Chondrus retortus (Gigartinales, Rhodophyta) in Hawai‘i: a taxonomic and biogeographic puzzle

Members of the genus Chondrus are well-known from temperate and cold waters. Chondrus ocellatus Holmes was reported from Hawai‘i Island (19° N latitude) in 1999 as a new record based on vegetative and tetrasporangial characteristics. The first specimens were collected by Setchell in 1900 in Hilo, HI. The presence of a Chondrus species in the subtropics has been a phycological enigma for over 100 years. We addressed the question of species identity and biogeographic affinities of the Hawaiian Chondrus with fresh cystocarpic material, DNA samples, and phylogenetic analyses. Analysis and comparison of five genes (nuclear: EF2; plastid: psbA, rbcL, and 23S/UPA; mitochondrial: COI) from Hawaiian Chondrus and holotype and topotype material of 10 of the 11 accepted Chondrus species indicate that Hawaiian specimens are C. retortus Matsumoto et Shimada. However, unlike type material, the Hawaiian specimens are commonly pinnulate, vary significantly in secondary medullary filament density, and have mature cystocarps filling the entire medullary space. This study shows the value of using multi-gene loci and comparing multiple sequences of several species to confirm taxonomic conclusions. Our findings suggest that C. retortus may have immigrated via rafting on natural floating material or on ships’ hulls. Solving this old puzzle adds new insight into Hawaiian phytogeography.