Authority: (Wulfen in Jacquin) Lamouroux
Hawaiian name: none
Characteristic feature: Hooks on the ends of branches.
Description: Plants often in clumps or masses 10-20 (50) cm high of loosely intertwined cylindrical axes and branches becoming progressively slenderer distally, infrequently coarser; axes 0.5-1.0 (2) mm diam. Below, tapering to apices; branching irregular and variable, in part from percurrent axes; ends of many axes and primary branches expanded and with broadened hook (hamate), or tendril-like, often twisted around axes of other algae; branches with small spinelike branchlets 50-200 mm at base, also tapered, simple or bifurcate and ± numerous on older or younger (basal or upper) parts of same plant; primary holdfast lacking or difficult to recognize, tangled with other larger algae (epiphytic). Tetrasporangial sori on branchlets. Spermatangial plants not detected. Cystocarps globular, usually crowded toward upper parts.
Habitat: Low intertidal and shallow subtidal reef flats, frequently entangled on Sargassum, also epiphytic on various other algae or directly attached to sandy flat rock.
Hawaiian distribution: All main Hawaiian Islands, except Hawai`i.
Other: Highly successful invasive species, forming large floating blooms in coastal Maui, and creating large biomass washes on the beaches of O‘ahu and Maui. Intentionally introduced in to Kane‘ohe Bay in January 1974 from Florida for kappa carageenan production.
Hypnea musciformis has been identified as a significant component of the diet of green sea turtles.