Common Name(s): False whip plexaura
Colony Form: Large, dichotomously branched, bushy or in one plane; to 1 m tall.
Branches: Long, thin, tapering, ~ 2-3 mm diameter.
Apertures: Small, smooth, round to slightly oval, ~ 1mm across, in irregularly scattered vertical rows; distance between apertures equal to or greater than aperture diameter; no projecting calices.
Color: Living specimens beige to purple. Dries purplish brown or light yellowish-brown. In alcohol pale gray or white.
Sclerites: Polyp armature: few flat little rods. Axial layer: small multi-radiate dark purple capstans (chiefly <0.15 mm in terminal branches). Outer layers: small leaf-clubs (0.1-0.15 mm), slender, both purple and white, often bent spindles with prominent but not crowded tubercles, to 0.5 mm long, but usually smaller.
Habitat: Most reefs in 2.5-50 m; most common in 15-20 m.
Distribution: Bermuda, South Florida, Caribbean Sea.
Notes: Distinguished by short, narrow spindles, leaf-clubs and capstans on outer cortex (Bayer, 1961). Wide spacing of apertures distinguishes this species from P. porosa (Bayer, 1961). Flamingo tongue gastropods, Cyphoma spp., are common predators of P. flagellosa (Harvell and Suchanek, 1987).