Relations of the new phylum Cycliophora

The Cycliophora is the most recently described animal phylum and is based on a single species, Symbion pandora, which was discovered on the mouthparts of the Norway lobster Nephrops norvegicus1. Because only a few morphological data1,2,3 are available for Symbion, the precise nature of its phylogenetic relationships is highly controversial2,4,5. Here we present a phylogenetic analysis of 18S ribosomal RNA sequence data, including a new Symbion sequence, which places Symbion in a lophophorate-aschelminth-protostome clade and which suggests a sister-group relationship between Cycliophora and a Rotifera-Acanthocephala clade.