Ceramium excellens J.Agardh

Scientific Description

Habit and structure. Thallus medium to dark red, 5–20(–30) cm high with a single or several erect axes (or occasionally a slight prostrate base) producing several long lateral branches, usually complanately branched and regularly alternate in younger parts but below often more irregularly branched, often with small, proliferous, often fertile branchlets along the lower main branches. Attachment by a tuft of uniseriate-celled rhizoids; epilithic. Structure. Branches 0.5–1.5 mm in diameter below, decreasing gradually to 150–250 µm in diameter just below the relatively straight, markedly tapering apices; proliferous branchlets on older branches 120–180 µm in diameter, often basally constricted. Axial cells L/D about 1 above, extending to L/D 2 in older branches, becoming corticated except for a narrow, annular, internodal space (occasionally almost as long as the nodes in upper parts) present throughout the thallus, but with the cortication usually closing completely on older branches and extending as the axial cells enlarge. Periaxial cells usually 8, each cutting off a regular pattern of 2 cells acropetally and 2 (rarely 3) basipetally, these continuing as corticating filaments usually each with two divisions, occasionally one; divisions normally synchronous, producing a straight edge to the cortication (except where axial cells have elongated relative to cortical development), with smaller terminal cells on the acropetal filaments than on the basipetal ones, ultimately often with further production of small cells closing the internodal space completely. Outer cortex produced from the periaxial cells and some of the larger inner cortical cells (which elongate to L/D up to 5), often with well developed rosettes over these cells, and on older parts a further cortical layer of small cells may be cut off, forming a distinctly thicker cortex over the central part of the nodal cortication; in some specimens, scattered small outer cortical cells become gland-like. Rhodoplasts discoid in very small cells, ribbon like and longitudinal in axial cells.

Reproduction. Gametophytes dioecious. Carposporophytes near the ends of short lateral branchlets, globular, 200–400 µm across, with 2–4(–5) short, relatively stout, involucral branches; carposporangia ovoid, 22–30 µm in diameter. Spermatangial patches covering several segments, largely in the axils of upper branches, sometimes surrounding small lateral branches. Tetrasporangia formed mainly in stichidiose branch ends of proliferous laterals which are basally constricted, derived from periaxial cells and forming a ring of 8–12 lying within and bulging the acropetal cortical filaments, thus producing a torulose appearance to the branch; tetrasporangia subspherical to slightly ovoid, 30–50 µm in diameter, decussately divided.

Distribution. Head of the Great Australian Bight, S. Aust., to Western Port, Vic., and around Tas.

Habitat. Usually a deep water species.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 398–400 (1998)]