Authority: (J. Agardh) Silva et DeCew
Hawaiian name: limu ‘aki‘aki ('bite, bite as a fish')
Characteristic feature: Forms a distinct band at the high intertidal level in dense, extensive patches.
Description: Gametophytes large, erect, growing in dense tufts from prostrate crusts, sometimes forming more or less continuous bands on midtidal basalt rocks; plants dark reddish black to red-brown, yellowing when growing in direct sunlight, to 60 cm tall and 1.5 mm diameter; terete, lower portions elongate, frequently unbranched for up to half their lengths, with several dichotomies in upper portions. Medulla pseudoparenchymatous, with all cells approximately the same shape and size; cortex thick, composed of small-celled filaments. Spermatangia forming transverse bands in outer cortex, giving striped appearance to fertile branch tips. Cystocarps formed in small groups near branch apices, to 2mm diam., often strongly projecting from surface, forming 3-4 carposomes. Tetrasporophytes maroon-colored crusts, with coalescing margins, without attaching rhizoids, to 5-6 cm across and 100 µm thick; tetrasporangia borne in intercalary chains within cortex.
Habitat: Intertidal, on basaltic rocks.
Hawaiian distribution: All main Hawaiian Islands, including the Northwestern Hawaiian Islands.
Other: Known to be cooked by Hawaiians in an underground oven with other foods. Releases carrageenan (a gel) when cooked.