Calcified, fragile, white, pink or purple; repeatedly branched. Branches fine, circular in cross section, jointed. A. fragilissima more delicate, dichotomously branched segments shorter than 1 mm and slightly swollen at each end. Joints of A. rigida longer than 1 mm and not swollen at ends. Isolated clumps in exposed places; great carpets in shallow sand coves; also common on patch reefs.