Critically Endangered elasmobranchs of Australia: insights into kinship, population genetics and symbionts

Jack D Ingelbrecht

Understanding the genetic composition and variability of populations is fundamental to developing effective conservation strategies and preserving biodiversity. Despite this, genetic information is rarely integrated into species assessments and management. One group of taxa that could benefit substantially from the integration of genetic data into conservation planning is the elasmobranchs, which are globally in decline, primarily as a consequence of intense fishing pressure. Improving our understanding of genetic factors in wild populations of elasmobranchs would not only lead to more effective conservation of sharks and rays, but also improve outcomes for any dependent species, such as parasites, via flow-on protection. Although parasites are the most abundant form of metazoan life on Earth, they are also among the most threatened and under-protected, and a loss of parasite diversity could have far-reaching consequences for the composition and function of ecological comunities. However, accounting for parasite taxa in estimates of coextinction remains a challenge due to the large portion of species that are unknown to science. There were two aims associated with this thesis. First, to assess kinship and genetic variability within key populations of two Critically Endangered batoid elasmobranchs, the green sawfish Pristis zijsron and the giant shovelnose ray Glaucostegus typus (Glaucostegidae), in order to gain insight into reproductive behaviour in sampled assemblages. Second, to characterise the ectoparasite community of these rays, in order to better understand the downstream community implications should either host become extinct. Furthermore, characterising the ectoparasite communities of P. zijsron and G. typus will provide the foundations for areas of future study, including rates of speciation, patterns in the distribution of parasite diversity and ecosystem health monitoring (i.e., use of parasites as bioindicators), which could lead to more effective conservation efforts for these hosts. Kinship and genetic variability were investigated using single nucleotide polymorphisms (SNPs), generated from genotype sequencing of tissue samples. Tissue samples were collected from P. zijsron in the Ashburton River and adjacent tidal creeks, and from G. typus in the southern Pilbara region, the Exmouth Gulf, and Shark Bay, Western Australia. Screenings for ectoparasites were also conducted from all fish captured in each of these areas. Kinship was inferred from pedigrees reconstructed from the genotypes of 60 P. zijsron and 90 G. typus. Three matrilineal generations were detected in P. zijsron reconstructed pedigrees, suggesting female philopatry to parturition sites. Only two generations of G. typus (i.e., parent and offspring) were present in reconstructed pedigrees. A total of 33 full sibling and 38 half sibling dyads were identified amongst sampled P. zijsron, including both maternal and paternal half siblings pupped in separate years, demonstrating polygamy in both sexes of this species, and suggesting philopatry and limited dispersal in female and male P. zijsron. Evidence of P. zijsron multiple paternity was also detected. By contrast, four full sibling and six half sibling dyads were identified amongst sampled G. typus, including a pair of half siblings where one individual was captured in the Exmouth Gulf and one captured in Shark Bay, over 600 km apart, providing evidence of long distance movement for G. typus. Population structure analyses indicated that sampled G. typus likely originated predominantly from a single breeding population. Observed heterozygosity and gene diversity for P. zijsron and G. typus were moderately low compared to other rays; levels of inbreeding were relatively low for P. zijsron and moderate to high for G. typus. Six parasite taxa were encountered on P. zijsron and G. typus during screenings, with four species common to both: Caligus furcisetifer (Copepoda); praniza larvae of unidentified gnathiid isopods; and Branchellion plicobranchus and Stibarobdella macrothela (Hirudinida). The remaining two species were both monogeneans and appeared to be hostspecific: Dermopristis cairae, encountered exclusively on G. typus, and Dermopristis pterophila n. sp., encountered exclusively on P. zijsron. Several intrinsic and extrinsic factors affected the abundance of encountered parasites: host total length and sex, freshwater river discharge, and sample region and season. The conservation implications for parasites are discussed, including how host-specific parasites are likely to be more endangered than their host, which could lead to an underestimation of the extinction risk faced by dependent parasites if their threatened status is defined solely by reference to the status of their host. This thesis will contribute to more effective management for these highly threatened elasmobranchs and their symbionts.

Critically Endangered elasmobranchs of Australia: insights into kinship, population genetics and symbionts

Files and links (1)

Abstract

Details

UN Sustainable Development Goals (SDGs)

Metrics