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Pomacea canaliculata is a highly successful invasive snail that shapes freshwater communities in both native and invaded habitats. We studied its digenean parasites from three freshwater bodies in its native distribution area in Buenos Aires Province, Argentina. An integrated approach was used to determine and describe the larval stages of digenean, including morphological, molecular, and histopathology analyses. We provide the first record of P. canaliculata as the first intermediate host of Stomylotrema vicarium (Stomylotrematidae), two species of the family Phaneropsolidae, and one species of the family Cyclocoelidae. This is also the first record of a species of the family Cyclocoelidae parasitizing snails of the genus Pomacea, with the apple snail acting as both the first and second intermediate host. The digestive gland was identified as the target organ of infection for all species. Stomylotrema vicarium and Phaneropsolidae gen. et sp. 2 alter the structure of the gonads, causing indirect parasitic castration, and, through mechanical compression, destroying also the digestive gland. Adequate knowledge of the identity and dynamics of the parasites affecting P. canaliculata in its native range and the damage they cause is key to explaining the success of this invasive species. The lack of parasite records in invaded areas supports the "enemy release" hypothesis, which could explain the apple snail's success in these environments. Parasitic castration reduces the reproductive potential of hosts, limiting the expansion and competition of invasive species, making it crucial to understand these impacts for their conservation and control., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025 Elsevier Inc. All rights reserved.)

Pomacea canaliculata is one of the most dangerous invasive species. Morphological and molecular analyses have revealed that a digenean species belonging to the family Echinostomatidae parasitizes this snail at two sites in Buenos Aires Province, Argentina, South America. Molecular results confirmed that the species belongs to a genus closely related to Patagifer . Analysis of the 28S rDNA showed that the sequences of the rediae and metacercariae are identical, indicating that the apple snail acts as the first and second intermediate host. The cercariae may encyst as metacercaria inside the redia and also emerge and re-infect the same snail or another snail. The prevalence of digeneans was higher in one of the sampling locations (15.1% vs. 0.72%), probably because the bird species that acts as the definitive host is more abundant in that area. Histopathological examination showed that the parasite quickly invades multiple host organs (gills, intestines, albumen gland, lung, kidney, and mantle border) besides the gonad and digestive gland, as is usual in digeneans. In addition, the partial or total castration of snails was observed in cases of moderate and high infection intensity. In males, there was loss of integrity in testicular tubules, while in females, the replacement of ovarian tissue by rediae was found.

In South America, the knowledge of trematode diversity parasitizing freshwater fishes is still scarce, as less than 5% of the freshwater fish fauna has been examined for parasites. A similar situation applies to studies on digenean life cycles, which have become increasingly rare. Among the digenean families parasitizing freshwater fishes in the region, Haploporidae is considered the richest in species diversity. However, information about the developmental stages of haploporid life cycles remains fragmentary. Particularly, in Argentina, nine cercariae attributed to the family Haploporidae have been described using morphological analysis, and only two life cycles of this family have been completely elucidated. In this study a new type of cercaria, morphologically assigned to the family Haploporidae and collected from the snail Heleobia parchappii (Cochliopidae) in Los Padres shallow lake, Buenos Aires province, was identified using morphological and molecular techniques. The molecular analysis, based on 28S and ITS2 sequences, revealed that the cercariae were 100% identical to adult specimens of Saccocoelioides nanii (Haploporidae) parasitizing the fish Prochilodus lineatus (Prochilodontidae) from Los Talas, Buenos Aires province. Our results not only provide information about the life cycle of S. nanii but also show that a molecular and morphological approach can be extremely useful in identifying the developmental stages of digeneans and elucidating their life cycles.

Temporary ponds are ecologically valuable habitats and useful sites for studies of diversity, ecology, evolution, and climate change. Organisms inhabiting these environments have developed strategies to ensure their survival. However, little is known about the temporal dynamics and strategies of parasites in these habitats. A 4-year study was carried out in a temporary pond to analyze the larval digenean assemblage in Biomphalaria peregrina, a potential vector of Schistosoma mansoni, a zoonosis of global importance. This environment had intermittent and irregular hydroperiods, and the overall prevalence showed differences throughout the years. After drought seasons, eight out of a total of nine species of digeneans were observed, and almost half of the snails were parasitized. In addition, six of the nine species of digeneans showed some type of abbreviation of their life cycles. These results suggest that digeneans have certain degree of plasticity in their life cycles in response to environmental changes in this pond, and the abbreviation of the digenean life cycle could be regarded as a parasite strategy to resist prolonged periods of desiccation.

In the last years, a growing number of studies have focused on the search for biological indicators of human impact in freshwater environments. Given their susceptibility to different types of impact, larval digeneans have been considered as useful tools for the study of temporal variations in parasite communities. However, few studies have been carried out in lotic environments. For that reason, the spatio-temporal variations of the larval digenean assemblages parasitizing the gastropod Heleobia parchappii were analysed in four human-impacted streams, located on the coast of Argentina. In total, 4800 specimens of H. parchappii were collected seasonally, and 12 digenean species belonging to eight families were registered. Three of the streams showed similar species presence but, during spring and summer, the streams located within the city presented lower species presence than the streams located in the urban periphery. By contrast, the spatial prevalence values evidenced a higher variation between the environments. Streams located in the urban periphery showed higher prevalence values throughout the year, compared to those calculated for streams located within the city. The spatio-temporal variations of larval digenean assemblages parasitizing H. parchappii seem to be mainly influenced by the diversity and vagility of definitive hosts, the types of digenean life cycles and habitat characteristics. However, environmental disturbances derived from anthropogenic activities are highlighted as the probable main factors that may affect the composition and dynamic of these parasite assemblages.

