Barnacle settlement: Field experiments on the influence of larval supply, tidal level, biofilm quality and age on Balanus amphitrite cyprids
A set of 3 field experiments lasting 24 h was conducted during April 1998 at the Duke University Marine Laboratory (Beaufort, North Carolina, USA) to: (1) assess the influence of larval supply, intertidal height, quantity and quality of biofilm and age of the larvae on the settlement of Balanus amphitrite Darwin and (2) examine the correspondence between small-scale planktonic distribution of larvae, the initial spatial pattern of newly settled larvae and the vertical distribution of adult barnacles. Precolonized methacrylate (Plexiglas) disks, arranged within 3 blocks and established so as to eliminate edge effects within 3 large experimental panels, were placed at 3 predetermined tidal heights (High, Medium, Low) corresponding to the upper limit, modal zone and the lower limit of adults of B. amphitrite. Split-split-plot ANOVAs were performed on densities of newly attached larvae (metamorphosis not completed) to test their habitat selection behavior to surfaces which had been precolonized by microbiota (bacteria and diatoms) at 3 heights (origin factor) for 0, 7, 14 or 21 d (age factor). The physical environment (salinity, temperature, current flow) was stable and comparable during the 3 experiments. B. amphitrite cyprids were uniformly distributed in the water column. Larval supply was poorly correlated with the intensity of settlement over the 1 wk experimental period. In fact, the same larval supply could induce either high (4x) or low (1x) settlement after 2 tidal cycles, and, inversely, similar settlement intensities were associated with planktonic larval abundance varying significantly at 2 d intervals (109 to 171 cyprids 923 l-1). Settlement was homogeneous on each experimental unit (no significant block effect). Tidal height, however, was a significant factor in determining the vertical patterns of newly settled larvae during the first experiment where larvae were abundant but not during subsequent experiments for which fewer larvae were collected. The degree of microbial precolonization was the main parameter affecting the settlement of B. amphitrite. For the first 2 experiments, 'weighed cyprid settlement' significantly decreased as the age of the biofilm increased, revealing a strong preference of settlers for clean surfaces and avoidance of biofouled surfaces of all intertidal origins. Further analysis of biofilm samples showed that free-space availability in the microbial film and bacterial densities were significantly inversely correlated to settlement intensity. Moreover, settlement to 'favorable' substrata decreased by nearly 1/2 during our experimental period, suggesting changes in the selectivity of settling larvae. Our experiments confirm the role of larval supply in determining the vertical intertidal distribution of adults of B. amphitrite, but the short-term variability in the larval supply/settlement coupling observed over a 1 wk period must be integrated in models of recruitment dynamics of barnacles.
Olivier, F; Tremblay, R; Bourget, E; Rittschof, D
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