Habit and structure. Thallus white, pulvinate, surface smooth (tetrasporangial) or verrucose (cystocarpic), 0.5–3 mm across, growing on Gracilaria ramulosa (or G. flagelliformis). Structure multiaxial, with slender filaments penetrating between the host cells and pit-connecting with them, externally with a mass of compact filaments forming the pustules with a thick gelatinous outer layer below 1–2 layers of host cortical cells raised up by the parasite.
Reproduction. Gametangial thalli dioecious. Carpogonial branches 2-celled, borne on a supporting cell which is an intercalary cell of cortical filaments. Gonimoblast filaments radiating horizontally as a basal layer of hyaline filaments with frequent fusions with gametophytic cells, and erect clusters of gonimoblast filaments which produce chains of carposporangia, 8–12 µm in diameter, amongst a lax palisade of cortical filaments. Spermatangia borne on outer cortical cells in chains of 2–6, each about 4 µm in diameter. Tetrasporangia developed in terminal cortical cells scattered between sterile cortical filaments, ovoid, 30–40 µm long and 15–20 µm in diameter, decussately divided.
Distribution. Whitford Beach, Perth, W. Aust., and from Portland to San Remo, Vic.; Orford, Tas.
Habitat. H. australis is a small but distinctive parasite, mainly on Gracilaria ramulosa on Victorian coasts but also known on G. flagelliformis near Perth, W. Aust.
[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 13 (1996)]
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Western Australian Herbarium (1998–). FloraBase—the Western Australian Flora. Department of Biodiversity, Conservation and Attractions. https://florabase.dpaw.wa.gov.au/