Between 2019 and 2025, metacercariae of the trematode genus Lecithocladium (Hemiuridae) were found in gelatinous zooplankters occurring in Japanese coastal waters. To identify these metacercariae and elucidate their life cycles, fish hosts were simultaneously examined for adult trematodes. Molecular analyses of the nuclear 28S ribosomal RNA (rRNA) and mitochondrial cytochrome c oxidase subunit 1 (COI) regions, comparing them with adult Lecithocladium species from fish hosts, confirmed the metacercariae as those of L. magnacetabulum and L. cf. excisiforme. Metacercariae of L. magnacetabulum were detected in the comb jelly Beroe cucumis and the jellyfish Aequorea coerulescens; adults of this species had previously been recorded in the Japanese butterfish Psenopsis anomala and the red seabream Pagrus major. For L. cf. excisiforme, metacercariae were detected in the arrow worm Aidanosagitta crassa and an adult in the whitetongue jack Uraspis helvola. These predatory gelatinous zooplankters are presumed to be the third (or higher) intermediate or paratenic hosts, given that their prey, planktonic copepods, are typically reported as the second intermediate hosts for hemiurid species. Furthermore, adults of two additional hemiurid species, L. excisum and Ectenurus trachuri, were also isolated from the fish hosts. Cumulatively, this work and previous research suggest that gelatinous zooplankters represent crucial transmission routes for hemiurid trematodes.

Gymnophallidae is one of the digenean families featuring bivalves as first intermediate hosts. However, the exact bivalve host species remain unknown for most members of this family. Gymnophallids have been one of the targets in our continuous efforts to reveal the diversity of digeneans in the higher north. Here, we focus on Gymnophallus minor, which we found in eiders from various locations in the Arctic and sub-Arctic. Sexual adults (maritae) of G. minor can be easily identified because they have a distinctive character: the roughly equal size of the pharynx and the ventral sucker. We also matched them, using DNA markers, with the intramolluscan stages (sporocysts, cercariae, and metacercariae) from the bivalve Liocyma fluctuosa collected on Spitsbergen. Taken together, we compile the first data on the life cycle of G. minor and discuss them in the context of other gymnophallids.

In the digenean life cycle the cercaria ensures an important transmission stage, from the first intermediate host to the second or the definitive host. In regions with pronounced seasonality, this process occurs within a certain interval, the transmission window. In high latitudes, the size of transmission window has previously been determined only by comparing data on seasonal dynamics of infection level in various categories of hosts or extrapolating the results of laboratory experiments on cercarial biology to natural conditions. In this study, we evaluated the dynamics of infection of the second intermediate hosts (mussels Mytilus edulis) with cercariae of two digenean species, Himasthla elongata (Himasthlidae) and Cercaria parvicaudata (Renicolidae), at a littoral site at the White Sea by exposing cages with uninfected mussels during the warm season. This is the first such study in a subarctic sea. Mussel infection was observed from May to mid-September, but its intensity was the greatest only for approximately a month, from some moment after 10 July to mid-August, when water temperature was within the optimal range for cercarial emergence in both studied species (15‒20°C). During this time, the mussels accumulated 66.3 ± 6.2% metacercariae of H. elongata and 79.7 ± 5.3% metacercariae of C. parvicaudata out of the total number accumulated during the experimental period. We suggest that climate warming at high latitudes may prolong the period when the water temperatures are optimal for cercariae emergence, thereby intensifying digenean transmission in coastal ecosystems.

During the 1990 grazing season, ants found attached to plants were collected from sheep pastures at monthly intervals and examined for Dicrocoelium metacercariae. The species Formica pratensis and F. rufibarbis showed average metacercarial intensities of 76 and 38 respectively. In addition to the ant species, the intensity of infection was found to be dependent on the length of the ant. The volume of metacercarial cysts was influenced by the intensity per ant relative to ant length but not by the ant species or the time of ant collection.

‘O’ group mullet, Chelon labrosus. were experimentally infected with Cryptocotyle lingua (Heterophyidae) by tail dip in a suspension of cercariae. Metacercariae were excised after 1 and 24 hours and prepared for TEM and post-embedding immunogold labelling. Antisera to cercariae of C. lingua were raised in adult mullet by natural infection via the skin and by intra-peritoneal injection of sonicate. The membrane-bound vesicles within the syncytial lining of the metacercarial excretory vesicle were found to be intensely antigenic with both antisera: the epidermal secretory bodies type 5 within the cystogenousglands gave a positive response. Penetration gland contents were not found to be antigenic with either antiserum. Discharge of the membrane-bound vesicles coinciding with both the reorganization of the lining of the metacercarial excretory vesicle and with cyst wall formation appears to be of significance in the initiation of the host immune response. That the term ‘excretory vesicle’ in Digenea may be a misnomer is discussed in the light of current information regarding the wide range of functions attributed to this structure.

Susceptibility and development of Clinostomum complanatum in the chicken (Gallus gallus domesticus) have been studied. The progenetic metacercariae of C. complanatum developed into ovigerous worms in 3 days in the buccal cavity of the chicken and survived only 6–7 days in this experimental host. The susceptibility is influenced by the age, sex and breed/type of the host. Both percent susceptibility and recovery of the worms decreased with increase in the age of the host. The egg laying chickens were found to be least susceptible to the infection. Mature male chickens were more susceptible than the females. The single comb white Leghorn (SCWL) was found to be susceptible while the local type (LT) and the cross breed (SCWL×LT) were not susceptible to C. complanatum. The possible reasons for these variations have been discussed.