Newly discovered associations between peritrich ciliates (Ciliophora: Peritrichia) and scale polychaetes (Annelida: Polynoidae and Sigalionidae) with a review of polychaete–peritrich epibiosis

In this research, we report the presence of two ciliate protozoans of the subclass Peritrichia, Cothurnia amphicteis and C. peloscolicis, as epibionts on the chaetae of scaled polychaetes Malmgrenia lilianae, M. andreapolis (fam. Polynoidae) and Sthenelais boa (fam. Sigalionidae), from the north Adriatic (Mediterranean Sea). Both ciliate species are herein found for the first time after their original description and are redescribed, based on light and scanning electron microscopy analyses. This is the first record of an association between ciliates and polychaetes of the family Sigalionidae. Our results suggest that these host–epibiont relationships might be highly specific. We also present the first review of epibiosis between polychaetes and peritrich ciliates, indicating that this relationship is more diverse than previously thought. Forty taxa of peritrich ciliates from 12 genera and seven families are recorded as epibionts on polychaetes, while 48 polychaete taxa are known as their hosts. The relationship can be considered ectocommensalism, where the ciliates have the advantages of increased food availability. This association might be a more widespread phenomenon than currently known, because it could be easily overlooked or misinterpreted. It, therefore, deserves careful attention and further investigation.