Cryptic diversity in the euryhaline red alga Caloglossa ogasawaraensis (Delesseriaceae, Ceramiales)

Abstract

:Caloglossa ogasawaraensis is one of the most widespread species of its genus, having been reported from marine to freshwater habitats in tropical to temperate regions of both hemispheres. Because little phenotypic variation has been reported for this species, we investigated genetic, physiological, and/or morphological variations in 18 strains of C. ogasawaraensis isolated from 14 countries. The combined data of plastid-encoded ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) gene and its spacer, nuclear-encoded large subunit (LSU) rDNA, and mitochondrion-encoded cox spacer region suggested that this species is divided into three groups; Group I contains Australian, Melanesian, and Micronesian strains; Group II includes strains from western Pacific coasts, Micronesia, and Madagascar; and Group III consists eastern Pacific–Atlantic and Malaysian strains. Considering wide distribution range and close genetic similarity within the groups, a long-distance dispersal event does not seem to be rare in this species. Growth of the strains was compared between brackish (salinity = 8) and seawater (salinity = 32) media; most Group I strains grew well in both brackish and seawater media; whereas, all the Group II strains showed better growth in the brackish medium. Morphological analyses of the strains incubated under the same conditions revealed that the Group III strains had wider rhizoids than the other strains. However, this feature was highly variable and easily affected by environmental and algal conditions. Although such morphological and physiological variations suggest speciation of the three phylogenetic groups, more clear-cut features are required to describe them as distinct morpho-species.