Molecular phylogenetic and morphological analyses of Hawaiian members of the red algal family Dumontiaceae were used to clarify the species diversity of Dudresnaya and Gibsmithia from Hawaiʻi. Although no new species of Dudresnaya were detected in the analyses, D. babbittiana is newly recorded from Lalo, Manawai, and Oʻahu. A new species of Gibsmithia, G. punonomaewa A.R.Sherwood, is described here and reported from mesophotic depths (79-104 m) of the Papahānaumokuākea Marine National Monument, Hawaiʻi. This new species differs from others in the genus based on the following combination of characters: moderate thallus size (up to 11 cm), smooth and terete gelatinous lobes, presence of a stipe (which is often branched), globose carposporangia, and a non-isodiametric shaped cell subtending the tetrasporangia. This new taxon increases the number of Gibsmithia species recorded from Hawaiʻi to three. Phylogenetically, G. pun onomaewa is most closely related to G. dotyi and G. larkumii, which are both widespread in distribution. The relatively dark habitat of the mesophotic in Papahānaumokuākea Marine National Monument contrasts with the surf ace waters of tropical and subtropical habitats where most Gibsmithia species are found, further highlighting the uniqueness of the species.
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