Mesophyllum engelhartii (Foslie) W.H.Adey

Scientific Description

Habit and structure. Thallus normally pinkish, encrusting to warty, layered or foliose, mostly 6–85 mm across and 0.05–22 mm thick or tall, epigenous and partially to completely ventrally affixed by cell adhesion; protuberant branches erect, mostly simple, 1–4 mm in diameter and 2.3–10 mm long; lamellate branches applanate or ascending, sometimes interwoven, simple or furcate, mostly 3–10 mm across and 5–16 mm long. Structure pseudoparenchymatous; organisation dorsiventral in crustose portions and lamellate branches but 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 3–19 µm in diameter and 9–33 µm long; epithallial cells 3–5 µm in diameter and 4–9 µm long, terminating most filaments at the thallus surface, with distal walls rounded or flattened but not flared; cell elongation occurring mainly within actively dividing subepithallial initials that are usually as long as or longer than their immediate inward derivatives; cells of adjacent filaments joined by cell-fusions; secondary pit-connections, haustoria, and trichocytes unknown.

Reproduction.Vegetative reproduction unknown. Gametangia and carposporophytes produced in uniporate conceptacles; tetrasporangia and bisporangia produced in multiporate conceptacles. Gametangial thalli monoecious or dioecious; carpogonia and spermatangia produced in separate conceptacles. Carpogonia terminating 2–4-celled filaments arising from the female conceptacle chamber floor. Mature female-carposporangial conceptacle roofs protruding above surrounding thallus surface, 100–180 µm thick, composed of 13–38 layers of cells above the chamber, conceptacle chambers 185–500 µm in diameter and 45–230 µm high. Carposporophytes lacking a conspicuous central fusion cell and apparently consisting of an irregularly shaped fusion cell (not always evident) that may look discontinuous in section or a several-celled fusion cell complex (evident only in young stages), and several-celled gonimoblast filaments bearing terminal carposporangia 30–95 µm in diameter. Spermatangial filaments unbranched, arising from the floor, walls and roof of male conceptacle chambers, mature male conceptacle roofs protruding above surrounding thallus surface, (32–)40–55(–135) µm thick, composed of 6–16 layers of cells above the chamber, conceptacle chambers (54–)108–175(–284) µm in diameter and 20–110 µm high. Tetrasporangia/bisporangial conceptacle roofs protruding above surrounding surface, not differentiated into a peripheral rim and a central sunken pore-plate, 3–10 cells thick above the chamber, pore canals lined by cells that are similar in size and shape to other roof cells, conceptacle chambers 160–500 µm in diameter and 65–260 µm high; tetrasporangia scattered across the conceptacle chamber floor, each mature sporangium 24–81 µm in diameter and 59–173 µm long, containing zonately arranged tetraspores and possessing an apical plug that blocks a roof pore prior to spore release; bisporangia occasional, 22–65 µm in diameter and 68–110 µm long.

Distribution. Eagle Bluff, Shark Bay, W. Aust., to Kitty Miller Bay, Phillip I., Vic., and the eastern and southern coasts of Tas. New Zealand, Auckland Islands.

Habitat. M. engelhartii has been found intertidally on reef edges and in pools, and subtidally to depths of 15 m in southern Australia. Thalli occur on rock, molluscs, sponges and various green, brown and red algae.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 193–197 (1996)]