Phylogenetic relationships in the genus Astropecten Gray (Paxillosida: Astropectinidae) on a global scale: molecular evidence for morphological convergence, species-complexes and possible cryptic speciation

Abstract

With over 150 described species, Astropecten Gray (Paxillosida:Astropectinidae) is one of the most species-rich genera among sea stars. This diversity is remarkable, because most species of Astropecten have a long-lived planktotrophic larval stage, which would be expected to lead to a low speciation rate. The taxonomy of this genus is complex and not well resolved, and phylogenetic relationships have only been addressed in the beginning of the last century. In order to resolve general taxonomic issues, identify speciation patterns and estimate species diversity within the genus Astropecten, we inferred a molecular phylogeny of 117 specimens of Astropecten belonging to 40 species from around the world using mitochondrial DNA (mtDNA) sequences of 12S rRNA, 16S rRNA and cytochrome oxidase subunit I (COI). We compared the resulting molecular phylogeny to a previously published morphological one by Döderlein and investigated the possibility of morphological convergence in species from different geographic regions. Finally, we also aimed at identifying potentially problematic descriptions and/or signs of cryptic speciation in Astropecten. The global molecular phylogeny exhibited three main clades, each containing specimens of the same geographic region: 1. the IndoPacific; 2. the Neotropics; and 3. the eastern Atlantic and Mediterranean. Phylogenetic inferences based on mtDNA indicate that morphological and ecological convergence has taken place in Astropecten, resulting in allopatric non-sister taxa with similar morphologies and habitat preferences. The comparison to Döderlein’s morphological phylogeny reveals congruence on the whole but many discrepancies on a local scale, indicating that meaningful morphological characters are not easily identified and categorized in Astropecten. Our results also reveal that A. polyacanthus Müller & Tröschel and A. indicus Döderlein are species-complexes; cryptic speciation may have occurred within each of these morphospecies. Furthermore, several variants, previously presumed to be conspecific, exhibit genetic distances large enough to justify recognizing them as separate species.