Centroceras clavulatum (C.Agardh) Mont.

Scientific Description

Habit and structure. Thallus dark red-brown, usually densely tufted, 2–8(–15) cm high, fastigiate and usually even topped with straight, terete, largely sub dichotomous filaments 100–180 µm in diameter, completely corticated, with few adventitious laterals, apices involute with short abaxial spines. Attachment by rhizoids with multicellar pads, arising from slight prostrate filaments; epilithic or epiphytic on larger algae or seagrasses. Structure. Apical cells shortly conical, 10–15 µm in basal diameter, enlarging rapidly to axial cells 40–90 µm in diameter and L/D (1–)2–3, with central longitudinal protoplasmic strands, and with 12–14 periaxial cells formed in alternating sequence, these each cutting off 2 short cells acropetally and 2 (sideways and basipetally), rarely 3, which form 24–29 unbranched closely adjacent, corticating rows of small, rectangular cells 8–12 µm in diameter and L/D 1–2(–3). Spines abaxial and unilateral near apices, sometimes opposite, originating from a periaxial cell, 1–3(–4) cells and 20–45 µm long, usually lost from lower nodes. Elongate-clavate hairs frequent to absent. Branching subdichotomous at apical cells or lateral from periaxial cells. Rhodoplasts discoid.

Reproduction. Gametophytes dioecious. Carpogonial branches on the first formed periaxial cells. Carposporophytes with 1–2 rounded gonimolobes 250–450 µm across, carposporangia ovoid, 25–45 µm in diameter, with 2–5 involucral branches from below. Spermatangia in dense sori around the nodes, borne on branched filaments from the periaxial cells. Tetrasporangia first abaxial but soon becoming whorled at the nodes, cut off from periaxial cells, without or with short involucral filaments, subspherical to pyriform, 40–70 µm in diameter, tetrahedrally divided.

Distribution. Widely distributed on most tropical to temperate coasts. Generally around Australia.

Habitat. C. clavulatum is a common epilithic alga in the lower eulittoral zone and just below, forming dense, dark red-brown tufts, and also occurs as smaller plants on a variety of larger algae (e.g. Hormosira) and robust seagrasses.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 417–419 (1998)]