New records of <i>Atherospio</i> (Annelida: Spionidae) and phylogenetic relationships of <i>A. aestuarii</i>

Abstract Here, we summarize and map the previous and new findings on three species of spionid polychaetes. Atherospio disticha is first recorded for the southern and southeastern parts of the UK, and Atherospio guillei is first recorded for the UK and Norway. Atherospio aestuarii is discovered in South Korea, for the first time outside of its type and two other localities in Japan. Adult morphology, mode of life, and reproductive biology of Atherospio species are discussed, and a key to their identification is provided. The phylogenetic relationships of A. aestuarii with other spionids are investigated further based on an expanded set of molecular data, including fragments of four genes: 16S, 18S, 28S, and Histone H3. The Bayesian analysis of the combined dataset nested A. aestuarii as a sister to a large group comprising Marenzelleria, Rhynchospio, and Spio species, while this whole group, including A. aestuarii, appeared sister to Scolelepis spp. This hypothesis of phylogenetic relationships differs from a previous molecular analysis, in which A. aestuarii was grouped together with the species of Dispio and Scolelepis. This is also completely different from earlier hypotheses, according to which the group of Atherospio–Pygospiopsis species was placed in the most basal part of the Spionidae tree in accordance with the available morphological data. We highlight, for the first time, the main drawback of studies on the phylogeny of Spionidae conducted at the end of the 20th century, which were based on external morphology and ignored the main features of conservative internal anatomy, which were used by zoologists in the first half of the 20th century.