Colonies form encrusting fleshy patches with a velvety appearance. They are a light grey in colour.
In habitats where the water flow is strong, the colonies will overhang the substrate and develop into elaborate, elongated finger-like projections. Autozooids are variable in shape, approximately 0.4 x 0.4 mm in size and are interspersed by numerous conical kenozooids. These kenozooids protrude above the surface of the autozooids and impart a velvet texture to the colony.
This species occurs commonly around all British coasts.
A. hirsutum grows on a wide range of algal substrates, most commonly Fucus serratus and Chondrus crispus. They occur on sheltered shores and down into the shallow sublittoral, as deep as the algal substrates occur.
A. hirsutum broods its white embryos in a distinctive circlet within the autozooid. These are a useful distinguishing feature when present.