Amoenothamnion planktonicum E.M.Woll.

Scientific Description

Habit and structure. Thallus medium red, erect, 0.5–1.5 cm high or in balls 0.5–1 cm across, with subdichotomous axes bearing whorls of 4(–5) small whorl-branchlets per axial cell. Attachment by branched rhizoids from basal axial cells or from the basal cells of the lowest whorl-branchlets, forming a rhizoidal holdfast; planktonic (free floating) form without attachment; epilithic, epiphytic or free floating. Structure. Apical cells tapering, 5–8 µm in basal diameter and L/D 1–1.5, enlarging rapidly (within 4–6 cells) to axial cells 30–50 µm in diameter, then to 120–180(–220) µm in diameter and L/D 1–2(–2.5), branched every 2–4 cells. Whorl-branchlets unbranched or with 1–3 short branches from the basal cells, occasionally digitately branched, 60–90 µm and 4–7 cells long, basal cell largest (20–40 µm in diameter and L/D 0.5–1), then tapering markedly, terminal cell acute, (4–)6–10 µm in basal diameter and L/D 1–1.5; hairs frequent on any cells of whorl-branchlets; gland cells absent. Lateral branches from just subapical cells, the axes becoming subdichotomous, or (especially in female plants) from the basal cells of whorl-branchlets below the procarps. Cells uninucleate; rhodoplasts discoid to elongate in smaller cells, becoming ribbon like in axial cells.

Reproduction. Gametophytes dioecious. Carpogonial branches borne on the basal (= supporting) cell of whorl-branchlets 3–5 cells long, close to apices of axes. Post-fertilization the supporting cell enlarges upwards and cuts off the auxiliary cell, which after fusion with the carpogonium divides to form a foot cell and a central cell which produces a terminal and then lateral gonimolobes 200–300 µm across with ovoid carposporangia 20–35 µm in diameter; the axial cell, residual supporting cell and foot cell fuse, with a broader connection to the central cell. Lateral branches from the axial cell below loosely surround the carposporophyte. Spermatangia occur on short special adaxial branches on the basal and lower cells of whorl-branchlets, each branch with 2 or 3 tri- or quadrichotomies. Tetrasporangia occur on basal cells of whorl-branchlets usually singly, sessile, subspherical to slightly ovoid, 40–50 µm in diameter, decussately to tetrahedrally divided.

Distribution. Dongara, W. Aust., to Gabo I., Vic., and SE Tas. Twofold Bay, N.S.W.

Habitat. A. planktonicum occurs on a variety of larger algae and seagrasses, or as a free-floating tetrasporangial form.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 185–188 (1998)]