Hydrolithon cymodoceae (Foslie) Penrose

Scientific Description

Habit and structure. Thallus encrusting, up to 1 mm thick, epiphytic 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 unicellular or 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 5–20 µmin diameter and 5–30 µmlong; 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 in horizontal rows and singly at thallus surface, not becoming buried within thallus.

Reproduction.Vegetative reproduction unknown. Gametangia, carposporangia, tetrasporangia and bisporangia produced in uniporate conceptacles. 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 protruding above or flush with surrounding thallus surface, composed of 2–4 cells above the chamber, pores orientated obliquely to surrounding thallus surface; conceptacle chambers 90–150 µmin diameter and 65–150 µmhigh. Carposporophytes developing within older female conceptacles after karyogamy, when mature composed of a large central fusion cell and gonimoblast filaments bearing terminal carposporangia. Spermatangial filaments unbranched, arising from the floor of male conceptacle chambers, mature male conceptacle roofs protruding above or flush with surrounding thallus surface, conceptacle chambers 90–105 µm in diameter and 50–85 µm high. Tetrasporangial and bisporangial conceptacle roofs protruding above or flush with surrounding thallus surface, 2–4 cells thick above the chamber, pore canals lined with non-protruding cells, conceptacle chambers 30–260 µm in diameter and 80–160 µm high; a single sporangium usually formed in the conceptacle chamber, sometimes 2 sporangia formed across the conceptacle chamber floor, each mature sporangium 45–135 µm in diameter and 80–185 µm long, usually zonately divided, sometimes irregularly divided.

Distribution.Shark Bay, W. Aust., to Walkerville, Vic.

Habitat. H. cymodoceae is found predominantly growing on the leaves and stems of the seagrass, Amphibolis antarctica.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 256–258 (1996)]