Hydrolithon onkodes (Heydr.) Penrose & Woelk.

Scientific Description

Habit and structure. Thallus encrusting, warty or lumpy 0.1–25 mm thick, epilithic or epizoic and affixed by cell adhesion. Structure pseudoparenchymatous; organisation dorsiventral; construction dimerous or monomerous, dimerous portions consisting of a single ventral layer of branched filaments composed of non-palisade cells, and multicellular simple or branched filaments that arise more or less perpendicularly from cells of ventral layer filaments, monomerous portions consisting of a single system of branched 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 5–15 µm in diameter and 5–22 µm long; 1–4 epithallial cells terminating filaments at the thallus surface, distal walls rounded or flattened but not flared; cells of adjacent filaments joined by cell-fusions, secondary pit-connections absent; trichocytes occurring commonly at thallus surface, arranged in horizontal rows, horizontal fields and singly, commonly becoming buried within thallus.

Reproduction.Vegetative reproduction unknown. Gametangia, carposporangia and tetrasporangia produced in uniporate conceptacles; bisporangia unknown. Gametangial plants monoecious; carpogonia and spermatangia produced in separate conceptacles. Carpogonia terminating 3-celled filaments arising from the female conceptacle chamber floor. Mature female-carposporangial conceptacle roofs flush with surrounding thallus surface, composed of 6–8 cells above the chamber, conceptacle chambers 136–191 µm in diameter and (74–)123–150 µm high. Carposporophytes developing within older female conceptacles after karyogamy, when mature composed of a large central fusion cell and gonimoblast filaments bearing terminal carposporangia, pore canals formed perpendicularly to thallus surface. Spermatangial filaments unbranched, arising from the floor of male conceptacle chambers, mature male conceptacle roofs flush with surrounding thallus surface, composed of 2 or 3 cells, conceptacle chambers 69–82 µmin diameter and 55–61 µmhigh. Tetrasporangial conceptacle roofs flush with surrounding thallus surface, 3–6 cells thick above the chamber, pore canals lined with non-protruding cells, conceptacle chambers 164–250 µmin diameter and 109–150 µmhigh; tetrasporangia usually occurring peripheral to a central columella, occasionally occurring across chamber floor, each mature sporangium 15–40 µm in diameter and 25–75 µm long, zonately divided.

Distribution.Known from numerous subtropical and tropical localities including, Bikini Atoll, Canary Islands, Sri Lanka, Chile, China, Clipperton Island and Costa Rica, Cuba, Ellice Islands, Hawaii, Indian Ocean, Kenya, New Guinea, North Atlantic Ocean, northern Australia, Philippines, Red Sea, Solomon Islands, Tahiti, West Africa, subtropical and tropical W. Aust. In southern Australia, known only from Point Valliant, W. Aust.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 261–263 (1996)]