Ditria expleta Huisman

Scientific Description

Habit and structure. Thallus brown-red, entirely prostrate on the host, axes 2–6 mm long, dorsiventrally organised with pairs of simple, terete, determinate laterals 300–800 µm long, usually separated by a dorsal segment bearing only a single cell. Attachment by rhizoids from ventral pericentral cells; epiphytic on Lobophora variegata and Zonaria spp. Structure. Apical cells hemispherical, 14–18 µm in diameter, segmenting transversely to give short axial cells, subapical cells dividing obliquely to cut off in a spiral sequence initials of determinate laterals which lie in alternate pairs separated usually by a segment with only a single dorsal cell, occasionally lacking the latter. Pericentral cells 5, ecorticate. Axes compressed, (50–)70–110 µm broad, segments L/D 0.6–1.0(–1.5), determinate laterals 15–20 segments long, (40–)50–85 µm in diameter, segments L/D 0.5–0.8. Lateral axes developed by continued growth of determinate laterals. Trichoblasts on occasional upper segments of determinate laterals, 400–1000 µm long, branched, basal cells 25–35 µm in diameter, L/D 2–5. Rhizoids cut off from pericentral cells, unicellular with multicellular haptera. Cells uninucleate; rhodoplasts discoid to elongate, often transversely orientated in axial cells.

Reproduction. Gametophytes dioecious. Procarps borne on the second cell of trichoblasts on apices of determinate lateral branches, with a 4-celled carpogonial branch and 2 sterile branches. Carposporophytes with a basal fusion cell and branched gonimoblast with clavate terminal carposporangia. Cystocarps terminal on determinate laterals, ovoid; pericarp ostiolate, ecorticate. Spermatangial organs arise on trichoblasts on determinate laterals, lanceolate, mostly 180–300 µm long and 35–45 µm in diameter, with 1–2 basal and 1–4 apical sterile cells. Tetrasporangia in linear series in swollen and often curved ends of determinate laterals, fertile segments 70–90 µm in diameter; 40–110 µm in diameter, with 3 cover cells.

Distribution.Houtman Abrolhos and Rottnest I., W. Aust., and from Cootes Hill, Sturt Bay, Yorke Pen., to Sellicks Reef, and Rocky Point, Kangaroo I., S. Aust., but probably more widespread.

Habitat. D. expleta is an inconspicuous alga, being entirely prostrate on Lobophora and Zonaria.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 310–312 (2003)]