A calcareous encrusting species, sea mats form star-shaped or broad sheet colonies on the fronds of large algae, small irregular patches on stones and shells, narrow tufts (independent of the substratum), or cylindrical encrustations around the fronds of small red algae. The zooids are ovate to oblong. Approximately one half of the front of the zooid is calcified but translucent and perforated by large pores that leave a distal oval, membranous, frontal area surrounded by four to 12 (often nine) spines. Spines vary in length, but the median, proximal spine is always present and usually larger than the others. In some cases the proximal spine becomes well developed and longer than the zooid, giving the colony a hairy appearance.
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