Phylogeography of the Christmas Tree worm Spirobranchus corniculatus (Annelida, Serpulidae) from the Coral Triangle, Australia, and Fiji

Abstract

Christmas Tree Worms is a common name given to a group of large and colourful serpulid polychaetes of the genus Spirobranchus that are obligate symbionts of hermatypic corals commonly found in tropical and subtropical regions world-wide. While current taxonomic species delimitations of these worms are based almost exclusively on the morphology of opercular structures, particularly the number and shape of calcareous opercular horns, high levels of intraspecific variation in this morphological feature have made species boundaries within the genus difficult to resolve. Molecular work to differentiate serpulid worms has long been hampered by the availability of informative markers. Recent work on Hydroides, however, has facilitated the use of COI barcoding in serpulids, which, in conjunction with existing nuclear and mitochondrial markers, can provide a robust interpretation of phylogenetic relationships within Spirobranchus. Here, we present a multi-gene phylogenetic reconstruction of the Spirobranchus corniculatus- a species complex that tentatively includes three morphospecies: S. corniculatus S. str., S. cruciger, and S. gaymardii. Samples spanning the Coral Triangle (Indonesia and the Philippines) as well as Australia and Fiji were included to determine the geographical distribution of putative lineages. Morphological analyses of opercular structure variability were used to test the hypothesis that the S. corniculatus-complex includes three genetically distinct lineages identifiable by their opercula. Haplotype diversity measures revealed higher levels of diversity within the central Coral Triangle region and lower diversity measures at peripheral locations, indicating recent colonization of these areas or, alternatively, a high extinction rate of haplotypes at these sites.