Species of the genus Tylodelphys (Diplostomidae) have a cosmopolitan distribution. Metacercariae of these species infect the eye, brain, pericardial sac or body cavity of fish second intermediate hosts, and the adults are found in piscivorous birds of many orders. An unnamed species of Tylodelphys from the eyes of bullies (Gobiomorphus cotidianus) was characterized molecularly and morphologically as a metacercaria in a previous study, in which it was predicted that the adult of this species would be found in the Australasian crested grebe. Two specimens of this bird became available and specimens of the unnamed Tylodelphys species were, indeed, found in them, confirmed by identity of genetic sequence data. Found to differ morphologically from its congeners, the new species is here described as Tylodelphys darbyi n. sp. Three species are closest to the new species in morphology: Tylodelphys glossoides, T. immer and T. podicipina robrauschi. Compared with T. darbyi n. sp. these three species are slightly larger and possess longer eggs. Tylodelphys glossoides also differs in having a wider oral sucker and T. podicipina robrauschi in having comma- or kidney-shaped pseudosuckers and an ovary that reaches a larger size, along with higher upper limits for body width, hind body and sucker width, holdfast and oesophagus length, and pharynx, pseudosucker and testes length and width. Tylodelphys immer also differs from T. darbyi n. sp. in having a shorter ventral sucker and the largest pseudosuckers of any Tylodelphys species.

The supralittoral amphipod Traskorchestia ditmari (Derzhavin, 1923) was identified as the intermediate host for Antechiniella septentrionalis Ivanova, Dokuchaev & Spiridonov, 2019, a parasite of the tundra vole Microtus oeconomus and Skrjabinocerca sp. (both Spirurida: Acuariidae) in Magadan Oblast in north-eastern Russia. Joint infection by both larval spirurids was not observed. The infective stage of A. septentrionalis was the encysted larvae, while larvae of Skrjabinocerca sp. were free in the amphipod's coelom. The identity of A. septentrionalis was confirmed using cox1 mtDNA gene analysis, performed on adult stages from a tundra vole and on larvae from amphipods. Possible transmission routes of A. septentrionalis are discussed.

We describe Hedruris dratini n. sp. (Nematoda, Hedruridae) from Hydromedusa tectifera and Phrynops hilarii in Argentina based on morphological and molecular characters. Also, we provide information about its life cycle. The new species differs from other species of the genus by possessing the excretory pore, nerve ring and deirids at equal distance from the anterior end. Additionally, H. dratini n. sp. has mammilated eggs and males possess nine pairs of caudal papillae. The subadults and adults of H. dratini n. sp and H. orestiae were characterized by sequencing the small subunit ribosomal DNA (18S). We present for the first time a life cycle of a species of Hedruris that includes an amphipod as intermediate host and a reptile as definitive host. Furthermore, we analysed the host and geographic distribution of all Hedruris species. Although the genus has a cosmopolitan distribution and parasitizes a great host diversity, the majority of species have a Gondwanian distribution, with amphibians being the preferred hosts.

The adult and metacercaria life stages of a new species of the microphallid genus Atriophallophorus Deblock & Rosé, 1964 are described from specimens collected at Lake Alexandrina (South Island, New Zealand). In addition to molecular analyses of ribosomal and mitochondrial genes, metacercariae of Atriophallophorus winterbourni n. sp. from the snail host Potamopyrgus antipodarum (Gray) were grown in vitro to characterize internal and external morphology of adults using light and scanning electron microscopy and histological techniques. Atriophallophorus winterbourni n. sp. is readily distinguishable from Atriophallophorus coxiellae Smith, 1973 by having a different structure of the prostatic chamber, sub-circular and dorsal to genital atrium, rather than cylindrical, fibrous, elongate and placed between the seminal vesicle and the genital atrium. The new species is most similar to Atriophallophorus minutus (Price, 1934) with regards to the prostatic chamber and the morphometric data, but possesses elongate-oval testes and subtriangular ovary rather than oval and transversely oval in A. minutus. Phylogenetic analyses including sequence data for A. winterbourni n. sp. suggested a congeneric relationship of the new species to a hitherto undescribed metacercariae reported from Australia, both forming a strongly supported clade closely related to Microphallus and Levinseniella. In addition, we provide an amended diagnosis of Atriophallophorus to accommodate the new species and confirm the sinistral interruption of the outer rim of the ventral sucker caused by the protrusion of the dextral parietal atrial scale at the base of the phallus.

