Skeletal modification in response to flow during growth in colonies of the sea whip, Junceella fragilis

Sessile marine organisms depend on water motion for important physiological functions yet face dislodgement or breakage caused by hydrodynamic forces. During growth, these organisms are subjected to increasing bending moments as height increases and they may modify their mechanical supports accordingly. Here we used the sea whip Junceella fragilis as a model species to examine how sessile organisms modify their skeletal supports to cope with hydrodynamic forces during growth. Eighty-one colonies of J. fragilis (height 5–156 cm) were collected from two populations in southern Taiwan. Within-colony variations in skeletal elements, namely the axial skeleton and sclerites, as well as the coenenchyme and water content were investigated by measurements taken from the base, middle, and top of colonies. The typological distribution of sclerites within colonies was examined in another 31 colonies. The results showed that the relative weight of axial skeleton increases while that of sclerites decreases with colony height, which suggests that the colony switches from using sclerites to axial skeleton as the main support system during growth. The axial skeleton at the colony base thickens in such a way as to maintain or slightly decrease its bending stress. A greater density of sclerites, mostly double-heads, found at the colony base also adds to the resistance to bending. Moreover, colonies living in environments with greater flows seem to incorporate more skeletal materials. This study demonstrates how sessile marine organisms cope with increasing hydrodynamic forces during their life history by modifying the constitution and construction of their skeleton elements.