Gigartina densa Edyvane & Womersley

Scientific Description

Habit and structure. Thallus dark red, fading to yellow-brown, erect and rigid, 2–6(–10) cm high, cartilaginous, with clumped axes densely branched above. Fronds with a terete to compressed percurrent axis 1.5–2(–3) mm in diameter, with a few subdichotomous to lateral main branches, usually becoming naked below but with crowded pinnate (when well developed) to radial branches, usually with densely clustered branch ends; upper laterals and branchlets terete to slightly compressed, 0.5–1 mm broad, with acute apices. Holdfast discoid, 2–4(–6) mm across; epilithic or on mussels. Structure of a dense cortex of slender, anticlinal, subdichotomous filaments, end cells 1–2 µm in diameter and L/D 2–3, and a medulla of anastomosing filaments 5–9(–12) µm in diameter; the central filaments tend to be mainly longitudinal and broader.

Reproduction. Sexual thalli monoecious. Carpogonial branches 3-celled, situated on an inner cortical (supporting) cell. Cystocarps 1–1.5 mm across, occurring laterally and subapically on slender, simple branchlets usually with a spinous apex, without involucral ramuli; carposporophyte consisting of a filamentous matrix of scattered larger cells, with pit-connections scarcely enlarged, and numerous clusters of ovoid carposporangia 11–14 µm in diameter, surrounded by enveloping tissue, with the inner cells connected to gonimoblast cells but without transverse filaments, with the cortex forming a distinct depression outside the cystocarp. Spermatangial sori inconspicuous, forming pale patches near young cystocarps, with elongate cortical cells each cutting off obliquely an elongate spermatangium which produces apically a single ovoid spermatium 1–2 µm in diameter. Tetrasporangial sori forming irregular patches on the surface and margins of branchlets, often as incomplete annuli; tetrasporangia transformed from inner cortical and outer medullary cells, and also cut off laterally from these cells, ovoid, 15–25 µm in diameter, cruciately divided.

Distribution.Recherche Arch., W. Aust., to Lawrence Rock, Portland, Vic., and E coast of Tas.

Habitat. G. densa is a distinctive, small but rigid, species found near low tide level on rough-water coasts.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 301–303 (1994)]