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Beard’s Provinces: Eremaean Province, South-West Province.
IBRA Regions: Esperance Plains, Geraldton Sandplains, Jarrah Forest, Pilbara, Swan Coastal Plain, Warren.
IBRA Subregions: Geraldton Hills, Perth, Recherche, Roebourne, Southern Jarrah Forest, Warren.
IMCRA Regions: Abrolhos Islands, Central West Coast, Eyre, Leeuwin-Naturaliste, Pilbara (offshore), WA South Coast, Zuytdorp.
Local Government Areas (LGAs): Albany, Ashburton, Augusta-Margaret River, Cambridge, Cockburn, Cottesloe, Dandaragan, Esperance, Greater Geraldton, Irwin, Joondalup, Karratha, Northampton, Rockingham.
Habit and structure. Thallus light to medium grey-red, often bleached, (2–)5–10(–12) cm high, usually in densely branched tufts of numerous erect fronds. Fronds pinnately branched, axes sparsely dichotomous with occasional main lateral branches, bearing short (largely determinate) branchlets distichously and often adventitiously, often denuded below. Axial intergenicula in mid and upper parts slightly to strongly compressed, subterete to cuneate, with branchlets from the opposite shoulders and often adventitious branchlets not in the main plane of branching. Determinate branchlets usually markedly slenderer than indeterminate axes, simple to di- or trichotomous 2–4 times, ultimately tapering to subacute apices. Axial intergenicula 300–700(–825) µm long, 300–900 µm broad, L/B 1–2(–3). Branchlet intergenicula terete to subterete, 200–600 µm long and 150–250 µm in diameter near base, 60–80 µm in diameter near apices. Holdfasts crustose when young, later developing entangling and attaching stolons with terete intergenicula 300–600 µm long and 200–400 µm in diameter; epilithic as low mats or epiphytic on larger algae or Amphibolis. Structure. Intergenicula with 3–5(–6) tiers of medullary cells, each (50–)70–100(–145) µm long, with lateral cell-fusions but without lateral pit-connections. Cortex 2–3 cells thick. Genicula 240–370 µm long in axes, less in determinate branchlets. Rhodoplasts discoid.
Reproduction. Gametangial thalli dioecious. Conceptacles swollen, axial and terminal. Carposporangial conceptacles usually with two simple surmounting branchlets each consisting of 1–3 intergenicula; external diameter 250–300 µm, chamber diameter 150–200 µm, fusion cells 50–70 µm in diameter and 20–30 µm thick, with carposporangial filaments marginal, carposporangia subspherical, (70–)80–100(–160) µm in diameter. Male conceptacles terminal on determinate branchlets, without surmounting branchlets, elongate-ovoid to lanceolate, 200–250 µm in external diameter, chamber 90–130 µm in diameter and 240–340 µm high, canal 50–90 µm long, with spermatangia produced on floor and walls of chamber. Tetrasporangial conceptacles at apices of determinate branchlets, swollen, surmounted by 1–3 small branchlets each with 1–3 intergenicula, (300–)400–500 µm in external diameter, chamber 280–330 µm in diameter, each conceptacle containing 6–10 mature tetrasporangia 160–330 µm long, zonately divided.
Distribution. Kalbarri, W. Aust., around southern Australia and Tas., to Port Denison, Qld. New Zealand; Juan Fernandez I. (?).
Habitat. H. roseum is a common species, ranging from the lower eulittoral zone where plants are turf-like, to the sublittoral where it occurs both on rocks and epiphytically on larger algae and the seagrass Amphibolis.
[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 310–313 (1996)]