Life cycle truncation in Digenea, a case study of Neophasis spp. (Acanthocolpidae)

Truncated life cycles may emerge in digeneans if the second intermediate host is eliminated, and the first intermediate host, the mollusc, takes up its role. To understand the causes of this type of life cycle truncation, we analyzed closely related species of the genus Neophasis (Acanthocolpidae) with three-host and two-host life cycles. The life cycle of Neophasis anarrhichae involves two hosts: wolffishes of the genus Anarhichas as the definitive host and the common whelk Buccinum undatum as the intermediate host. Neophasis oculata, a closely related species with a three-host life cycle, would be a suitable candidate for the comparison, but some previous data on its life cycle seem to be erroneous. In this study, we aimed to redescribe the life cycle of N. oculata and to verify the life cycle of N. anarrhichae using molecular and morphological methods. Putative life cycle stages of these two species from intermediate hosts were linked with adult worms from definitive hosts using ribosomal molecular data: 18S, ITS1, 5.8S-ITS2, 28S. These markers did not differ within the species and were only slightly different between them. Intra- and interspecific variability was also estimated using mitochondrial COI gene. In the constructed phylogeny Neophasis spp. formed a common clade with two other genera of the Acanthocolpidae, Tormopsolus and Pleorchis. We demonstrated that the first intermediate hosts of N. oculata were gastropods Neptunea despecta and B. undatum (Buccinoidea). Shorthorn sculpins Myoxocephalus scorpius were shown to act as the second intermediate and definitive hosts of N. oculata. The previous reconstruction of the two-host life cycle of N. anarrhichae was reaffirmed. We suggest that life cycle truncation in N. anarrhichae was initiated by an acquisition of continuous morphogenesis in the hermaphroditic generation and supported by a strong prey-predator relationship between A. lupus and B. undatum.