Systematics and taxonomy of <i>Codium</i> (Bryopsidales, Chlorophyta) in the Hawaiian Islands: Description of six new species

The well-known Bryopsidalean genus Codium has a worldwide distribution and contains almost 150 species, with cryptic diversity confusing the actual number. In the Hawaiian Islands, 15 species have been previously recorded, with several of these described in the past several decades, largely from specimens collected from mesophotic coral ecosystems. We assessed the diversity of Codium in Hawai'i from both shallow and mesophotic habitats by employing DNA barcoding and phylogenetic analyses of partial rbcL and tufA gen plastid markers and morphological characterization. DNA sequence analyses supported 18 species of Hawaiian Codium (eight of which are considered endemic), which is a 20% increase in recognized species richness for this genus in Hawai'i. Ten previously reported species were confirmed or provisionally confirmed, six new species have been described (C. pikoii sp. nov., C. upohoae sp. nov., C. hakakaupilii sp. nov., C. kanepohihiae sp. nov., C. torulosum sp. nov., and C. iolekaae sp. nov.), and two new records have been reported (C. "geppiorium4" and C. taylorii). Twenty-eight percent of Hawaiian Codium clades were mesophotic only, and 22% were shallow only, while 50% of clades were known from both shallow and mesophotic depths. Recent emphasis on the systematics of Hawaiian mesophotic algae has sufficiently increased specimen numbers from this habitat to allow a more complete assessment of the genus in this location, making it one of the most thoroughly collected and studied marine algal genera from Hawai'i and an excellent model for future examination of both horizontal (i.e., spatial) and vertical (i.e., depth) distributional trends.