Parasites can have strong effects on invertebrate host behaviour, fecundity and survival in marine ecosystems. However, parasites are often poorly documented and still rarely integrated into marine ecological modelling; comprehensive surveys of infection in marine invertebrates are sporadic at best. For example, rock crabs are an important part of Californian coastal ecosystems, both as regulators of mussel populations and non-native species, and as prey items for predators like sea otters, but their parasite communities and potential effects on crab population dynamics are seldom studied or understood. Here, we present the first report of infection by the trematode Helicometrina nimia in the economically and ecologically important red rock crab (Cancer productus) and Pacific rock crab (Romaleon antennarium). As intermediate hosts, they are a missing link for infection by H. nimia in Californian fish that was unreported until now. Based on these findings, we advocate for further research into parasite diversity and their potential effects on ecologically and commercially important species.

Few digeneans of the family Fellodistomidae are known from the Russian Arctic seas. The taxonomic status of these species, their life cycles and host range raised recurrent questions, some of which remain unanswered. To revise the species composition and life cycles of fellodistomids in the White Sea, we searched for them in several known and suspected hosts: wolffish, flatfishes (definitive), gastropods of the family Buccinidae (second intermediate) and protobranch bivalves (first intermediate). Species identification was based both on morphology and 28S ribosomal RNA gene sequences. We found Fellodistomum agnotum in the White Sea for the first time. Buccinum undatum was proved to be intermediate host of both F. agnotum and Fellodistomum fellis, and metacercariae of F. fellis were registered from two more buccinid species: Buccinum scalariforme and Neptunea despecta. We also found metacercariae of F. agnotum and F. fellis producing eggs in the second intermediate host. Two fellodistomids were found in protobranch bivalves: sporocysts and cercariae of Steringophorus furciger in Nuculana pernula, and sporocysts with large furcocercous cercariae in Ennucula tenuis. The latter were identified as F. agnotum by molecular analysis; thus, the entire life cycle of this species was reconstructed.

Aulonocephalus pennula is a heteroxenous nematode that commonly infects a declining game bird, the northern bobwhite quail (Colinus virginianus). There is a lack of information on the life cycle of A. pennula and the potential effects of infection on bobwhites. In order to better understand the life cycle of this parasite, various species from the order Orthoptera were collected from a field site in Mitchell County, Texas. Using polymerase chain reaction (PCR), nine potential intermediate hosts were identified from the 35 orthopteran species collected. Later, ten live specimens were collected to identify larvae within the potential intermediate hosts. Larvae were present in three of these and were sent for sequencing. Similarly, the presence of larvae was confirmed from extra tissues of samples identified as positive with PCR. This was the first study to document potential intermediate hosts, but future studies are needed to confirm that these species are capable of transmitting infection to bobwhite. However, this study demonstrates that PCR has increased sensitivity and may be a valuable tool when determining intermediate hosts.

Sequence data were combined with morphological analyses to identify two lepocreadiid trematode species from jellyfishes and fishes. Three species of jellyfish were captured within Port Phillip Bay, Australia, and three species of fish that feed on jellyfish were obtained from Moreton Bay (Queensland) and Port Phillip Bay and Portland (Victoria). The digeneans were distributed throughout most parts of the jellyfish. Opechona cf. kahawai Bray & Cribb, 2003 parasitized the scyphozoan jellyfish Aequorea eurodina and the scombrid fish Scomber australasicus. Cephalolepidapedon warehou Bray & Cribb, 2003 parasitized the scyphozoans Pseudorhiza haeckeli and Cyanea annaskala, and the centrolophid fishes Seriolella brama and Seriolella punctata. Intensities ranged from four to 96 in the jellyfish, and one to 30 in the fish. For both trematode species, internal transcribed spacer 2 of ribosomal DNA sequences from mature adults in the fishes matched those from metacercariae from the jellyfish. This is the first record of larval stages of C. warehou and O. cf. kahawai, and the first use of DNA sequencing to identify digenean trematode metacercariae from jellyfish. Three new host records are reported for C. warehou and two for O. cf. kahawai.

