Scytosiphon lomentaria is a littoral brown seaweed with an irregularly lobed many filamentous form.

It is a member of the Phaeophyta in the order Ectocarpales and grows attached to shells and stones in rock-pools and in near-shore waters.

Scytosiphon lomentaria has cylindrical, shiny, olive brown, unbranched fronds up to 400 mm long.

They have short stalks and a large number may arise from a single holdfast.

[2] It occurs in the littoral zone and favours wave-exposed shores and rock pools.