Ceramium monacanthum J.Agardh

Scientific Description

Habit and structure. Thallus dark red, mostly erect, 0.5–1.5 cm high, branches dense, subcomplanate near apices, usually becoming irregular below, alternate (to subdistichous), 3–8 axial cells apart. Attachment by tufts of rhizoids from the base of erect axes or from short horizontal filaments, rhizoids arising from periaxial cells, uniseriate, simple or sparingly branched (sometimes with a multicellular attachment pad on Corallina); usually epiphytic on Codium fragile and Corallina. Structure. Branches 200–550 µm in diameter below, tapering slightly to 100–150 µm near the involute apices. Axial cells more-or-less isodiametric, slightly shorter than broad in younger parts and slightly longer than broad in lower parts, with internodal spaces throughout. Periaxial cells 7–8, each cutting off 2(–3) smaller cortical cells acropetally and basipetally to form the nodal bands 4–6 cells long when young and bearing relatively coarse spines; node extending by fairly synchronous growth but short internodal spaces present throughout the thallus, usually with long internodes in basal filaments. Spines usually single at each node, occasionally less common, abaxial, with a multicellular base and 3–6 cells (30–100 µm) long, usually lost from older parts of thallus. Rhodoplasts discoid in cortical cells, linear in axial cells.

Reproduction. Gametophytes dioecious. Carposporophytes globular, 180–300 µm across, borne on upper branches and usually subtended by lateral branchlets, with angular carposporangia 20–35 µm across. Spermatangia in dense patches covering the adaxial side of the nodal band, later spreading around the node. Tetrasporangia cut off from periaxial cells, at first single and abaxial, with later sporangia arising near the first and sometimes extending around the node, partly to largely involucrate by the cortical cells, 35–60 µm in diameter, tetrahedrally divided.

Distribution. Cape Willoughby, Kangaroo I., and Robe in S. Aust., to Bridgewater Bay, Vic., and around Tas.

Habitat. C. monacanthum is a distinctive species in south-eastern Australia, and most records are epiphytic on Codium fragile or Corallina, at low eulittoral or uppermost sublittoral level.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 385–388 (1998)]