Physiological condition of Balanus amphitrite cyprid larvae determines habitat selection success

A field experiment was used to assess the relationship between the physiological condition of barnacle larvae and habitat selectivity at settlement. Three experiments were carried out on methacrylate (Plexiglas) disks precolonized with biofilms of different ages (0, 7, 14 and 21 d) that were arranged within 3 blocks and placed at 3 intertidal positions corresponding to the upper limit, mid zone and lower limit of the adult Balanus amphitrite (Darwin) habitat. To limit the effect of post-settlement mortality, each experiment was conducted over 2 full tidal cycles only. During these periods, larvae in the water column were sampled and abundance estimated at each high tide at the depths corresponding to the 3 interticlal positions. Larval physiological condition was assessed using lipid classes analyzed by liquid chromatography. Ratios of total triacylglycerols to sterol content (TAG/ST) were used to quantify the energy status of larvae. Our results indicate that the cyprids' TAG levels reflect the level of settlement success independent of cyprid abundance in the water column. Intertidal position and biofilm characteristics were significant factors determining the settlement success with respect to cyprid energy content. The number and the energy content of settling cyprids were maximal on clean surfaces and decreased gradually with the degree of biofilm precolonization. Our study shows (1) the importance of lipid levels, particularly TAG, in habitat discrimination at small temporal scales in B. amphitrite cyprids and (2) that larval energy content is a critical variable for understanding benthic–pelagic coupling.