Molecular confirmation and morphological reassessment of <i>Udotea geppiorum</i> (Bryopsidales, Chlorophyta) with ecological observations of mesophotic meadows in the Main Hawaiian Islands

<i>Udotea geppiorum</i> is reported to form meadows in deep-water, soft sediment habitats in the Main Hawaiian Islands. The identification of the species, initially published as <i>U. argentea</i> in the archipelago, is revised and confirmed based on morphological assessment and comparative DNA sequencing with western Pacific specimens. <i>Udotea geppiorum</i> is fully corticated and newly shown to display discrete concentric segments arranged in a ‘tongue and groove’ manner, which are connected by medullary siphons running along the entire segment interface. <i>Udotea geppiorum</i> is phylogenetically nested among a cluster of <i>Udotea</i> spp. recently circumscribed within <i>Udotea sensu stricto</i>. This cluster of species is separate from the remainder of the family Udoteaceae, which includes several genera and <i>Udotea</i> spp. <i>sensu lato</i> that lack or exhibit incomplete cortication. Based on field observations conducted in O‘ahu and Maui, <i>U. geppiorum</i> can be found from 20 m to &gt; 90 m depth with a peak in abundance at mesophotic depths of 60–85 m, where it forms extensive meadows that support associated diversity.