Ceramium pusillum Harv.

Scientific Description

Habit and structure. Thallus light to dark red, 1–5(–10) cm high, with one to a few linear, straight to slightly flexuous branches usually with alternate, flabellate groups of laterals, sometimes more irregularly branched or with a few lower proliferous branchlets. Attachment by a tuft of multicellular, branched rhizoids, some descending between the cortex and lower axial cells; epiphytic on various algae. Branches 400–1000 µm in diameter below, 100–200 µm in diameter in younger branchlets, tapering gradually until near the apices which are usually fairly straight. Structure. Axial cells L/D 0.6–1 (shorter near apices), becoming completely corticated close to the apices apart from a narrow separation marked by smaller cells terminating the acropetal growth, compared to slightly larger cells terminating the basipetal growth of the younger segment, this separation often visible for many segments from the apices, though the acropetal and basipetal cortical cells abut each other. Periaxial cells (7–)8–9(–10), each cutting off two cells acropetally and basipetally, these continuing as corticating filaments often with two divisions if space permits, the cells usually angular and isodiametric at first; terminal cells of the acropetal filaments sometimes projecting slightly; the inner cortical cells enlarge but remain subspherical to ovoid. Outer cortex of small cells present, commonly forming rosettes around many of the larger inner cortical cells as well as the periaxial cells. Rhodoplasts discoid in small cells, ribbon shaped and branched in larger cells.

Reproduction. Gametophytes dioecious. Carposporophytes usually near branch apices, globular, 200–450 µm across with 2–4(–6) short involucral branches, carposporangia ovoid, 25–30 µm in diameter. Spermatangia covering several segments, especially in axils of upper branches, later spreading around the branch. Tetrasporangia derived from the periaxial cells, forming prominent whorls of 10–15 (often including immature ones), often with some cut off later from inner cortical cells, mostly decussately divided, ovoid, 20–50 µm in diameter. Paraspores not infrequently occur on C. pusillum.

Distribution. Yanchep, W. Aust., to Gabo I., Vic., and around Tas.

Habitat. C. pusillum appears to be always epiphytic, on various algae (rarely on Posidonia) under conditions of strong water movement, especially at the outer edge of rock platforms, in the sublittoral fringe or uppermost sublittoral.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 393–395 (1998)]