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To address the taxonomic uncertainty of Sporolithon species named in the early to mid-20th century, targeted PCR sequencing was performed on eight historical type specimens and on recently collected specimens. Six type specimens amplified for the rbcL gene and were Sanger sequenced yielding sequences ranging in length from 118 to 280 base pairs (bp). One, S. australasicum, failed to amplify and another, S. howei, was amplified for the psbA gene yielding a sequence 544 bp in length. The 118 bp long rbcL sequence of the lectotype of S. crassiramosum showed that it is a later, heterotypic synonym of S. molle. The rbcL sequences of type specimens of S. episoredion, S. schmidtii, S. sibogae and S. timorense ranged from 118 to 228 bp, and each is a distinct species. The 544 bp long psbA sequence of S. howei is also unique. The 280 bp long rbcL sequence of the lectotype of S. durum did not match any sequence with that name in any public repository, including the previously published complete plastome and mitogenome sequences. However, it was identical in sequence to a specimen in GenBank from the southern coast of Western Australia as well as several other sequences generated from field-collected specimens from the states of South Australia and Western Australia. The rhodolith specimens from New Zealand previously called S. durum are S. nodosum sp. nov. The species is endemic to New Zealand. The epilithic specimens from New Zealand previously called S. durum are S. immotum sp. nov., which is also found along the southeastern coast of Australia. Sporolithon crypticum sp. nov. is described from the southern coast of Western Australia. RAxML and Bayesian phylogenetic analyses of Sporolithon psbA and rbcL sequences are congruent between the two plastid encoded genes. DNA sequencing of type specimens of species of corallines is demonstrated to be the only reliable method to correctly apply names. 

Phymatolithon  Foslie is one of the most studied and ecologically important genera of crustose coralline algae (CCA) due to their dominant abundance in various marine ecosystems worldwide. The taxonomy of the genus is complex and has been revised and updated many times based on morphological and molecular analyses. We report on a crustose coralline algal species collected in June 2011 via snorkeling in the subtidal zone along the beach Abu Qir on the Mediterranean coast of Egypt, as part of a larger macroalgal diversity survey in the region. The species shows significant sequence divergences (3.5%–14.8% in rbc L; 2.9%–11% in psb A) from other closely related Phymatolithon taxa. Morpho-anatomically, this species possesses the characters considered collectively diagnostic of the genus Phymatolithon , namely, thalli non-geniculate epithelial cells and non-photosynthetic and domed-shaped meristematic cells, usually as short with progressive elongation of their perithallial derivatives. Based on molecular and morphological analyses, we determined that these specimens encompass a new, distinct species that we herein name Phymatolithon abuqirensis. Including this new species, the total number of described Phymatolithon species found in the Mediterranean Sea is now six. 

The recently described crustose calcifying red algal species Ramicrusta hawaiiensis, known only from mesophotic depths off Lehua Island, west of Kaua’i Island, was found in shallow benthic reef habitats (3-18 m deep) along the western coast of Hawai’i Island. Molecular and microscopy techniques were used for genetic confirmation and for detailed morphological and anatomical examination. Two independent benthic cover survey datasets collected from west Hawai’i Island were used to investigate temporal and geographic distribution of Ramicrusta . In both datasets, we report Ramicrusta at approximately 60% of the sites surveyed. Benthic cover for this alga varies among sites and among years and its presence in west Hawai’i is evident since at least 2003. These findings help to document Hawaiian coral reef ecosystem change and benthic community composition reshuffling. This study also emphasizes the critical importance of taxonomy and proper identification of macroalgal species to understand the potential for phase-shifts of dominant taxa in coral reef ecosystems after environmental disturbances and fluctuations in abiotic factors. In the last decade, members of the red algal order Peyssonneliales have increased in abundance and overgrown other benthic species in reef ecosystems in the Caribbean and tropical Pacific. The novel aspect of finding abundant Ramicrusta in much shallower water than originally described, the decadal presence of Ramicrusta , and its potential for competition with other benthic organisms make this research valuable to coral reef ecology and justify further investigation of Ramicrusta ecology and biology in the Hawaiian Islands and globally. 

DNA sequence analysis of plastid-encoded psbA and rbcL loci, and nuclear-encoded LSU rDNA of rhodolith-forming specimens of Sporolithales from Brazil and the northwestern Gulf of Mexico reveal that they belong to an unnamed species of Sporolithon (Sporolithaceae). Sporolithon amadoi sp. nov. is morpho-anatomically characterized by a vegetative thallus reaching more than 20 cell layers, a tetrasporophyte with tetrasporangial sori slightly raised above the thallus surface that become overgrown and buried after spore release, and by cruciately divided tetrasporangia with pores surrounded by 9–13 rosette cells. Since these morpho-anatomical features are shared with some other Sporolithon species, identification of this species can only be confirmed by DNA sequences. 

DNA sequence analysis of plastid-encoded psbA and UPA, mitochondrion-encoded COI, and nuclear-encoded LSU rDNA of rhodolith-forming crustose coralline algal specimens from the northwestern Gulf of Mexico reveals that Mesophyllum erubescens (Foslie) Me. Lemoine is present in mesophotic rhodolith beds offshore Louisiana and Texas at 39–57 m depth. Morpho-anatomical characters viewed with SEM support the identification of these specimens. Mesophyllum erubescens is reported for the first time offshore Louisiana at Ewing Bank, the Louisiana–Texas border at Bright Bank, and Texas in the Flower Garden Banks National Marine Sanctuary. 

A coralline rhodolith was collected from a depth of 178 m, the lower end of the mesophotic zone, on Plantagenet Bank offshore of Bermuda by the submersible Nomad using a mechanical armature. DNA sequence comparisons of the plastid encoded psbA and rbcL loci of this specimen to Sporolithales from the tropical western Atlantic and worldwide revealed it to be an unnamed species of Sporolithon. Sporolithon mesophoticum sp. nov. possesses tetrasporangial sori flush to slightly raised above the thallus surface and has uniquely flattened perithallial and meristematic cells. Thus far, the new species is the deepest known living marine macroalga that has been sequenced and placed into a phylogenetic context. 

Nongeniculate coralline algae are difficult to identify based solely on morpho‐anatomy. To address the systematics of several taxonomically challenging taxa, we analyzed DNA sequences of a short portion (118–296 base pairs) of the 3′ end of therbcL gene from three type specimens. The analyses revealed thatHarveylithon munitum(basionym:Lithophyllum munitum), described in 1906 from Cave Cays, Exuma Chain, Bahamas, is conspecific with bothGoniolithon accretumandGoniolithon affine, described in 1906 from Sand Key, Florida and in 1907 from Culebra Island, Puerto Rico, respectively.Lithophyllum munitumandG. accretumwere described in the same 1906 publication and have equal priority. We have selected the currently accepted and most commonly used nameH. munitumto apply to this entity. Comparative analyses ofrbcL,psbA, UPA, COI, and LSU sequences from contemporary field‐collected specimens revealed thatH. munitumcurrently inhabits mesophotic rhodolith beds in the northwestern Gulf of Mexico, as well as the intertidal zone in the Florida Keys, Honduras, Atlantic Mexico, Caribbean Panama, and Guadeloupe, French West Indies. Species delimitation analyses reveal that the Western Atlantic and AustralianH. munitumpopulations may be separate species. Two new species ofHarveylithonfrom the northwestern Gulf of Mexico and one new species from the southwestern Gulf of Mexico, the Caribbean, and the Red Sea were also identified in the analyses and are described.