Melobesia membranacea (Esper) J.V.Lamour.

Scientific Description

Habit and structure. Thallus normally pinkish, encrusting, mostly 4–15mm across and 10–75 µm thick, epigenous and completely affixed by cell adhesion. Structure pseudoparenchymatous with dorsiventral organisation throughout; construction dimerous, consisting of a single ventral layer of branched, laterally cohering, filaments each composed of palisade or non-palisade cells 2–13 µm long and 2–30 µm high, and additionally of epithallial cells or multicellular simple or branched, laterally cohering, filaments that arise dorsally and more-or-less perpendicularly from most cells of ventral layer filaments and are each composed only of an epithallial cell 2–10 µm in diameter and 2–8 µm long or with additional subtending cells 2–15 µm in diameter and 2–15 µm long; distal walls of epithallial cells rounded or flattened but not flared; cell elongation characteristics uncertain; cells of adjacent filaments joined by cell-fusions; secondary pit-connections, haustoria, and trichocytes not observed.

Reproduction.Vegetative reproduction unknown. Gametangia and carposporophytes produced in uniporate conceptacles; tetrasporangia produced in multiporate conceptacles; bisporangia not observed. Gametangial thalli monoecious or dioecious; carpogonia and spermatangia produced in separate conceptacles. Carpogonia terminating 2- or 3-celled filaments arising from the female conceptacle chamber floor. Mature female-carposporangial conceptacle roofs protruding above or flush with surrounding thallus surface, 5–25(–40) µm thick, composed of 2–8 layers of short, squat cells, conceptacle chambers 50–250 µm in diameter and 25–80 µm high. Carposporophytes when mature apparently consisting of one (or several?) inconspicuous fusion cells bearing several-celled gonimoblast filaments with terminal carposporangia 18–60 µm in diameter. Spermatangial filaments unbranched, arising from the floor, walls and roof of male conceptacle chambers, mature male conceptacle roofs protruding above or flush with surrounding thallus surface, 7–20 µm thick, composed of 2–7 layers of more or less squat cells, conceptacle chambers 57–130 µm in diameter and 25–40 µm high. Tetrasporangial conceptacle roofs protruding above or flush with surrounding surface, commonly but not always differentiated into a peripheral rim and a central sunken pore-plate, pore-plate composed of cells that are similar in size and shape to other roof cells, roof 2–5 cells thick above the chamber, conceptacle chambers (35–)50–252 µm in diameter and 15–83 µm high; tetrasporangial initials situated one cell layer below the thallus surface, scattered across the conceptacle chamber floor, each mature sporangium 10–53 µm in diameter and 22–65 µm long, containing zonately arranged tetraspores and possessing an apical plug that blocks a roof pore prior to spore release; bisporangia not observed.

Distribution.Widespread, but many records require confirmation. In Australia, Flat Rock (30 km N of Dongara), W. Aust., to Gabo I., Vic., and around Tas.

Habitat. M. membranacea occurs epiphytically on a variety of green, brown and red algae in southern Australia and has been found intertidally on rocky reefs and in tide pools and subtidally to depths of 42 m.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 168–171 (1996)]