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ABSTRACT: 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 deriv-ing 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 cyclosys-tems 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 com-petition in marine benthic organisms. The colonies of Stylaster sp. grow using both strategies resulting in the asexual repro-duction 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. I N T R O D U C T I O N Stylasterid corals are colonial hydroids characterized by a rigid skeleton composed of calcium carbonate (coenosteum). Their colonies are generally erect and branched and only the Pacific genus Stylantheca shows an encrusting habitus. The colony is polymorphic, with gastrozooids and dactylozooids retractable into particular skeleton depressions named respectively gas-tropores and dactylopores (Moseley, 1881; Broch, 1942; Cairns, 1983; Bouillon et al., 2006). The distribution of gastro-zooids and dactylozooids over the colony may be irregular or limited to certain regions. In several genera such as Stylaster, they are arranged in circles (cyclosystems) where one gastro-zooid is surrounded by several dactylozooids and a central style (gastrostyle) is present on the bottom of the gastropore cavity (Moseley, 1881; Broch, 1942; Cairns, 1983; Bouillon et al., 2006). The gonophores develop inside internal or superficial globose skeletal structures named ampullae. The sperm are released from the male ampullae and enter into the female one; when mature the planula arises from the ampulla and settles on a suitable substrate leading to the formation of a new colony (Moseley, 1878; Hickson, 1890; England, 1926; Broch, 1942; Ostarello, 1973, 1976; Fritchman, 1974). The early development stages of the new coenosteum after the planulae settlement are known only in few species. Ostarello (1973) studied the natural history of Stylaster californicus (cited as Allopora californica) observing that, after its release, the planula generally crawls for a short time around the parental colony settling close to it. Nevertheless, some new colonies are born well away from the female, demonstrating that a certain degree of dispersal before settling is possible (Ostarello, 1976). A newly settled colony is flat. However, traces of the depression where the first cyclosystem will form are visible even at this early stage (Ostarello, 1973). Recruits of Stylaster roseus, showing gastropores and dacty-lopores, were recorded for the first time on experimental carbonate tiles in the Mexican Caribbean (Ruiz-Zárate & Arias-González, 2005). The most exhaustive study, concern-ing the planula structure, its metamorphosis and the develop-ment of the early stages of the colony, was carried out by Fritchman (1974). He observed that the larva of the encrusting species Stylantheca petrograpta (cited as Allopora petrograpta) attaches through the anterior side and rapidly flattens into a disc 1.5 –2 mm in diameter. A series of roughly concentric elevations appear on the upper surface two hours after the attachment. Once they disappear, a large number of internal vesicles arise within the ectoderm that is meanwhile emerging through the involution of the lower surface. The vesicles gradually disappear leaving the endoderm organized into a reticulum of cords sheathed with ectoderm. Skeletal material appears between the ectodermal layers within twenty-four
Journal of the Marine Biological Association of the UK 01/2010; 90(6):1145–1151. · 1.00 Impact Factor
