The Phylogeography and Demographic History of the Direct-Developing Coastal Gastropod Haustrum haustorium
Life history traits can be influential in a species’ evolution, particularly in regards to dispersal ability. Studies have attempted to find a generalized link between particular life history traits and levels of population genetic structure. In theory, organisms with life history traits such as having a pelagic larval duration will result in high levels of gene flow among populations. Often, this is not the case. In some species, high dispersal ability is not a good predictor for the level of genetic structure, and they are found to have highly structured populations. An interesting case to consider is that of direct-developing organisms which, theoretically, have a low dispersal ability and to consider whether this is typically associated with a high degree of population genetic structure. This question has been the topic of several recent phylogeographic studies of species found around the New Zealand coast. The overall goal of this thesis research was to investigate the phylogeographic structure of another similar direct-developing coastal species. The Haustrum genus is a group of four coastal gastropod species endemic to New Zealand and is composed of four species Haustrum haustorium, H. scobina, H. albomarginatum, and H. lacunosum. H. haustorium is the primary focus of the present study. H. haustorium is an endemic direct-developing carnivorous marine mollusc found in moderate abundances on rocky intertidal coasts across most of New Zealand. Due to its limited dispersal ability, direct-developing nature, and wide distribution across New Zealand H. haustorium is an ideal candidate for phylogeographic analysis. H. lacunosum is similar to H. haustorium in that it is a coastal gastropod with limited dispersal ability and is thought to be a direct developer, but has a smaller distribution limited to the South Island and other offshore islands.
This study determined 310 new partial cytochrome c oxidase subunit 1 (COI) mitochondrial DNA (mtDNA) sequences and used them to investigate the levels of genetic diversity and phylogeographic structure of H. haustorium. Eight partial COI sequences were collected to make some precursory observations about the mtDNA marker in H. lacunosum. These sequences were combined with the pre-existing COI data for H. scobina and H. albomarginatum reported previously by Logan (2019) and O’Mahony (2021). This combined dataset formed a genus-wide dataset totalling 984 partial COI mtDNA sequences.
The results of the phylogeographic analysis of H. haustorium are unusual when compared to other coastal gastropod species. Results from H. haustorium show that all populations, except those on the east and southern coasts of the North Island, had significant levels of genetic differentiation between them. Two common haplotypes, one eastern and one western, have become widespread along the respective coastlines of both the North and South Island. This east-west genetic discontinuity is somewhat unusual in similar New Zealand coastal taxa, which are more likely to exhibit a north-south split. This study finds evidence that H. haustorium had a relatively stable population size throughout the last glacial maximum (LGM), contrary to other coastal gastropod species, which tend to show genetic evidence of a population contraction during the LGM and subsequent population expansion. This supposed population resilience could be caused by various physiological traits and feeding behaviours of H. haustorium that allowed it to persist in most locations despite the harsh conditions during the LGM. Localized extinctions and subsequent recolonisation or bottleneck events explain the unusual genetic structure seen in H. haustorium better than a species-wide population contraction to lower latitude locations during the LGM. The phylogeny of the Haustrum genus reported here attempts to also address a highly divergent clade of COI haplotypes reported in unpublished H. scobina and H. albomarginatum sequences collected and analysed by Logan (2019) and O’Mahony (2021) The findings reported in this study add to the growing number of studies that have now been conducted which investigate the phylogeographic structure of direct-developer marine snail species. This study shows that there is much variability in the patterns that these types of species have, and this reinforces the complexity of the natural world and the evolutionary forces that act to shape population genetic structure. This study also demonstrates that the responses of organisms to historic events like the LGM are species specific, and that genetic structure is not a result of one sole factor like life history. Instead, factors like demographic history and physiological traits like resilience to extreme conditions should also be taken into account when investigating genetic structure in a species.