Hydrodynamically induced synchronous waving of seagrasses: 'Monami' and its possible effects on larval mussel settlement

Underwater observations of eelgrass (Zostera marina L.) beds at the mouth of the Jordan River, Maine, USA, indicated that the eelgrass blades gently undulated with low-amplitude movements under low current speeds. When the above-canopy speeds exceeded 10 cm s(-1), dramatic large-amplitude waving of many blades in synchrony occurred. The eelgrass waving caused wide variability in horizontal water current speeds measured above the canopy. During three summers (1986, 1989, 1990), the blade tips (distal 30 cm) had an average of about 3 x more recently settled (plantigrade) blue mussels (Mytilas edulis) compared to regions lower (30-90 cm) on the blade. Because waving of seagrass blades results in the blade tips moving through much more of the water column than lower regions of the blade and in enhanced turbulent mixing above the plant canopy, we hypothesize that such movements increase the likelihood of blade encounter with mussel larvae, and explain enhanced mussel abundances on blade tips. We further hypothesize that the enhanced mixing may direct larvae into seagrass beds generally. Large-amplitude, synchronous waving of terrestrial grasses has been termed 'honami,' (Japanese: ho = cereal; nami = wave) and has been shown to dramatically alter aerodynamical conditions within and above the grass canopy. We suggest that 'monami' (mo = aquatic plant) is important in coastal hydrodynamics and has major implications for larval settlement and recruitment.