Vidalia spiralis (J.V.Lamour.) J.Agardh

Scientific Description

Habit and structure. Thallus dark red-brown, 10–30 cm high, erect and much branched irregularly for 3–5 orders with ultimate branches 1–8 cm long, flat, 2–3 mm broad, with regularly spaced (1–2 mm apart) marginal serrations 0.5–1 mm long, branches sparsely to frequently spirally twisted, apices dorsally revolute. Axes with a prominent midrib, denuded basally and 2–3 mm broad, 1–2 mm thick, serrations with a faint central vein. Lateral branches arising by continued growth of marginal serrations. Holdfast conical, becoming fibrous, 0.5–1(–4) cm across; epilithic, Structure. Apices revolute, broad; axes with 5 pericentral cells, 2 lateral ones on each side and a ventral pericentral cell, with the dorsal lateral cells each cutting off a pseudopericentral cell; a ventral pseudopericentral cell sometimes also formed. The lateral pericentral cells form the wings of the branches, 300–500 µm thick, with a medulla of 2 larger cells and a cortex (1–)2–3 cells thick, the midrib becoming thickened by proliferation from adjacent cortical cells. Marginal serrations occur 2–3 axial cells apart, have curved apices and are rapidly corticated. Trichoblasts occur subapically on marginal serrations and on short, dorsal, projections on the midrib, 0.5–1 mm long, basal cells 20–30 µm in diameter and L/D 1–1.5 with thick walls, upper cells 6–10 µm in diameter and L/D 6–10. Cells uni- or (larger) multinucleate; rhodoplasts discoid to elongate.

Reproduction.Reproductive organs produced mostly on the branch midrib. Gametophytes dioecious. Procarps not observed. Carposporophytes with a prominent basal fusion cell, much branched gonimoblast and clavate terminal carposporangia 30–50 µm in diameter. Cystocarps short-stalked, globular, 1–2 mm in diameter; pericarp with a small ostiole, wall 6–10 cells and 200–300 µm thick, cells irregular in section. Spermatangial organs clustered on branches of trichoblasts on marginal serrations or on the midrib, subspherical to ovoid, 80–130 µm in diameter. Tetrasporangial stichidia densely clustered on the midrib, sessile, compressed, elongate-ovoid to fusiform with curved apices, 180–220 µm broad and 0.4–1.5 mm long, with a narrow stalk, corticated, with 2 rows of tetrasporangia 50–90 µm in diameter.

Distribution.Houtman Abrolhos, W. Aust., to Port Phillip Heads, Vic.

Habitat. V. spiralis is a common species in shallow but shaded situations to deep water.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 397–399 (2003)]