Could possibly be confused with A. mamillatum, however A. parasiticum is dark brown where A.mamillatum is yellowish.
Colonies form a thick greyish brown cylindrical encrustation around the stems of hydroids and erect bryozoans. The colour of the colonies is largely due to the build up of fine silt and mud particles on the colony surface. When these particles are brushed away it is possible to see numerous hair like-filaments that are likely to be trapping the sediment particles. The borders of the autozooids are often unclear but the zooids appear to be roughly hexagonal in shape. They measure approximately 0.2 mm in diameter.
A. parasiticum is a sublittoral species and has been recorded around the British coastline from Northumberland, the Clyde Sea, the Isle of Man, Menai Strait, Lundy and East Anglia. Recent CEFAS surveys found it occurring abundantly in the southern North Sea.
A. parasiticum is often found growing on large hydroids such as Hydrallmania falcata and Tubularia indivisa. It can also be observed on colonies of the erect cheilostome bryozoan Cellaria and more unusually on the carapace of decapod crustaceans.
Little is yet known of its reproductive biology though specimens with an ITO were recently reported from the coast of Belgium.