Spiral cleavage and early embryology of a loxosomatid entoproct and the usefulness of spiralian apical cross patterns for phylogenetic inferences

By applying fluorescence staining in combination with confocal microscopy and 3D-imaging techniques, we analyzed early embryonic development of a basal loxosomatid entoproct. We found that cleavage is asynchronous, equal, and spiral. An apical rosette, typical for most spiralian embryos, is formed. We also identified two cross-like cellular arrangements that bear similarities to both, a "molluscan-like" as well as an "annelid-like" cross, respectively.A broad comparison of cleavage types and apical cross patterns across Lophotrochozoa shows high plasticity of these character sets and we therefore argue that these developmental traits should be treated and interpreted carefully when used for phylogenetic inferences.Currently, bilaterian animals are subdivided into four major groups: the supposedly basal Acoelomorpha, the Ecdysozoa (combining all molting animals such as arthropods and nematodes), the Lophotrochozoa with a trochophore-like ciliated larva (e.g., Annelida, Entoprocta, Mollusca, Platyhelminthes), and Deuterostomia (including chordates, hemichordates and echinoderms) [1-4]. Despite ongoing efforts, the interrelationships of the phyla that nest within the Lophotrochozoa remain unresolved [5,6]. Entoprocta is a phylum that has been proposed to belong to a clade of spirally cleaving animals, the so-called Spiralia, which together with its suggested sister group, the Lophophorata (Ectoprocta, Brachiopoda, and Phoronida), forms the Lophotrochozoa [1]. Typically, entoprocts are microscopic, mostly marine, sessile metazoan animals. Its approximately 150 hitherto described species are divided into four subgroups, the solitary (and supposedly basal) Loxosomatidae and the colonial Barentsiidae, Pedicellinidae, and Loxocalypodidae [7]. Their adult gross morphology is characterized by a ciliated tentacle crown, which surrounds both the mouth and the anus. The calyx houses the reproductive organs, mostly one pair of protonephridia, and the cerebral ganglion. Entoprocts