Macrothamnion pellucidum (Harv.) E.M.Woll.

Scientific Description

Habit and structure. Thallus medium to dark red-brown, erect, 10–30 cm high, much branched with several axes and long laterals, each axial cell with 3 whorl-branchlets, axes corticated by entwined rhizoids over basal parts. Holdfast rhizoidal, 2–10 mm across; epilithic. Structure. Apices densely branched, apical cells 8–10 µm in diameter and L/D 0.6–1, enlarging to 200–400 µm in diameter and L/D 1.5–2(–3) in mid and lower axial cells. Whorl-branchlets 1–2.5 mm long, simple, basal cell 75–120(–150) µm in diameter and L/D 0.8–1.2, cells above L/D 2–2.5, tapering to subterminal cells 25–35 µm in diameter and L/D 1–1.5, terminal cells mucronate; some young whorl-branchlets have small, unilateral or opposite, cells with extended curved walls ("hooked spines") on their upper cells, these later lost; gland cells borne on special short branches on lower cells of whorl-branchlets, 1 to a few (but up to 14) gland cells on one branch cluster, 25–45(–70) µm in diameter. Lateral branches arise on basal cells of whorl-branchlets at irregular intervals along the axes. Cells uninucleate when small; rhodoplasts elongate in small cells, becoming ribbon like in larger cells.

Reproduction. Gametophytes dioecious. Carpogonial branches borne on basal (=supporting) cells of whorl-branchlets on 8–20 successive axial cells. Post-fertilization the auxiliary cell cuts off an upper gonimoblast cell which develops a terminal gonimolobe, and later lateral gonimolobes, 100–500 µm across of ovoid to angular carposporangia 20–25(–30) µm across; fusions occur between the axial cell, residual supporting cell and the foot cell, and the carposporophyte is surrounded by longer whorl-branchlets from below. Spermatangia occur terminally on the special tufted branches 70–120 µm across, which also each bear a single gland cell on their basal cells. Tetrasporangia occur on the special, short, gland cell bearing, branches, sessile, ovoid, 45–55 µm in diameter, decussately or cruciately divided.

Distribution.Lancelin, W. Aust., to Sorrento, Vic., and around Tas.

Habitat. M. pellucidum is usually a deep water species on rough water coasts, but does occur on calmer coasts where there is strong current flow.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 123–124 (1998)]