A giant form of Anadyomene, most similar to Anadyomene pavonina (J. Agardh) Wille, a rare and diminutive alga endemic to Florida, appeared as up to 10 m long net-like strands covering 10%-80% of a 0.5 km region of the 25-50 m deep Belizean outer reef slope where none had been present up to 12 months earlier. This new species, described herein as Anadyomene gigantodictyon Littler et D. S. Littler, is characterized by a unistratose blade or cluster of blades formed by the polychotomous branching of uniseriate veins, with the interstices, or spaces between the veins, completely or partially filled with cells that are smaller than those of the veins, with cylindrical to ovate cells. The cells at mid-blade are 1.7-2.0 mm in length and 0.2-0.3 mm diameter; interstitial cells are parallel and not juxtaposed. All cells are joined in one plane and form species-specific, fan-shaped patterns with secondary interstitial cells loosely or tightly woven. [ABSTRACT FROM AUTHOR]