Xenoturbella bocki

For the genus name Xenoturbella, Ancient Greek xénos, means foreign or strange, and Latin turbela, means a bustle or turbulence in water. Genus Xenoturbella is a member of sub-phylum Xenoturbellida, which are known as paradoxmaskar[2] – Swedish for "paradox worms" (a term that some popular media have applied to the species), because if it is classified as a deuterostome, it would be more closely related to humans than other, more complex, invertebrates such as lobsters.[3] Deuterostomes are a superphylum of animals whose anus forms before their mouth does during embryonic development. It includes humans, other chordates, echinoderms and hemichordates.

The species signifier bocki refers to Sixten Bock, who first collected the organism in 1915.[4] It was assigned by Swedish zoologist Einar Westblad, who described the species in 1949.

In 1999, examination of X. bocki specimens held at the Swedish Museum of Natural History showed that a small subset of them must belong to another species.[5] This population differed from specimens identified as X. bocki in internal fertilization, its small size of 12 mm (0.47 in) at most, and its pink coloration – in contrast to yellow-white coloration identified for X. bocki. The new taxon was named after Westblad, who collected the specimens from coarser and shallower habitats in the same range as X. bocki. However, mitochondrial DNA sequencing from the specimens identified with both species suggested that the two populations belonged to the same species, involving that X. westbladi is a junior synonym to X. bocki.[6]

Comparison of mitochondrial DNA and protein sequences showed that the species Xenoturbella bocki – often found off the coast of Sweden – is the sister group to X. hollandorum, a species discovered in 2016 in eastern Pacific Ocean.[7] In turn, these two species share evolutionary affinities with X. japonica into a clade of 'shallow-water' taxa.[8]

The two competing hypotheses as to the phylogeny of Xenoturbella are shown in red. One suggests it is a deuterostome (right), while the other suggests it is a basal bilaterian (left).

When it was discovered, X. bocki was placed in a new genus Xenoturbella. Above the genus level, the classification of this animal is controversial. Westbald placed it in the phylum Platyhelminthes in the class Turbellaria (free-living flatworms).[9] In 1999, based on genetic analysis, it was placed in Protostomia by Israelsson, grouped with the bivalves.[10][11] Protostomia is a large clade including worms, mollusks and arthropods. In embryonic development, their mouth develops prior to the development of the anus for most protostomes, though some have evolved other developmental pathways. If placed in this clade, Xenoturbella would also be among these exceptions.[12]

However, today, this is understood as a misclassification due to contaminating DNA from its shellfish food. Swedish scientist Sarah J. Bourlat and her coauthors in 2006 placed it in its own phylum, Xenoturbellida. More recent studies suggest on the basis of genetic and developmental evidence (e.g. Hox genes) that it should be grouped with Acoela and Nemertodermatida into Acoelomorpha. These three taxa are sometimes placed within the deuterostomes (a large clade that includes humans and other chordates, sea stars and others),[12] while others classify these organisms as a basal offshoot that resembles a common ancestor of deuterostomes and protostomes.[11] A 2016 analysis of many genetic data sets supports the latter, and suggests that, like Xenoturbella bocki, the common ancestor of protostomes and deuterostomes likely had one opening, ciliated locomotion and a wormlike body.[13] However, if the deuterostome hypothesis is correct, then Xenoturbella must have lost many ancestral traits, such as an anus.[14]