Narrow compressed stipe expands into strap-like blade, usually inrolled to form a channel, with thickened margins. Repeatedly dichotomously branched, axils acute. Upper part of frond with papillae to 10 mm or more long on surfaces and margins on female plants.
Male plants lack papillae but are generally rare.
Tetrasporophyte a purplish-black crust. On rocks in lower intertidal, often in large continuous mats, widespread and abundant. On rock, lower intertidal, widely distributed, common. Generally associated with Mastocarpus stellatus , of which it is the tetrasporophyte, in the lower interntidal and shallow subtidal. Commonest on southern and western coasts.