Unravelling the mystery of the shark genus Mustelus in southern Africa using a multidisciplinary approach

[摘要] ENGLISH ABSTRACT: Multidisciplinary approaches have previously offered some alternative innovative ways of addressing classical ecological questions while providing novel insights into behaviour and biology of elasmobranch species. The species-rich shark genus Mustelus, or smoothhounds (smoothhound sharks), is one of the most bio-economically important groups of elasmobranchs in the world's oceans. Despite the commercial value of Mustelus, its systematics remains largely unresolved and the knowledge on the copulating and dispersal strategies of species of Mustelus is scarce. Here, a multidisciplinary approach – molecular, morphology and histology – with different methods of analysis on various spatial scales was used.First, this study investigated the evolutionary origin of the shark genus Mustelus in southern Africa using molecular phylogenetic and statistical biogeography approaches. Results gave strong support for a northern hemisphere origin of southern African Mustelus species, and that the radiation of Mustelus in this region was primarily driven by long-distance dispersal. The monophyly of expanded Mustelus indicated that southern African species of the genus arose from at least two separate colonisation events from the Northern Hemisphere.On a microevolutionary scale, a comparative population genetics approach was used to gain insight into spatial genetic structure and dispersal patterns in species of Mustelus (M. mustelus and M. palumbes) and other co-distributed demersal sharks (Galeorhinus galeus and Triakis megalopterus) characterised by assorted life histories, habitat preferences, and dispersal behaviour. Based on novel Next Generation Sequencing (NGS)-mined microsatellites, the null hypothesis of genetic homogeneity was rejected for all species investigated except for T. megalopterus. Most noteworthy is that the coalescent analysis of migration supported asymmetric gene flow from the Indian to the Atlantic Ocean, concordant with the Atlantic Ocean–Indian Ocean connection via Agulhas leakage proposed for many marine species along the South African coast.In terms of fisheries forensics, a dermal denticle identification key guide and two molecular assays (a microsatellite panel and High-resolution melting assay) were successfully developed for species identification of southern African Mustelus (M. mosis, M. mustelus and M. palumbes) and three other shark species (Galeorhinus galeus, Scylliogaleus quecketti and Triakis megalopterus) commonly confused with species of Mustelus in the region. Additionally, a SNP discovery and genotyping pipeline was optimised that could in future be used to obtain genome-wide data that will enable population genetic and demographic processes of the study species to be assessed more accurately. Lastly, evidence of sperm storage in female common smoothhound sharks was reported for the first time using a histological approach. The molecular analysis of a single common smoothhound litter also hinted at the within-species variation in the presence and frequency of multiple paternity previously reported for elasmobranchs.Overall, this study provides the most comprehensive set of conservation genetic resources for the common smoothhound shark to date. The results provide novel insights into the conservation biogeography, species identification and ecology of dispersal as well as mating behaviours in species of Mustelus. This will help inform existing and ongoing management and conservation efforts for smoothhound sharks occurring in southern Africa.

[发布机构] Stellenbosch University