Heterostroma nereidiis Kraft & M.J.Wynne

Scientific Description

Habit and structure. Thallus medium to dark brown-red, with decumbent fronds 1–4 cm long arising from prostrate, imbricate to expanded, bases to 280 µm thick. Fronds complanately and pinnately branched for 3–4 orders, branches (1–)2–3(–4) mm broad and 60–130 µm thick, margins irregularly serrate with alternate, pointed, determinate laterals, all with distinct apices and some becoming indeterminate. Attachment by clumped rhizoids from the ventral surface; epilithic or epiphytic on Amphibolis or on encrusting corallines on larger algae. Structure. Apical cells short-conical to dome-shaped, 15–20 µm in diameter, subapical cells cutting off first lateral branch initials then 4 pericentral cells with 2 lying on each of the dorsal and ventral surfaces which become heteromorphic by different divisions but remain ecorticate. The axial cells branch alternately laterally every 2 cells and also produce unicellular primordia from alternate cells on alternate sides; however, inner parts of secondary laterals may be branched 4–7 axial cells apart. Dorsal pericentral cells divide transversely and longitudinally to produce angular cells 15–30 µm across and L/D 1–2, lying over the larger ventral cells which divide only longitudinally, remaining 40–75(–140) µm in diameter and L/D (2–)3–4. Trichoblasts occur only associated with reproductive organs but are well developed, branched and 0.5–1.5 mm long. Rhizoids unicellular, cut off from the pericentral cells and with multicellular haptera.

Reproduction.All reproductive organs borne on short terete marginal processes with 5 pericentral cells and trichoblasts. Gametophytes dioecious. Procarps on the second cell of trichoblasts, with a 4-celled carpogonial branch and 2 sterile groups. Cystocarps (juvenile) ovoid; pericarp ostiolate, with about 11 erect filaments, probably ecorticate. Spermatangial organs borne as one basal branch of trichoblasts, elongate-ovoid, 130–200 µm long and 50–60 µm in diameter, with 7–9 fertile segments each with 3 pericentral cells, producing a layer of initials and a surface layer of spermatangia. Tetrasporangial stichidia occur marginally, single or grouped and bearing terminal trichoblasts, irregularly twisted or distorted by the sporangia, stichidia (0.25–)0.5–1.2 mm long and 150–200 µm in diameter, each segment with 5 pericentral cells and a single tetrasporangium, spirally arranged, 50–100 µm in diameter, with 3 cover cells.

Distribution.Houtman Abrolhos to Rottnest I., W. Aust., and Elliston, S. Aust.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 343–344 (2003)]