Bostrychia radicans (Mont.) Mont.

Scientific Description

Habit and structure. Thallus prostrate with suberect branches, robust, turf-forming, purple to brown; main axes indeterminate, 10–15(–20) mm long bearing determinate lateral branches 2–3 mm long, with 1–2(–3) orders of alternate branching (5–11 second order determinate branches); two tiers of pericentral cells per axial cell, with (5–)7–8 pericentral cells per tier around the main

axes and lateral branches, 5 pericentral cells per tier in fertile branches; ecorticate throughout; polysiphonous throughout; attached to substratum by cladohaptera. Indeterminate axes 120–200 µm in diameter, axial cells 80–140 µm long; cortex absent; branches arising subdichotomously at intervals of more than 10 axial cells; determinate branches arising laterally at intervals of (2–)4–6 axial cells. Determinate branches of 1–2(–3) orders, branching at intervals of 2–5(–10) axial cells; ultimate branches 25–35(–45) axial cells long. Cladohaptera 4–6(–7) axial cells long. (Specimens formerly referred to as B. bipinnata from Tonga produce cladohapteral branches in the usual position but these do not become attached to the substratum and have the appearance of short unbranched polysiphonous laterals of determinate growth.)

Reproduction. Carpogonial branches borne within 2–5 subapical axial cells of lateral branches; fertile region usually 2–3 consecutive axial cells long with 1(–3) procarps per axial cell; axial cells below the fertile region have 5–7 pericentral cells per tier and procarpial axial cells 4–5 pericentral cells per axial cell; the supporting cell produces a sterile group of 2 cells prior to fertilisation and the adjacent sterile pericentral cell produces a group of 2–3 cells; the carpogonial branch is 3 cells long. Cystocarps subapical, ovoid to globular, 300–500 µm long, 250–450 µm in diameter, with a 35–50 µm diameter ostiole; cystocarp walls have 8 longitudinal pericarp filaments with 2 two-tiered pericarp pericentral cells per filament cell and one order of cortical cells; carposporangia 100–125 µm long, 30–45 µm in diameter, 30–70 per cystocarp. Spermatangial branches 200–500 µm long, 65–75 µm in diameter, formed from 3–10 subapical axial cells of ultimate lateral branches; axial cells below the fertile region have 5–6 pericentral cells per tier; spermatangial branches have 4–5 pericentral cells around the axial cell, giving rise to two tiers of pericentral cells; each pericentral cell produces 0–1 order of cortical cells, and all superficial cells are potentially spermatogenous. Tetrasporangia borne in subapical stichidia on ultimate lateral branches; stichidia 800–1200 µm and (2–)5–7(–9) axial cells long, 120–140 µm in diameter; branches bearing stichidia have 5–6 pericentral cells per tier; stichidia have 4–5 pericentral cells per axial cell, each of which gives rise to a tetrasporangium and 2 cover cells; each cover cell divides once only without further cortication; tetrasporangia 50–80 µm in diameter. 

Distribution. Recorded from northern Australia (tropical coasts of north Qld., N. Territory and W. Aust., south to latitude 27"s.) and widely distributed in tropical seas. 

Habitat. Thallus forming a turf (often intermixed with Caloglossa leprieurii) on mangrove pneumatophores and prop roots and intertidal rock. Occurs in tidal rivers, inlets and bays; also epilithic.