Evolutionary biogeography and patterns of dispersal in the intertidal slug Onchidella (Systellomatophora: Mollusca)
Pulmonate slugs of the genus Onchidella commonly inhabit the intertidal rocky shores of southern continental landmasses (Australia, New Zealand, South America) and remote southern oceanic islands. Oceanic dispersal has been suggested to underpin the striking taxonomic similarities among these isolated southern populations. Such an explanation raises interesting questions about the ability of different taxa to disperse over transoceanic ‘barriers’ and the frequency and scales over which such connections occur. In this thesis, I use DNA sequences (COI, 16S, H3) and AFLP fingerprint data (215 loci) to elicit patterns of dispersal and connectivity in Onchidella spp. with contrasting reproductive development modes (direct-development vs. planktonic) and ecologies (kelp-dwelling vs. rock-dwelling). The existence of two widespread species (as opposed to four previously recognised) was inferred from these genetic data. Specifically, O. marginata (direct-developing/kelp-dwelling) occurs on east coast of New Zealand’s South Island, subantarctic islands and southern Chile; and O. nigricans (planktonic larvae/rock-dwelling) is present in southeastern Australia and New Zealand. Surprisingly, despite lacking a means of autonomous dispersal, O. marginata ranges over the broadest scales (up to > 8000 km), which I infer is facilitated by rafting on macroalgae (Durvillaea spp.) via ocean currents. Nevertheless, dispersal between populations is apparently infrequent enough to allow significant mesoscale genetic structure. In contrast, the planktonic O. nigricans showed very little population differentiation, even across the Tasman Sea, suggestive of regular and ongoing connectivity among populations. Reflecting the harsh climactic conditions experienced during the LGM, populations of O. marginata in lower latitudes show a possible signature of post-glacial recolonisation (i.e. low diversity and starburst pattern in mtDNA sequence data) compared to relatively diverse northern populations, or alternatively were recently colonised. In addition to trans-oceanic connectivity, O. marginata also shows evidence of fine-scale connectivity across soft-sediment ‘barriers’, with no genetic structure, although genetic patchiness was detected at very fine (metre) spatial scales in this species. In summary, dispersal - both planktonic and rafting - appears to have been fundamental in shaping the evolutionary biogeography of southern Onchidella and confirms the importance of ongoing marine biological connections among southern landmasses.
Advisor: Waters, Jon; Spencer, Hamish
Degree Name: Doctor of Philosophy
Degree Discipline: Zoology
Publisher: University of Otago
Keywords: Onchidella; planktonic; dispersal; kelp-rafting; colonisation; subantarctic; AFLP; mtDNA
Research Type: Thesis