Possible progenesis in Neophasis anarrhichae (Nicoll, 1909) Bray, 1987 in the White Sea

Neophasis anarrhichae (Nicoll, 1909) Bray, 1987, unlike the majority of acanthocolpid digeneans, has an abbreviated two-host life cycle. The reproduction of rediae, development of cercariae, and their transformation into unencysted metacercariae occur within the only intermediate host, the whelk Buccinum undatum. Normally, the metacercariae develop into sexual adults (maritae) and egg production starts when the infected whelk is eaten by a wolffish Anarhichas lupus. In the White Sea, we have found three cases of infection of B. undatum by progenetic metacercariae of N. anarrichae. These metacercariae had a fully developed and functioning hermaphroditic reproductive system, and eggs were found in their uterus. Most eggs observed in the histological sections were abortive, but some contained embryos at early stages of development. The progenetic metacercariae were similar in their morphometric characteristics to the sexual adults from the wolffish, the main differences being the size of the ovary and eggs. In order to confirm progenesis, and thus a facultative one-host life cycle in N. anarrichae, we need to prove that the eggs from metacercariae are viable.