Nematode species of the genus Contracaecum Railliet & Henry, 1912 have been reported around the world in many species of fish-eating birds and seals. Here, Contracaecum jorgei n. sp. is morphologically described using light and scanning electron microscopy for adults and fourth-stage larvae (L4) found in the bird Nannopterum brasilianus and third-stage larvae (L3) found in the freshwater fish Hoplias argentinensis, both from the province of Córdoba, Argentina. Additionally, sequences of cytochrome c oxidase subunit II were obtained from these specimens and molecular phylogenetic analysis was used to determine its relationships within the genus. The present species is distinguished from other species by the number and disposition of cephalic papillae; shape and size of the interlabia; length of the spicules; and number and arrangement of papillae in the posterior end of the male. Furthermore, in the molecular analyses, sequences obtained from adult L4 and L3 specimens of C. jorgei n. sp. were similar and grouped, forming an independent lineage, thus confirming it as a distinct species. Thus, morphological characteristics associated with molecular data support the proposal of a new species.

The life cycle of Pleurogenoides wayanadensis Shinad & Prasadan, 2018, infecting the frogs Hoplobatrachus tigerinus and Euphlyctis cyanophlyctis, is elucidated in this study. All the life cycle stages from egg to egg-producing adults were elucidated under natural conditions and successfully established in the laboratory. The life cycle took about 58 to 65 days for completion. Miracidia were released by teasing the eggs with fine needles. Sporocysts were found in the freshwater snail, Bithynia (Digoniostoma) pulchella, collected from paddy fields at Payode, Western Ghats, Wayanad region, in the months of October and November 2019. Cercariae were of the virgulate xiphidiocercous type. Metacercariae were recovered from the eyes of the damselfly naiads of the species Ischnura sp. and Copera sp., and the thorax and abdomen of the dragonfly naiads, Orthetrum sp. The metacercariae showed progenetic development. The growth and development of the metacercariae in the naiads that were exposed to cercariae, and development of the trematode in frogs that were force-fed with encysted metacercariae, have been studied at regular intervals. The prepatent period is 14–19 days. The present life cycle study of a Pleurogenoides spp. forms the seventh report from the world, fourth report from India and the third from Kerala.

We describe the larval developmental stages and life cycle of the dracunculid nematode Mexiconema cichlasomae in both the intermediate, Argulus yucatanus (Crustacea: Branchiura), and definitive hosts, Cichlasoma urophthalmus (Perciformes: Cichlidae), from the Celestun tropical coastal lagoon, Yucatan, Mexico. The morphological analyses showed significant differences between the total length of L1 found in M. cichlasomae gravid female and L2–L3 in A. yucatanus. This result indicates that the M. cichlasomae larval development occurs in the intermediate host. We obtained sequences from the small subunit (SSU) ribosomal marker from larval stages of M. cichlasomae in A. yucatanus and adult nematodes in C. urophthalmus. Our morphological and molecular results support conspecificity between M. cichlasomae larvae in A. yucatanus and the adult stages in C. urophthalmus. We briefly discuss the phylogenetic position of M. cichlasomae among the Daniconematidae, and provide evidence of the monophyly of the daniconematids associated with branchiurid intermediate hosts. Based on the phylogenetic results, we support the transfer of the Mexiconema genus to the family Skrjabillanidae and do not support the lowering of family Daniconematidae to subfamily.

Many helminth taxa have complex life cycles, involving different life stages infecting different host species in a particular order to complete a single generation. Although the broad outlines of these cycles are known for any higher taxon, the details (morphology and biology of juvenile stages, specific identity of intermediate hosts) are generally unknown for particular species. In this review, we first provide quantitative evidence that although new helminth species are described annually at an increasing rate, the parallel effort to elucidate life cycles has become disproportionately smaller over time. We then review the use of morphological matching, experimental infections and genetic matching as approaches to elucidate helminth life cycles. Next we discuss the various research areas or disciplines that could benefit from a solid knowledge of particular life cycles, including integrative taxonomy, the study of parasite evolution, food-web ecology, and the management and control of parasitic diseases. Finally, we end by proposing changes to the requirements for new species descriptions and further large-scale attempts to genetically match adult and juvenile helminth stages in regional faunas, as part of a plea to parasitologists to bring parasite life-cycle studies back into mainstream research.