SEASONAL AND BETWEEN‐PLANT VARIABILITY IN THE MORPHOLOGY OF SARGASSUM MATHIESONII SP. NOV. (PHAEOPHYTA) FROM THE GULF OF MEXICO 1

Sargassum mathiesonii sp. nov. is described from a population at the mouth of the Homosassa River, Homosassa, Florida. Patterns of morphological variation were quantified as well as the plant's phenology and ontogeny. Sargassum mathiesonii is dioecious (50/50 sex ratio), produces receptacles basipetally along laterals, and has vesicle pedicels that average 1–1.7 mm in length. As judged from coefficients of variation and measurements of between‐plant variation, vesicle diameter was the most stable continuous trait, followed by vesicle length, blade length: width ratio, blade width, and blade length. Anatomical variation was attributed primarily to ontogeny. Plants were smallest in March and April and grew to their maximum size in June (ca. 2 m). Biomass was augmented by additional main axes and primary laterals in October and receptacles in December. At maturity, male receptacles were longer and more loosely branched than female receptacles. A few vesicles developed viable oogonia in their cryptostomata and gave rise to fertile branches, situations previously unknown for Sargassum. Sargassum mathiesonii is determined to be morphologically and developmentally distinct from the lectotype and other North American populations of S. filipendula C. Ag. A more utilitarian approach to Sargassum taxonomy is achievable if species descriptions incorporate an understanding of morphological variability, reproductive periodicity, and ontogenetic patterns.