Two New Mesophotic Species of the Red Algal Genus Chrysymenia (Rhodymeniaceae, Rhodymeniales) from the Gulf of Mexico: C. stanlaurelii sp. nov. and C. oliverhardyi sp. nov.

Two new mesophotic species of Chrysymenia are reported for the Gulf of Mexico. Chrysymenia oliverhardyii sp. nov. is a broadly foliose species growing attached to rhodoliths (free-living carbonate nodules predominantly accreted by crustose coralline algae) at 58–66 m depth offshore Louisiana (northwestern Gulf) and at ~64–68 m depth in the vicinity of the Dry Tortugas, Florida (southeastern Gulf). A multi-marker phylogenetic analysis based on chloroplast-encoded rbcL and UPA, as well as nuclear LSU rDNA sequences, indicates that this taxon is most closely related to C. stanlaurelii sp. nov., a slender, branched species from the vicinity of the Florida Middle Grounds, Florida (northeastern Gulf), growing at 60 m depth. These two species from the eastern Gulf of Mexico share a basal most recent common ancestor with respect to the other known species of Chrysymenia. Illustrations and discussion are provided for the new species, as well as for the other Gulf of Mexico members C. planifrons, C. littleriana, C. halymenioides and C. nodulosa. A nomenclatural list, morphological figures, a dichotomous key, and a phylogenetic tree of pertinent Chrysymenia are provided.