Application of molecular techniques to the study of sawshark biology

Sawsharks are a distinctive group of demersal sharks characterised by a tapering, saw-like rostrum and a pair of long barbels attached to the rostrum’s ventral surface. Although their unusual appearance might be expected to attract attention, this group has been largely neglected in the scientific literature. In Australia, two species of sawshark, the common (Pristiophorus cirratus) and southern (Pristiophorus nudipinnis), have been frequent by-catch in commercial fisheries since their establishment in the early 20th century but little dedicated research has been conducted on these species. This is problematic as insufficient knowledge of biology prevents adequate assessment of conservation status and the development of effective management plans. This thesis aimed to provide new insights into the biology of sawsharks through molecular analysis and improve the knowledge surrounding this understudied group.

Molecular analyses were used to address several aspects of sawshark biology, including distribution, population structure and effective population size, and reproductive biology. To resolve uncertainties in the native range of P. cirratus, the cytochrome oxidase 1 (CO1) barcoding technique was used on a sawshark specimen collected outside the recognised range for the species to confirm its identity. In combination with historical catch data, the native range of P. cirratus was updated to include southern Queensland.

The population structure of P. cirratus and P. nudipinnis was assessed with mitochondrial and nuclear DNA (single nucleotide polymorphisms, SNPs). Both markers showed that P. cirratus consists of at least two distinct populations, whilst P. nudipinnis is comprised of only one population. Mitochondrial DNA was also used to investigate the demographic histories of these species and showed that the opening and closing of Bass Strait during the Pleistocene Era is likely to have caused population expansions in the past and separated P. cirratus into two mitochondrial lineages. Estimation of effective population size from SNP data indicated that both P. cirratus and P. nudipinnis could be at risk of losing genetic diversity in the long-term and need to be regularly monitored to ensure that their harvest remains sustainable.

Multiple paternity and hybridisation were investigated in P. cirratus and P. nudipinnis with nuclear SNPs. Analysis of single litters of each species demonstrated that both species exhibit multiple paternity. Additionally, tests for hybridisation on a P. nudipinnis individual which had the appearance of P. nudipinnis but the mitochondrial DNA of P. cirratus identified this individual as a first-generation hybrid.

The results from this thesis help fill knowledge gaps in the biology of P. cirratus and P. nudipinnis and are an important step in furthering our understanding of the biology of these unique elasmobranchs.