Dictyota acutiloba J.Agardh

Scientific Description

Habit and structure. Thalli mahogany-brown to tan, 4.5–16.5 cm long, usually in clusters from a consolidated fibrous basal clump to 9 mm in diameter. Fronds 0.3–1.5(–2.0) mm wide; blades flat or irregularly to regularly spiralled; margins smooth, weakly to strongly sinuous, generally lacking proliferations or with a few simple scattered marginal or submarginal spines to 2 mm long. Angles of forking mostly less than 45°, with acute or rounded axils; dichotomies mostly concentrated in the distal 15–20 mm, commonly 5 or 6; interdichotomies 15–25 mm long. Frond apices tapering abruptly from 400–500 µm to 60–80 µm within the distal 400 µm, the tips occupied by a bilenticular to convex apical cell. Marginal patches of rhizoids uncommon. Hair tufts at first confined to the midline, rounded, 60–100 µm in diameter, becoming more dispersed, longitudinally elongate, 120–200 µm long and 40–60 µm wide, the compact rows of precursor cells tending to grow out into true trichothallic hairs. Cross-sections distally 70–100 µm thick, 110–170 µm proximally, the cortex 1-layered throughout, the surface often an irregular mixture of short and longer cells 10–25 µm and 20–25 µm tall and 10–25 µm wide, mostly uniformly wide around blade edges, some 8–10 µm wide cells intercalating. Medial medullary cells of mixed sizes, not strongly transversely aligned, 100–275 µm long, 50–80 µm wide, without dark central contents showing up as transverse spots visible in surface view.

Reproduction. Sporangia mostly concentrated along blade midlines, rarely at margins, arising in non-soral aggregates in varying stages of maturity, borne on a single stalk cell, non-involucrate, very rarely cruciately divided. Spermatangia in oval to longitudinally elongate sori 180–1000 µm long and 150–300 µm wide; sori confined to the distal 1–2 cm of the frond, bounded by 1–4 rows of 1- or 2-celled involucral filaments, the plurangia in densely aggregated palisades of 16–32 columns per precursor cell. Oogonia clavate, on a single stalk cell, 60–120 µm long, 55–100 µm wide, in sori 60–450 µm in diameter which can coalesce into compound midline structures to > 2 mm long.

Distribution. Recorded from Micronesia, the Hawaiian Islands, French Polynesia, Easter Island, Chile and, in Australia, from Qld., including the southern Great Barrier Reef, and Lord Howe I.

[After Kraft, Algae of Australia: Marine Benthic Algae of Lord Howe Island and the Southern Great Barrier Reef, 2: Brown Algae:158–159 (2009)]