Morphology and development of the early growth stages of an Indonesian Stylaster (Cnidaria: Hydrozoa)

The aim of this work is to describe the skeletal morphology of the early stages of the colonies of Stylaster sp., settled on artificial panels placed along a coral reef in the Bunaken Marine Park (North Sulawesi, Indonesia). The youngest observed stage deriving from the planula settlement is represented by a well developed primary cyclosystem symmetrically budding two secondary cyclosystems in a very early phase of growth. Successively each cyclosystem starts its vertical growth producing new cyclosystems sympodially arranged. At the same time the basal disc of the first cyclosystem enlarges producing a flattened crust from which new colonies arise. Vertical and encrusting forms are considered different modalities to face the problem of space competition in marine benthic organisms. The colonies of Stylaster sp. grow using both strategies resulting in the asexual reproduction of several colonies from a single settled planula. The new cyclosystems originate as small volcano-like elevations having the centre filled with calcium carbonate. Gradually this calcareous matrix dissolves starting from its perimetric zone and leads to the formation of the gastrostyle and the ring palisade. It is plausible that the cells composing the stolon nets deeply pervading the calcareous structure are responsible for this skeletal plasticity.