Platoma magnum Huisman

Scientific Description

Habit and structure. Thallus (gametophyte) light pink to pale yellow, mucilaginous, erect, to 20 cm tall, with a small discoid holdfast. Branching irregular, with limited compressed portions and numerous terete to subterete branches. Axes 5–15 mm wide, tapering to 0.5–2.0 mm near the apices. Structure multiaxial, with a diffuse apical meristem. Medullary filaments with cells 35–250 ìm long and 5–10 ìm diam. Cortex filamentous, dichotomously (rarely trichotomously) branched, 6–12 cells thick, with ellipsoidal to pyriform inner cells 15–25 ìm long and 7–12 ìm diam. and subspherical to pyriform surface cells 4–6 ìm diam. Intercalary subspherical to obovoid vesicular cells (20–45 ìm diam.) common in the cortex.

Reproduction. Plants monoecious. Spermatangia in sori, subspherical, 1–4 on outer cortical cells, 2–3 ìm diam. Carpogonial branches 3-celled. Generative auxiliary cells transformed mid-cortical cells. Gonimoblast arising from upper surface of generative auxiliary cell, producing a spherical carposporophyte to 200 ìm diam. composed entirely of angular or spherical carposporangia 12–35 ìm diam. Mature cystocarps immersed in the thallus. Tetrasporophytes and tetrasporangia not observed.

Distribution. Known from the Dampier Archipelago and Scott Reef, Western Australia.

Habitat. Epilithic in the subtidal to depths of 20 m.

[After J.M. Huisman in Algae of Australia: Marine Benthic Algae of North-western Australia, 2. Red Algae: 272–274 (2018)]