Settlement of abalone larvae ( Haliotis laevigata Donovan) in response to non-geniculate coralline red algae (Corallinales, Rhodophyta)

Abstract

Settlement trials with larvae of the abalone Haliotis laevigata and three species of non-geniculate coralline red algae (NCA) revealed a species-specific response. The number of settled larvae was significantly greater on Sporolithon durum than on two other species of NCA ( Mesophyllum engelhartii and Hydrolithon rupestre) co-occurring in the habitat. Settlement on Sporolithon durum commenced immediately after competent larvae were added. When offered a choice between two growth-forms of S. durum, larvae initially preferred the more complex lumpy growth-form to the encrusting growth-form, but this result was not significant after 48 h. This indicates that surface characteristics of the species influence settlement of Haliotis laevigata, but are not the main factors. When the algal thallus was damaged, the number of settled larvae was greater on damaged than on undamaged pieces of Sporolithon durum and Mesophyllum engelhartii, but less on damaged pieces than on undamaged pieces of Hydrolithon rupestre. Settlement on Sporolithon durum was also significantly greater when the photosynthetic pigments in the outermost cells of the thallus were present. These results indicate that inducers of settlement are highly variable and dependent on both the NCA species and species-specific characteristics. No larvae settled on the bottom or sides of the jars, suggesting that the inducers of settlement may not be soluble in water after their release from the algal thalli. Alternatively, inducers are not released in a large enough concentration, even when the algal thallus has been damaged. We conclude that the inducers are algal in origin. Larvae of the abalone Haliotis laevigata do not settle gregariously in response to recently-settled conspecific larvae. Gregarious settlement behaviour was, however, observed with 7-day-old conspecifics.