Coelarthrum opuntia (Endl.) Børgesen

Scientific Description

Habit and structure. Thallus mediumred to dark red-brown, (5–)10–30 cm high, much branched with even-topped branches of similar height (growth usually synchronous), branches segmented with slender constrictions mostly 0.5–1 mm long and ovoid to elongate, hollow, segments bearing (1–)2–4 branches from the apex of most segments; mature segments increasing in length below, 0.5–3(–4) cm long and 2–7 mm in diameter, lowest segments (stipe) terete, 1–2 mm in diameter. Holdfast crustose and thick, 2–10 mm across, bearing one to several fronds; epilithic. Structure multiaxial, with a cortex 30–50 µm and 2–3 cells thick, outer cells laxly arranged, often in slight rosettes, ovoid and 5–8(–10) µm in diameter, and a large-celled medulla 1–2 cells thick bordering the central weakly mucilaginous cavity, with the inner cells ovoid and 50–100(–220) µm in diameter, walls 4–10 µm thick, with frequent secondary pit-connections and bearing 1–3 ovoid or pyriform secretory cells 20–30 µm in diameter, on cells on the inner medullary cells; long surface hairs frequent. Constrictions with pseudoparenchymatous partitions 6–12 cells thick, of large and small cells intermixed with a cluster of secretory cells borne on the upper side of the partition. Rhodoplasts discoid to elongate, in chains or ribbon like in inner cells.

Reproduction. Gametangial thalli dioecious. Carpogonial branches 4-celled, borne on mid cortical cells; auxiliary cell branches 2-celled, auxiliary cell with a large, globular inclusion. Carposporophyte with a basal fusion cell and all gonimoblast cells becoming ovoid to angular carposporangia 20–30 µm in diameter. Basal nutritive tissue moderate, erect filaments disintegrating but walls of lower cells remaining. Cystocarps scattered, external, 1–1.5 mm in basal diameter, conical to hemispherical and often beaked around the ostiole; pericarp 130–200 µm and 6–12 cells thick, cells laxly arranged, ostiolate. Spermatangia in sori, cut off from outer cortical cells, ovoid, 3–4 µm in diameter. Tetrasporangia scattered, transformed from inner cortical intercalary cells, subspherical to ovoid and 25–50(–55) µm in diameter, decussately divided.

Distribution.N Indian Ocean. Darwin, N.T., around W. Aust., and S. Aust., to Walkerville, Vic., and the N coast of Tas.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 55–57 (1996)]