SUCCESSFUL EXTERNAL FERTILIZATION IN TURBULENT ENVIRONMENTS

Mathematical and experimental simulations predict that external fertil ization is unsuccessful in habitats characterized by high water motion . A key assumption of such predictions is that gametes are released in hydrodynamic regimes that quickly dilute gametes. We used fucoid seaw eeds to examine whether marine organisms in intertidal and subtidal ha bitats might achieve high levels of fertilization by restricting their release of gametes to calm intervals. Fucus vesiculosus L. (Baltic Se a) released high numbers of gametes only when maximal water velocities were below ca. 0.2 m/s immediately prior to natural periods of releas e, which occur in early evening in association with lunar cues. Natura l fertilization success measured at two sites was always close to 100% . Laboratory experiments confirmed that (i) high water motion inhibits gamete release by F. vesiculosus and by the intertidal fucoids Fucus distichus L. (Maine) and Pelvetia fastigiata (J. Ag.) DeToni (Californ ia), and (ii) showed that photosynthesis is required for high gamete r elease. These data suggest that chemical changes in the boundary layer surrounding adults during photosynthesis and/or mechanosensitive chan nels may modulate gamete release in response to changing hydrodynamic conditions. Therefore, sensitivity to environmental factors can lead t o successful external fertilization, even for species living in turbul ent habitats.