How Many Sipunculan Species Are Hiding in Our Oceans?

Abstract

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

Segmented worms (Phylum Annelida): a celebration of twenty years of progress through Zootaxa and call for action on the taxonomic work that remains

“…It is known that this group of marine invertebrates is poorly understood, and the lower rates of sipunculans' new species descriptions reflect the absence of specialists capable of identifying and describing them. Schulze & Kawauchi (2021) summarized the state of knowledge of the number of species in Sipuncula and reinforced the detection of many lineages in different papers with molecular tools that likely represent new species or species synonymized in the past that have to be reinstated. However, they also warn that this gap may stay hidden due to the small number of specialists worldwide and the lack of investment and recognition for taxonomic work.…”

Annelid Diversity: Historical Overview and Future Perspectives

“…The current ranges of valid nominal species in the literature go from 14,000 to 20,000 [7,[86][87][88], and databases such as WoRMS currently considers 23,774 accepted species of extant annelids [89]. Recounting the number of species after the latest revisions, such as the Handbook of Zoology chapters [29][30][31] and the present special issue ( [90][91][92][93][94][95][96][97][98][99][100][101]), there seems to be around 20,000 currently accepted nominal species (Figure 3). There is a continuous documentation of new species and diversity patterns as new taxonomic surveys are carried out in poorly explored geographic areas and localities, in new environments, such as the deep-sea and, surprisingly, also in apparently well-known zones when using different collecting gear, sorting methods or identification techniques, such as SEM and molecular taxonomy.…”

“…com. [94]; Terebelliformia [93]; Arenicolidae [89]; Opheliidae [90], Salibregmatidae and Travisia [89,90]; Capitellida [89]; Spionida [103][104][105][106]; Sabellariidae [89,107]; Sabellida [99]; Siboglinidae [108]; Cirratuliformia [89,100]; Orbiniida [95]; Phyllodocida [101]; Eunicida [91]; Sipuncula [97]; Amphinomida [89].…”

On the occurrence of Antillesoma antillarum (Grube, 1858) (Sipuncula, Antillesomatidae) on the south-central part of the Colombian Caribbean coast

“…The body is formed in two main parts: a larger trunk without segmentation and a retractable introvert, with a tentacular arrangement at the most distal part, either arranged in a circle around the mouth or in a horseshoe shape around the chemosensory nuchal organ. This tentacular crown is more or less developed depending on the taxonomic group (Spongberg 2006;Schulze and Kawauchi 2021). The introvert is often adorned with shields, papillae, and proteinaceous hooks.…”

“…Some species, particularly in the genera Aspidosiphon Diesing, 1851 and Cloeosiphon Grube, 1868, have proteinaceous shields at the anterior and sometimes at the posterior end. Antillesoma antillarum (Grube, 1858) have some purple pigment on their tentacles or at the tentacle base (Rice 1975;Cutler 1994;Adrianov et al 2006;Adrianov and Maiorova 2012;Schulze and Kawauchi 2021).…”