A new brown alga Petalonia tatewakii sp. nov. (Scytosiphonaceae) is described from the Hawaiian Islands based on morphological observations and phylogenetic analyses of DNA sequences of the chloroplast-encoded rbcL gene and the ITS1-5.8S-ITS2 region of the nuclear ribosomal cistron. The thalli are linear and complanate, up to 15 cm in length and 0.8 cm in width with rhizoidal filaments in the medulla. This species is morphologically distinguishable from other Petalonia species by having ascocysts among plurilocular zoidangia. Molecular phylogenetic analyses revealed that samples of P. tatewakii form a distinct clade that is sister to the clade of P. binghamiae (J. Agardh) Vinogradova. All our collections of Hawaiian Petalonia from the main Hawaiian Islands were assigned to P. tatewakii.
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