Plesiothoa gigerium is found in the south-west of England and the Channel Islands, and extends to South Africa. This species is the only British representative of the genus, which is otherwise known from Chile, New Zealand, the South Shetland Islands and Antarctica.
Plesiothoa gigerium forms adnate runner-like colonies, composed of branching chains of autozooids, closely adhering to the substrate. Autozooids are pear- or club-shaped with a short thread-like proximal portion. They are approximately 0.4 mm long.
New autozooids bud distally (furthest from the colony origin) and disto-laterally of preceding zooids on one or both sides. The daughter zooids usually come to lie parallel to the parent.
The species is able to colonise a range of substrates including red algae (Palmaria palmate) and Laminaria hyperborea. The species is found in shallow subtidal waters often in association with similar bryozoan species such as Celleporella hyalina, Haplopoma impressum and Haplopoma bimucronatum, as well as Electra pilosa.
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