetic structure of the global population of this species to improve management prospects. Therefore, this study aimed to use molecular genetic techniques to determine the population structure, connectivity and patterns of seascape genetics in P. ornatus throughout its global range. A total of 298 specimens were sampled from 17 locations covering the West Indian Ocean, Western Australia, Southeast Asia, and the Northwest and Southwest Pacific. From sequencing a 461 bp fragment of mitochondrial control region from these samples, high levels of haplotype diversity (H=0.99) and overall significant population differentiation (Φst = 0.113, P < 0.001) was found. The West Indian Ocean and Western Australia populations were the most genetically divergent, with little differentiation among the remaining Southeast Asian and West Pacific populations. The Western Australia population showed evidence of being a peripherally-isolated population, with significantly lowered effective population size and asymmetric gene flow, likely due to prevailing ocean currents. Thirteen polymorphic microsatellite loci were also used to examine fine-scale genetic structure among the samples of P. ornatus from throughout its range. Significant overall population differentiation was observed (F’ST = 0.051, P < 0.001). The patterns of microsatellite variation confirmed that the Western Australian population was significantly diverged with reduced genetic diversity, likely due to a bottleneck, and also suggested a previously unrecognised genetic divergence of the New Caledonia population. The previously identified strong mitochondrial DNA divergence of the West Indian Ocean populations was only minimally supported in the microsatellite data, suggesting recent gene flow across this region has eroded past lineage divergence. Overall, the microsatellite data supported genetic homogeneity of the central Southeast Asian populations, with varying degrees of genetic divergence and reduced diversity of a number of peripheral populations. Seascape genetic analyses were conducted by testing observed patterns of genetic variation for association with environmental variables in order to gain some initial insight into the possible environmental processes that shape the population genetic structure of P. ornatus. The results indicate that common physical and environmental variables, especially sea surface temperature and convergence, may be involved in structuring the populations. Furthermore, life history characteristics of the species, in combination with environmental factors, appear to play a role in driving the small scale patterns of genetic variation seen within the population of this species. Overall, this research provides additional insights into the processes structuring marine populations that extend across the Indo-West Pacific marine environment, and those processes contributing to spiny lobster population structure. The results from this thesis provide valuable information to assist in more effective fisheries management and commercial aquaculture development of this important spiny lobster species. Several genetically independent stocks are identified, each of which should receive separate fisheries management. The presence of multiple genetically diverged lineages also offers the possibility of differentially adapted genetic lines for future breeding programs. This highlights the potential commercial importance of the conservation of these existing stocks.