Sporolithon durum (Foslie) R.A.Towns. & Woelk.

Scientific Description

Habit and structure. Thallus normally pinkish to grey-pink, encrusting to warty, lumpy or fruticose, mostly 30–195 mm across and 1–115 mm thick or tall, epigenous and completely affixed by cell adhesion or unattached and forming rhodoliths mostly 16–114 mm in greatest dimension; protuberant branches simple or branched, free from one another or fused to varying degrees, mostly 5–26 mm in diameter and 5–23 mm long. Structure pseudoparenchymatous; organisation dorsiventral in crustose portions and radial in protuberant branches; construction monomerous, consisting of a single system of branched, laterally cohering, filaments that collectively contribute to a ventrally or centrally situated core and a peripheral region where portions of core filaments or their derivatives curve outwards towards the thallus surface, each filament composed of cells 5–12 µm in diameter and 6–23 µm long; epithallial cells 2–5 µm in diameter and 7–10 µm long, terminating most filaments at the thallus surface, with distal walls flattened and flared; cell elongation occurring mainly behind actively dividing subepithallial initials that are most commonly as short as or shorter than their immediate inward derivatives; cells of adjacent filaments joined by cell-fusions, secondary pit-connections apparently uncommon and not observed in most (southern Australian) thalli; haustoria absent; one trichocyte recorded.

Reproduction. Vegetative reproduction by thallus fragmentation. Gametangia and carposporophytes produced in uniporate conceptacles; tetrasporangia produced on separate thalli in calcified compartments that are aggregated into sori and are derived from the walls of tetrasporangial initials. Bisporangia unknown. Gametangial thalli probably dioecious. Carpogonia terminating 2–4-celled filaments arising from the female conceptacle chamber floor and sometimes from the chamber walls. Mature female-carposporangial conceptacle roofs flush with or slightly protruding above the surrounding thallus surface, 13–215 µmthick, composed of 3–18 layers of cells above the chamber, conceptacle chambers 80–445 µmin diameter and 70–370 µmhigh. Carposporophytes apparently consisting only of carposporangia 16–30 µmin diameter and 105–119 µmlong, developing directly from fertilized carpogonia and subtended by one- or several-celled stalks that presumably are the remains of carpogonial filaments; fusion cells and gonimoblast filaments unknown. Spermatangial filaments branched, arising from the floor and roof of male conceptacle chambers, mature conceptacle roofs flush with or slightly sunken below surrounding thallus surface, 13–81 µmthick, composed of 3–11 layers of cells above the chamber, conceptacle chambers 67–245 µmin diameter and 67–122 µmhigh. Tetrasporangial sori irregularly shaped, (<1–)5–15(–40) mm in greatest dimension and very slightly raised above surrounding thallus surface, composed of sterile filaments and calcified compartments, each compartment with a tetrasporangium; each mature sporangium 38–60 µmin diameter and 80–110 µmlong, containing cruciately arranged tetraspores and possessing an apical plug that blocks the compartmental pore prior to spore release; older sori almost always sloughed off.

Distribution.Rottnest I., W. Aust., to Bowen I., Jervis Bay, N.S.W.

Habitat. S. durum occurs in intertidal reef pools and subtidally to depths of 30 m attached to rock or unattached and free-living as rhodoliths.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 155–157 (1996)]