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.

Studying complexes of cryptic or pseudocryptic species opens new horizons for the understanding of speciation processes, an important yet vague issue for the digeneans. We investigated a hemiuroidean trematode Lecithaster salmonis across a wide geographic range including the northern European seas (White, Barents, and Pechora), East Siberian Sea, and the Pacific Northwest (Sea of Okhotsk and Sea of Japan). The goals were to explore the genetic diversity within L. salmonis through mitochondrial (cox1 and nad5 genes) and ribosomal (ITS1, ITS2, 28S rDNA) marker sequences, to study morphometry of maritae, and to revise the life cycle data. Mitochondrial markers showed that L. salmonis is likely divided into six lineages (referred to as operational taxonomic units, OTUs), which often occur in sympatry, sometimes in a single host specimen. Variation in rDNA was not consistent with that in the mitochondrial markers. Morphometric analysis of maritae was performed for four out of six OTUs; it showed that some OTUs had significant differences from the others, but some did not. The effect of host species on the morphometric characteristics cannot be excluded. Intramolluscan stages were identified for two OTUs; they differed clearly by the structure of cercariae and also by the species of the first intermediate host. The case of L. salmonis is instructive in how different criteria for species delimitation can contradict each other. We regard this as a sign of recent or ongoing speciation and suggest using the name Lecithaster cf. salmonis. The most promising criteria to differentiate genetic lineages within L. cf. salmonis are first intermediate hosts and morphological characteristics of the cercariae: shape of the delivery tube and caudal cyst, and length of the filamentous appendage.

The family Brachycladiidae (Digenea) is a species-rich taxon restricted to marine mammals, well-known for its associated pathogenic load, but otherwise poorly understood. A major gap in our knowledge is their circulation pathways, as the life cycle has been elucidated only for a single brachycladiid species, Orthosplanchnus arcticus, a parasite of pinnipeds. Our goal was to determine whether small ocellate metacercariae previously reported from the Barents Sea edible cockles (Cerastoderma edule) belong to the brachycladiids. We searched for brachycladiid metacercariae in nine bivalve species across three sub-Arctic Seas. They were found in three cardiid species (Ciliatocardium ciliatum, C. edule, and Serripes groenlandicus) and a scallop, Chlamys islandica. Surprisingly, we discovered numerous brachycladiid metacercariae in a single infected whelk, Buccinum undatum. Obtained molecular genetic data (mitochondrial and nuclear ribosomal) indicated that all these metacercariae belong to O. arcticus. We used statistical analysis to explore potential preferences for the second intermediate hosts in this brachycladiid species. The results suggest that bivalve size, rather than species, more accurately predicts infection patterns with O. arcticus metacercariae. We also found no significant differences in morphology between metacercariae from different bivalve species. As such, O. arcticus exhibits broad specificity for the second intermediate hosts, with larger bivalve individuals showing higher infection rates. We discuss the factors underlying this broad specificity, the benefits of larger bivalve preference, and the unequal contribution of different hosts in the transmission of a parasite. Finally, we highlight the potential zoonotic risk associated with O. arcticus due to human consumption of its intermediate hosts.