A Nation-wide Phylogenetic and Phylogeographic Investigation
of the Endemic New Zealand Oyster Borers, Haustrum scobina and Haustrum
albomarginatum
Authors: O'Mahony Seamus 
Published: March 2021

The endemic New Zealand sea snails Haustrum scobina and Haustrum albomarginatum are rocky shore intertidal dogwhelks of the Muricidae family. They have direct developing young and are carnivores. Their radula is used to drill into the shells of their prey, and they are commonly referred to as oyster borers. The taxonomic status of these species is still unresolved and therefore the name Haustrum scobina sensu lato is used.<br><br>The overall goal of this thesis research was to investigate the phylogeny and phylogeography of Haustrum scobina sensu lato using mitochondrial DNA sequences. Comparisons made to phylogeographic studies of ecologically similar species such as Cominella spp. provide an opportunity to identify the common environmental determinates of population migration route, genetic differentiation and speciation whenever similar patterns are found.<br><br>A nation-wide collection of samples was used to generate 277 new sequences from a 610 bp portion of the cytochrome c oxidase subunit I (COI) gene. This enabled the formation of a dataset of 654 DNA sequences, which was comprised of the 277 new sequences, 16 retrieved from a published study that deposited them in GenBank, and 361 from a previous unpublished thesis study. An unexpectedly diverse phylogeny of 58 COI haplotypes from 31 sample sites was recovered. These formed three clusters using K-means clustering by pairwise mutational distance. The in-group species did not form reciprocal monophyly groups, and the expected closest outgroup species (Haustrum haustorium) appeared to be as similar to the in-group clusters as they were to each other. A dataset of 27 DNA sequences from an 827 bp portion of the large sub-unit 28S nuclear rRNA gene was produced with the intention of corroborating the findings from the analyses of the COI dataset. This consisted of 26 new sequences and one sequence from a published study that deposited the sequence on GenBank. The expected taxonomic arrangement of Haustrum scobina sensu lato could not be matched by COI sequences due to incongruence with the 28S phylogeny and shell morphology. <br><br>The 28S dataset and shell morphology indicated there are two species in Haustrum scobina sensu lato. These are most likely Haustrum scobina and Haustrum albomarginatum, but they could not clearly be identified in the COI data. As a result, the phylogeographic certainty was limited when using the COI dataset because of the lack of clarity between the haplotypes of the two putative species. Possible reasons for the complicated COI dataset are discussed. Phylogenetic analysis of both the 28S and COI datasets did not support the expected conclusion that members of Haustrum scobina sensu lato are each other’s closest relatives. Haustrum haustorium was the expected immediate outgroup species but formed a polytomy with the in-group. A decrease in COI haplotype diversity was observed in southern samples when they were compared to the samples collected at northern locations. Taranaki sites shared a haplotype with multiple South Island sites that had no haplotype diversity. This suggested post-glacial re-colonisation of southern sites after displacement by ice-age conditions from these locations, a hypothesis consistent with results from studies of the Cominella genus. Association between Purau Bay in Lyttleton Harbour, Titahi Bay, Port Ahuriri and Kawau Island with no associated haplotypes between these locations suggested human-mediated translocation events. A genetic disjunction was also apparent between the south Wellington/Wairarapa coast and the eastern Wairarapa coast. This pattern was consistent with one study of Cominella maculosa and other studies have attributed similar patterns of other species in the region to recent uplift events affecting coastal community composition. The phylogeny of Haustrum scobina sensu lato will require further investigation before it can be used to more confidently resolve the phylogeographic history of the species.