Biomechanics and mass mortality of erect bryozoans on a coral reef

The bryozoan Cigclisula sp., is a heavily calcified erect species which occurs abundantly in the shallow sublittoral coral reef at Quilaluia Island, Quirimba Archipelago, northern Mozambique. It grows in an arborescent bilaminate form termed adeoniform on the undersides of coral heads or other available bioconstructed hard substratum. The geography and bathymetry of the area results in periodic high and complex current regimes often carrying water borne debris. If this periodic force overcomes the structural integrity of Cigclisula sp. either the outermost branches break or the colony topples at the base. The local population was dominated by young individuals: nearly 40% of the sample population had ten branches or fewer and less than 10% had more than 50 branches (these could be ascertained as young due to the lower proportion of damaged colonies with less than or equal to 10 branches; Figure 1). The only size group with no damaged specimens (branch breakage) were those with less than or equal to 5 branches. The mean proportion of branches broken significantly increased with increasing colony size. After one year a population of Cigclisula sp., initially comprising 30 colonies (encompassing the entire size spectrum of colonies), consisted of just one small specimen.