A gelatinous, non-calcified bryozoan that is yellow or pale brown in colour. Colonies are stalked and stand erect from the substrate. The colony grows in finger-like lobed branches, and frequently exceeds 30 cm in height. When undisturbed, small white lophophores emerge from the surface. A. diaphanum is widely distributed and abundant in the coastal waters of the British Isles and the southern North Sea. Colonies are often dragged up in fishing nets, especially in the "weedy" areas of Dogger Bank, where this bryozoan is particularly abundant.

Colony erect; cylindrical, laminar or irregularly branched, firm textured, up to 30 cm tall. Body wall of autozooids bounded by a thickened epicuticle, folded and papillate, with adjacent folds fusing to enclose exterior spaces, incorporating them within the colony structure. Autozooids with papillate orifices opening on all surfaces of the colony, the polypides orientated perpendicular to colony surface. Colony growth proceeds through intercalary frontal budding. Lophophore with 14 to 16 tentacles. Embryos lecithotrophic, brooded in multiples within the maternal autozooid. No intertentacular organ. (Porter et al, 2001).

May be confused with Alcyonidium hirsutum.   A. hirsutum tends to be less gelatinous and will not directly attach to rock.

A. diaphanum colonies exhibit morphological plasticity. Six colony
morphotypes are recognised:

1. The knobbly column morphotype which has short tapered cylindrical lobes, up to 20 mm long, that are closely spaced and radially distributed along the entire length of the colony

2. The cylindrical morphotype is cylindrical along its entire length

3. The flat laminar morphotype is approximately cylindrical at the attachment point, but broadens into a flat section before tapering towards the distal tip.

4. The bushy morphotype

5. The sausage morphotype

6. The bootlace morphotype

The six morphotypes show geographic variation in their distribution, with the bushy and sausage forms thought to be limited to particular geographic areas.

Distributed around the whole of the British Isles. Particularly abundant in the North Sea. Common along the English and French coasts of the English Channel and throughout the Irish Sea. Dense populations have been reported from the greater Thames Estuary, the coasts of the Netherlands and Belgium.

Extends from extreme low water spring tides to about 100 m. Occurs only rarely in the intertidal on British coasts. Tolerant of estuarine conditions.

A. diaphanum
commonly causes an unpleasant allergic dermatitis. This condition is referred to as "Dogger Bank Itch" and produces large painful, weeping blisters. Sensitization, through frequent contact, results in increasingly painful recurrent attacks. Carle et al (1982) have isolated the allergen responsible and characterised the condition as a type 4 allergic reaction. Fishermen are prone to the dermatitis if they are not wearing the correct gloves.