Gelidium pusillum (Stackh.) Le Jol.

Scientific Description

Habit and structure. Thallus medium to dark red-brown, cartilaginous, forming sparse to dense turfs, (2–)5–20(–30) mm thick and commonly 1–4 cm in spread, with prominent prostrate stolons bearing simple to irregularly marginally branched, compressed, erect branches; stolons terete to slightly compressed, prominent, frequently branched, (80–) 200–400 µm in diameter. Erect blades at first simple, becoming basally branched to form clusters of simple to subdistichous, flat, blades, (2–)5–20(–25) mm high, 200–1000(–1500) µm broad and 50–100 µm thick [B/T 4–8(–10)], broadening upwards to a rounded, sometimes slightly depressed apex with a slightly protuberant apical cell, margins of blades straight to slightly undulate. Holdfasts of massed rhizoids 200–600 µm across, sessile or terminating short branches and often below an erect blade; epilithic or on molluscs in pools or subtidally. Structure. Outer cortical cells crowded, tending to lie in rows splaying outwards, more or less isodiametric with the protoplast 3–5 µm across in surface view, wall relatively thin; medulla narrow in the flat blades; rhizines present throughout the medulla, mainly in the outer medulla in older branches.

Reproduction. Sexual thalli dioecious. Cystocarps usually single in the mid to upper part of erect blades, round to ovate, 300–400 µm across, with small ostioles on both surfaces. Carposporangia single, pyriform to ovoid, 30–45 µm long and 15–25 µm in diameter; elongate arachnoid cells and nutritive cell chains present. Spermatangia covering the surface of upper parts of relatively narrow erect blades, with 4–8 initials replacing each outer cortical cell, cutting off terminally ovoid spermatangia 1–2 µm in diameter. Tetrasporangial sori in upper part of erect blades or laterals, ovate to elongate, 200–500 µm across and 250–1200(–1500) µm long, with a sterile margin. Tetrasporangia of various ages forming mixed sori, derived from the inner to mid cortical cells in two layers, ovoid, 20–35 µm in diameter, irregularly cruciately (mainly decussately) divided; rhizines confined to edges of tetrasporangial branchlets.

Distribution. Widely distributed. In southern Australia, from Rottnest I., W. Aust., to Collaroy, N.S.W., and around Tas.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 133–136 (1994)]