The comparative phylogeography of shore crabs and their acanthocephalan parasites

Abstract

Comparing the genetic structure of host populations with that of their parasites can shed light on the efficiency and independence of their respective dispersal mechanisms. The degree of congruence between host and parasite genetic structure should reflect to what extent they share dispersal mechanisms. Here, we contrast the genetic structure of the acanthocephalan parasite Profilicollis novaezelandensis with that of its intermediate host, the hairy-handed shore crab Hemigrapsus crenulatus, along the east coast of New Zealand’s South Island. We expected no congruence in their phylogeographic patterns because of the very different modes of dispersal used by the crabs (planktonic drift) and the acanthocephalans (bird-mediated dispersal). Based on analysis of cytochrome oxidase subunit I gene sequences, we found no significant genetic structure among isolated populations of the crab and those of their parasite, along a roughly 600 km stretch of coastline. Surprisingly, based on a distance-based co-evolutionary analysis statistical tool (PACo), we observed an overall significant level of congruence between host and parasite population-level phylogenies. The most parsimonious interpretation is that statistical significance does not translate into biological significance, with the result likely due to chance, possibly because bird movements that disperse parasites coincidentally match patterns of crab dispersal by ocean currents in parts of our study area. In this system, the connectivity among different localities and the apparent genetic mixing among populations may have implications for host–parasite co-evolution.