Dictyota mertensii

Thallus robust, erect, bushy, light- to dark-brown, greenish-brown, (5-)10-20 cm high. Branching repeatedly alternate-dichotomous. Branches strap-shaped, 2.5-6(-10) mm wide, broadest below the dichotomies, narrowest near apices, 80-120 mm thick, with spines at margins. Segments in between the dichotomies decrease in length from the base to the distal ends of the thallus. Apices rounded to obtuse in young thalli and aculeate or dentate in mature thalli. Surface cells rectangular, arranged in regular rows. In transverse section, medullarly cells rectangular, to 100 mm thick, cortical cells square-roundish, 15-20 mm thick. Hairs in tufts, scattered. Sporangia solitary or in linear clusters. Oogonia 80-100 mm diam., solitary or in groups of 2-3. Attachment by discoid holdfast. Growing on hard substrate, low intertidal to subtidal.

Tropics and subtropics of Atlantic and Pacific Oceans.