Paul W. Gabrielson

Phymatolithon acervatum is an encrusting, non-geniculate coralline alga from South Africa. While morpho-anatomical details of the species are well documented, no genetic analyses have been performed on P. acervatum . Here, we analyzed rbc L and psb A gene sequences, and they showed that two species were passing under the name Phymatolithon acervatum in South Africa. A partial rbc L sequence from the lectotype of Lithothamnion acervatum , basionym of P. acervatum , showed to which species the specific epithet applies, and furthermore that both species belong in Phymatolithopsis. We transfer Phymatolithon acervatum to Phymatolithopsis , as Phymatolithopsis acervata comb. nov., and propose Phymatolithopsis roseola sp. nov. These species overlap in morpho-anatomy, habitat and distribution and can only be distinguished based on DNA sequences. Previous misinterpretations of conceptacle development are corrected for species of Phymatolithopsis. The main character used to define the family Mesophyllumaceae is shown to have arisen several times in independent lineages in the order Hapalidiales. At present, only one family, Hapalidiaceae, thus merits recognition in the Hapalidiales.

Porolithon is one of the most ecologically important genera of tropical and subtropical crustose (non‐geniculate) coralline algae growing abundantly along the shallow margins of coral reefs and functioning to cement reef frameworks. Thalli of branched, fruticose Porolithon specimens from the Indo‐Pacific Ocean traditionally have been called P. gardineri , while massive, columnar forms have been called P. craspedium . Sequence comparisons of the rbc L gene both from type specimens of P. gardineri and P. craspedium and from field‐collected specimens demonstrate that neither species is present in east Australia and instead resolve into four unique genetic lineages. Porolithon howensis sp. nov. forms columnar protuberances and loosely attached margins and occurs predominantly at Lord Howe Island; P. lobulatum sp. nov. has fruticose to clavate forms and free margins that are lobed and occurs in the Coral Sea and on the Great Barrier Reef (GBR); P. parvulum sp. nov. has short (<2 cm), unbranched protuberances and attached margins and is restricted to the central and southern GBR; and P. pinnaculum sp. nov. has a mountain‐like, columnar morphology and occurs on oceanic Coral Sea reefs. A rbc L gene sequence of the isotype of P. castellum demonstrates it is a different species from other columnar species. In addition to the diagnostic rbc L and psb A marker sequences, the four new species may be distinguished by a combination of features including thallus growth form, margin shape (attached or unattached), and medullary system (coaxial or plumose). Porolithon species, because of their ecological importance and sensitivity to ocean acidification, need urgent documentation of their taxonomic diversity.