Hitchhiking consequences for genetic and morphological patterns: The influence of kelp-rafting on a brooding chiton

Abstract

Onithochiton neglectus is a morphologically variable, brooding chiton inhabiting coastal reefs throughout New Zealand and its Sub-Antarctic Islands. Southern O. neglectus populations are typically associated with buoyant kelp (Durvillaea spp.) and are potentially connected via kelp-rafting. Northern O. neglectus populations are less likely to raft, due to lower numbers of Durvillaea in northern New Zealand. To test for the impact of kelp-rafting on the spatial distribution of variation in O. neglectus, we undertook a combined analysis of morphological and genetic variation across the range of the species. Geometric morphometrics were used to assess shell shape. We detected a northern vs. southern split in shell shape, corresponding to the frequency of the O. neglectus/Durvillaea spp. association. To assess O. neglectus genetic patterns across New Zealand, we estimated phylogenetic trees with nuclear (ITS) and mitochondrial (COI and 16S) markers, which revealed distinct northern and southern lineages, and an additional lineage in central New Zealand. Neither the morphological nor genetic groups match existing O. neglectus subspecies, but are concordant with the patterns of association of O. neglectus with Durvillaea. We suggest that shell shape may be linked to O. neglectus’ regionally variable ecological association with kelp holdfasts.