Genetic structure of the grey side-gilled sea slug (Pleurobranchaea maculata) in coastal waters of New Zealand

Pleurobranchaea maculatais a rarely studied species of the Heterobranchiafound throughout the south and western Pacific (and recently recorded in Argentina) whose population genetic structureis unknown. Interest in the species was sparked in New Zealand following a series of dog deaths caused by ingestions of slugscontaining high levels of the neurotoxin tetrodotoxin. Here we describe the genetic structureand demographic historyof P. maculata populations from five principle locations in New Zealand based on extensive analyses of 12 microsatellite loci and the COI and CytB regions of mitochondrial DNA (mtDNA). Microsatellite data showed significant differentiation between northern and southern populations with population structure being associated with previously described regional variationsin tetrodotoxinconcentrations. However, mtDNA sequence data did not support such structure, revealing a star-shaped haplotype network with estimates of expansion time suggesting a population expansion in the Pleistocene era. Inclusion of publicly available mtDNA sequence from Argentinian sea slugsdid not alter the star-shaped network. We interpret our data as indicative of a single founding population that fragmented following geographical changes that brought about the present day north-south dividein New Zealand waters. Lack of evidence of cryptic species supports data indicating that differences in toxicity of individuals among regions are a consequence of differences in diet.