The role of larval settling behaviour in determination of the specific habitat of the hydrozoan Dynamena pumila (L.) larval settlement in Dynamena pumila (L.)

Dynamena pumila (L.) lives on the brown alga Laminaria saccharina and Fucus serratus, in areas exposed to waves and currents. An experimental study of settling behaviour and metamorphosis of the planulae was conducted. Reconstruction of larval settlement showed that the colonies of D. pumila developed and fed in high flow environments, due not only to various adaptations of their morphology, but in great extent, due to the specific behaviour of larvae at, and prior to settlement and metamorphosis. Positive phototaxis by newly liberated planulae of D. pumila leads to colonization of illuminated zones on fucoid thalli and laminarian stipes and blades, where they metamorphosed in response to microbial films covering the algal surface. Crawling planulae of D. pumila were far more resistant to rapid water flows than the planulae of other hydroids, what allows this hydroid to establish its colonies in places with strong water movements, where the other species could not develop. Temporary exposure to air facilitated attachment and metamorphosis of the planulae, which probably settle and undergo metamorphosis during the periods of low-tidal emersion.