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.

Two genera of the Rhodymeniales, Halopeltis and Leptofauchea, are here reported for the first time from the Hawaiian Islands and represent the deepest records for both genera. Molecular phylogenetic analyses of cytochrome oxidase subunit I (COI), rbcL, and large subunit ribosomal DNA (LSU) sequences for Hawaiian specimens of Leptofauchea revealed one well-supported clade of Hawaiian specimens and three additional lineages. One of these clades is described here as Leptofauchea huawelau sp. nov., and is thus far known only from mesophotic depths at Penguin Bank in the Main Hawaiian Islands. L. huawelau sp. nov. is up to 21 cm, and is the largest known species. An additional lineage identified in the LSU and rbcL analyses corresponds to the recently described L. lucida from Western Australia, and is a new record for Hawai‘i. Hawaiian Halopeltis formed a well-supported clade along with H. adnata from Korea, the recently described H. tanakae from mesophotic depths in Japan, and H. willisii from North Carolina, and is here described as Halopeltis nuahilihilia sp. nov. H. nuahilihilia sp. nov. has a distinctive morphology of narrow vegetative axes that harbor constrictions along their length. The current distribution of H. nuahilihilia includes mesophotic depths around W. Maui, W. Moloka‘i, and the island of Hawai‘i in the Main Hawaiian Islands. Few reproductive characters were observed because of the small number of specimens available; however, both species are distinct based on phylogeny and morphology. These descriptions further emphasize the Hawaiian mesophotic zone as a location harboring many undescribed species of marine macroalgae.

Algal biogeography in Hawai'i has not been studied in detail for many species. The islands are home to swift-current deep-water channels separating suitable habitats. The ability of these channels to act as barriers to dispersal has been studied in several animal lineages and is assessed here using the widespread red alga Amansia glomerata C.Agardh. A variety of analytical techniques based on 129 mitochondrial COI barcoding sequences collected across c. 2500 km were used to assess the genetic diversity of A. glomerata in Hawai'i. Haplotype network analyses demonstrated that the species is split into four main lineages which overlap in large parts of their range, yet there is insufficient support to recognize the lineages as separate species. Measures of haplotype diversity, nucleotide diversity, and neutrality tests suggest that at least three of these lineages have undergone recent population expansion. Biogeographic barriers were found to largely match those of marine animal groups in the archipelago. No evidence was found for distinct haplotypes or lineages between shallow and mesophotic reefs. A number of potential collection locations are suggested for the 1822 lectotype of the species, which was included in the molecular analyses. Potential scenarios leading to observed diversity patterns in the archipelago are presented. Amansia glomerata exhibits a high degree of haplotypic variation in Hawai'i, suggesting it may exhibit vast molecular divergences across its broader range, which extends from Hawai'i to southeastern Africa.