Authority: (Vahl) Børgesen
Hawaiian name: none
Characteristic feature: Solid cylindrical branches covered with many small spine-like branches.
Description: Plants to 25 cm, 2-3 mm wide, sparingly to repeatedly branched, with lateral spinose determinate branches, no spines on main axes; base a thickened crust, coalescing at times with lower axes; apices pyramidal, with incurved trichoblasts. Pericentral cells corticated densely, the central axial cells usually evident; in older axes, central axial filaments surrounded by small-celled adventitious filaments. Tetrasporangia ca. 75 mm diam. In short, determinate, spinose branches. Spermatangial heads plate-like, on single-celled stalks near branch apices. Cystocarps on adaxial sides of spines. Red, brown, or dark green in colour, but yellow if exposed to high sunlight.
Habitat: Saxicolous, epiphytic, or epizooic; intertidal and subtidal, or free floating. Often a dominant intertidal species, on calm shallow reef flats, in tidepools, and rocky benches swept by small waves.
Hawaiian distribution: All main Hawaiian Islands.
Other: Highly successful invasive species on Hawaiian reef flats. Unintentionally introduced after 1950, entering on a barge from Guam. The brittle nature of the branches often results in fragmentation, contributing to frequent, large free-floating populations and widespread distribution.