Halimeda gracilis J.Agardh

Scientific Description

Habit and structure. Thallus erect or decumbent, to 11 cm tall, green, drying pale green to white, arising from a single small holdfast. Branching dichotomous or trichotomous, often sparse and with sequences of up to 10 unbranched segments. Segments flattened (rarely subcylindrical), ribbed, heavily calcified, mostly trilobed, rarely cuneate, 5–8 mm long, 4–11 mm wide. Cortex with 2 utricle layers; peripheral utricles hexagonal in surface view, 30–45 µm diam., 50–75 µm long in section, up to 9 supported by each secondary utricle; secondary utricles clavate, 25–60 µm diam., 170–300 µm long, extending to the medulla. Medullary siphons 25–50 µm diam., 2 or 3 fusing at nodes for 300–400 µm; siphon wall thickening at nodes.

Distribution. Possibly pantropical, but a complex of species might be involved, two of which occur in the Indo-Pacific.

Habitat. Epilithic in the subtidal.

[After Huisman & Verbruggen, Algae of Australia: Mar. Benthic Algae of North-western Australia, 1. Green and Brown Algae 130-131 (2015)]