Tandem host‐parasite dispersal inferred from similarities in phylogeographical patterns among Little Penguins and their ‘terrestrial’ ectoparasites

Aim: Organisms with poor intrinsic dispersal capacity, such as parasites, often rely entirely on transport with host species that have a greater dispersal capacity. Penguins, for example, are exploited by terrestrial ectoparasites when they come ashore to breed. Recent research indicates that Little Penguin (Eudyptula minor and E. novaehollandiae) hard ticks (Ixodes eudyptidis and I. kohlsi) may be capable of surviving short periods (days) at sea with their hosts, but their capacity to survive longer voyages (weeks) is not known. We here aimed to assess whether phylogeographical patterns in Little Penguins and their ticks indicate that the terrestrial ectoparasites are able to disperse long distances at sea with their swimming hosts. Location: Southern Australia and New Zealand. Taxon: Ixodes eudyptidis and I. kohlsi ticks. Methods: We conducted a broad-scale genomic assessment of Little Penguin ticks from across their hosts' ranges in Australia and New Zealand. Using genotyping by sequencing, we generated SNP data sets from ticks from 14 penguin colonies, and analysed phylogeographical structure. We included ticks from some sympatric flighted seabirds to verify host specificity. Results: We resolved two distinct lineages of Ixodes from Little Penguins, with one restricted to Australia, and the other found throughout New Zealand and in low numbers at some eastern Australian sites. Both lineages exhibited phylogeographical structure consistent with patterns observed in their hosts, with some evidence of occasional oceanic dispersal, including across the Tasman Sea between Australia and New Zealand. Ticks from sympatric short-tailed shearwaters (Ardenna tenuirostris), which disperse aerially, were genetically distinct from those collected from Little Penguins, supporting prior evidence of host specificity in seabird ticks. Main conclusions: The most parsimonious explanation for our results is that ticks can travel at sea with Little Penguins. We infer that some terrestrial ectoparasites associated with aquatically dispersing hosts have evolved the capacity to survive oceanic voyages.