How Many Sipunculan Species Are Hiding in Our Oceans?

Sipuncula, long considered a separate phylum, are now commonly included in the Annelida based on phylogenomic analyses. The sipunculan body consists of an unsegmented trunk and retractable introvert, usually with set tentacles at its anterior end. Unlike other annelids, they have no chaetae, but introvert is often adorned proteinaceous hooks that can be important taxonomic characters. Other external characters include (number, shape arrangement), papillae and, some cases, hardened shields, as well length ratios. Many species require dissection for correct identification to reveal internal characteristics, such retractor muscles, nephridia contractile vessels. Here we summarize state current knowledge diversity sipunculans. We emphasize molecular studies, conducted over past two decades, revealed multiple complexes cryptic or pseudocryptic species. It has become obvious significantly higher than scheme accounts for, formal descriptions lagging behind. Although major branches phylogeny increasingly consolidated, relationships within most still flux.