Molecular-assisted identification using plastid-encoded rbcL and mitochondrion-encoded COI loci identified five species of Polysiphonia sensu lato from 16 Florida and Caribbean Mexico samples. Morphological character states were examined and used to identify these species as Neosiphonia bajacali comb. nov., N. echinata comb. nov., N. sphaerocarpa, N. tepida, and Polysiphonia anomala. Descriptions are provided and the phylogenetic relationships of the five species were determined through maximum likelihood analyses of rbcL and nuclear-encoded SSU sequence data. Neosiphonia bajacali and N. echinata had a combination of character states described for Neosiphonia: rhizoids cut-off from pericentral cells, lateral branch or trichoblast initials on every segment in a spiral pattern, tetrasporangia in spiral series, and spermatangial stichidia developing as bifurcations of trichoblasts, and these new combinations are proposed. Examination of N. echinata, P. fracta and North Carolina specimens identified as P. breviarticulata revealed no significant morphological differences. Polysiphonia fracta is proposed as a synonym of N. echinata and the presence of P. breviarticulata within the western Atlantic is questioned. This is the first report of N. bajacali from the Caribbean and the first report of N. echinata from Caribbean Mexico.
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