Distribution,delimitation and description of species of Archaphanostoma (Acoela)

Acoela is a small group of microscopic marine worms that live interstitially in sandy or muddy sediments. Although Acoela is an abundant group of the meiofauna, knowledge of their diversity is incomplete. The Swedish fauna with 66 nominal species is one of the best studied in the world. Despite this new species and species new to the area are still found in Swedish waters. In this study, we describe four new species belonging to the family Isodiametridae: Archaphanostoma fontaneti sp. n., Archaphanostoma occulta sp. n., Archaphanostoma sublittoralis sp. n. and Archaphanostoma ylvae sp. n. The new species were collected on the Swedish west coast and in northern Germany during 2011. Each description is accompanied by drawings and microphotographs. Nucleotide sequences for the ribosomal genes 18S rRNA and 28S rRNA as well as for the mitochondrial COI mtDNA gene have been determined for the new species and used in Bayesian and maximum‐likelihood analyses to further support their classification. Diagnostic molecular characters are presented for each species, and we use Bayesian methods and statistical parsimony to delimit the new species within Archaphanostoma. We also use species of Archaphanostoma to test patterns of acoel diversity.