Antithamnion hanovioides (Sond.) G.De Toni

Scientific Description

Habit and structure. Thallus medium to dark red-brown, with branched prostrate axes with more-or-less erect unattached ends 1–3 cm long, densely covered with pairs of opposite, decussate, whorl-branchlets (pinnae) which are upwardly curved and overlap. Attachment by branched clusters of rhizoids from the basal cells of pinnae, with terminal digitate haptera; epiphytic on Posidonia, Amphibolis and various larger algae. Structure. Apical cells 8–15 µm in diameter and L/D 1–1.5, densely surrounded by young whorl-branchlets, increasing to 100–300 µm in diameter and L/D 1–1.5 in mature axial cells which have prominent pit-connections. Pinnae rigid, distichously branched, (0.5–)1–2 mm long, rachis cells 35–70 µm in diameter, basal cells L/D 0.8–1.2, then cells L/D 1.2–1.6, each cell bearing a single, alternate, usually branched pinnule, lower cells 25–55 µm in diameter and L/D 1–2, tapering near their apices to a mucronate end cell; gland cells prominent, ovoid, borne on short 2–3-celled branches of the pinnules, 25–40(–50) µm in diameter. Lateral branches occur irregularly, arising on the small basal cells of the pinnae. Cells uninucleate when small, probably multinucleate when larger; rhodoplasts discoid in small cells, becoming elongate and ribbon like in axial cells.

Reproduction. Gametophytes dioecious. Carpogonial branches borne in series of 10–16 on basal cells of young pinnae, with axial elongation ceasing on development of the first carposporophyte, and pinnules usually absent on the rachides. Post-fertilization the supporting (basal) cell enlarges upwards and cuts off the auxiliary cell and fusion occurs via a connecting cell. Fusions occur between the axial cell, residual supporting cell and foot cell, and several gonimolobes 150–550(–900) µm across of ovoid-angular carposporangia 25–45 µm in diameter develop, protected by surrounding pinnae from lower axial cells. Spermatangia occur terminally on dense, branched, clusters on the upper side of lower pinnule cells. Tetrasporangia occur on the pinnules or their branches, sessile, ovoid and 45–65 µm in diameter, decussately or cruciately divided.

Distribution. Houtman Abrolhos, W. Aust., to Barranjoey Heads, N.S.W., and around Tas.

Habitat. A. hanovioides is the most common species of Antithamnion on southern Australian coasts, epiphytic on a wide variety of larger algae and seagrasses.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 110–112 (1998)]