Relative importance of natural cues and substrate morphology for settlement of the New Zealand Greenshell™ mussel, Perna canaliculus

Production from aquaculture of the New Zealand Greenshell™ mussel, Perna canaliculus, is worth more than US$190 M annually. However, expansion of the industry is currently limited by the number of naturally occurring seed available for growout. Previous work suggests that natural chemical cues and settlement surface topography may be important factors in the settlement and retention of P. canaliculus larvae. This study investigated the effects both of these factors separately and in conjunction on the settlement of P. canaliculus larvae under laboratory conditions. Eight species of marine algae (5 red and 3 brown) and one species of hydrozoan that have high numbers of mussels attached in the field were targeted as sources of chemical cues. In laboratory assays, non-polar extracts from four of the eight species (Osmundaria colensoi, Ballia callitricha, Melanthalia abscissa and Scytothamnus australis) increased settlement relative to controls, although differences were non- or marginally significant. Polar extracts generally decreased settlement relative to controls. Further assays using a concentration gradient of non-polar extracts of these same four species showed weak positive effects on larval settlement. The four non-polar extracts were further fractionated and subsequently tested on both rough and smooth surfaces. Settlement was significantly higher on rough compared to smooth surfaces either with, or without extracts. Although the application of extracts did not significantly enhance settlement, the hexane and 10% EtOAc:hexane fractions of S. australis had a positive effect on settlement when added to rough surfaces. The strong effect of surface structure, and the weak effect of natural chemical cues, supports future studies concentrating on the manipulation material surface properties and surface structure to enhance the settlement of larvae and retain Greenshell™ mussel spat.