FUNCTION OF FUNNEL-SHAPED CORAL GROWTH IN A HIGH-SEDIMENTATION ENVIRONMENT

Advantages and disadvantages of a funnel-shaped growth in 2 coral spec ies (Acropora clathrata, Turbinaria peltata) in a high-sedimentation e nvironment (Natal, South Africa) were observed in the field and modele d in a flow tank. Funnel-shaped growth serves different purposes in di fferent hydrographic settings. In calm waters with little currents (in our case deep reef areas, 18 to 25 m) funnel-shaped colonies served a s 'sacrificial sediment traps': all sediment trapped inside the funnel was directed towards the centre, where it was concentrated. There, ti ssues underwent necroses, but all other tissues remained sediment free and healthy. In areas with high currents (in our case shallower reef areas with high surge, 8 to 14 m) funnels tended to be self-cleaning. By a process of vortex shedding, mass replacement of fluid within the funnel also led to the removal of all sediment. Current speeds between 30 and 90 cm s(-1) were enough to clean the funnels of 3 experimental grain sizes (coarse, fine, medium sand).