The microphallids Microphallus szidati and Levinseniella cruzi parasitized at least three crustacean species as second intermediate host in Mar Chiquita coastal lagoon: the crabs Neohelice granulata and Cyrtograpsus angulatus and the grass shrimp Palaemonetes argentinus. Palaemonetes argentinus acts as host for both metacercariae in brackish and freshwater environments; however the seasonal variations of these digeneans in freshwater habitats remain unexplored. The aim of the present study was to analyze the seasonal dynamic of the larval digenean populations parasitizing P. argentinus in a lentic environment. From the total sample of 600 P. argentinus collected seasonally, 28.370 and 194 metacercariae of M. szidati and L. cruzi, respectively, were recovered. Microphallus szidati was characterized by high prevalence, 100% in all seasons. In contrast, L. cruzi presented low prevalence values, which increased towards summer. The two species of digeneans exhibit seasonal fluctuations in intensity of infection, which was related to host size. Temporal variation of larval digenean assemblage in the crustacean host is probably associated with the presence of the other hosts included in their life cycles. The high values of prevalence found for M. szidati in Nahuel Rucá lagoon suggest that P. argentinus could favor the contact between the larval digenean and the definitive hosts, establishing the presence of M. szidati also in freshwater environments and, thus, increasing the presence of the parasite in different areas of Mar Chiquita coastal lagoon.

The importance of small-scale heterogeneity in local factors which overrides that of larger-scales factors, suggest that local factors play a major role in determining the richness and prevalence of larval digeneans in intertidal gastropods. The restricted distribution of the snail Heleobia australis along a 500 m transect in Cangrejo creek (Mar Chiquita, Argentina) provides a good opportunity to test the assumption that a study at spatial scale of 100s meters can detect spatiotemporal fluctuations of a larval digenean assemblage. To analyze that, 3600 specimens of H. australis were collected seasonally during the year 2011. A quantitative variation and a space-time interaction between sampling points and seasons of the year for the total prevalence of larval digeneans and snail's densities were found, as well as a positive correlation with abiotic factors. These results revealed that the fluctuations in the community of larval digeneans of the snail H. australis can be detected at small spatial scale, using its natural distribution of 500 m. This study also highlights the importance of seasonality as a factor that must be considered in studies focused on the search for patterns structuring the communities of larval digeneans, at medium and large scales.

In Mar Chiquita coastal lagoon (Argentina), the reefs of the invasive polychaete Ficopomatus enigmaticus (Serpulidae) serve as concentration areas for invertebrates and vertebrates and as potential foci of parasite transmission (mainly digeneans). To analyse the role of F. enigmaticus as facilitator of parasite transmission, and to evaluate the influence of the habitats selected by 2 species of cochliopid snails (Heleobia conexa and Heleobia australis), on the richness and on the prevalence of the digenean assemblages that parasitize them, 1945 snails were collected from 2 sampling sites. The comparisons between larval digenean communities parasitizing both snail hosts revealed significant differences related to the overall prevalence and the prevalence contributed by birds in the snails collected from reefs. These results support the idea that the reefs may increase the number of links between intermediate and definitive hosts of digeneans, facilitating parasite transmission only when the aggregates of F. enigmaticus become the unique habitat of the snail host. This first report of an exotic species acting as facilitator of parasite transmission in an estuarine area highlights the importance of analysing the effects of the assimilation process of exotic species by recently colonized environments (including the possible effects on parasite transmission).

Larval digeneans have been proposed as indicators of abundance and diversity of vertebrate and other hosts as well as environmental disturbances. To evaluate its response to environmental changes and its potential use as an indicator of environmental fluctuations, the temporal stability of the community of larval digeneans in Heleobia conexa was comparatively analysed in 4 separate years (1996, 1999, 2004 and 2005) in Mar Chiquita coastal lagoon (Buenos Aires province, Argentina). In total, 4579 specimens of H. conexa were collected and 22 digenean species were observed. Overall prevalence presented inter-annual and seasonal differences. These differences correlate with seasonal changes in composition of the vertebrate definitive host community and with the elimination of the preferred habitat of H. conexa in 1999. In general, the larval digenean community of H. conexa showed a yearly re-establishment following the annual cycle of H. conexa and the presence of definitive hosts. This annual restructuring allowed inferences about the effects of short-term environmental changes in the lagoon. According to these observations, the larval digenean community of H. conexa could be considered as a good bio-indicator with quick response to environmental disturbances.

The nonparametric estimators of species richness are some of the most widely used extrapolation methods in studies of biodiversity. These studies need relative large samples to achieve total diversity (including rare species), and estimators are routinely used to alleviate this problem. In the existing literature, parasite communities have received little attention. In fact, the present study is the first dealing with parasites in invertebrate hosts. Real data on seasonal diversity of larval digeneans in the snails Heleobia conexa and Heleobia australis from Mar Chiquita coastal lagoon (Argentina) were used to evaluate seven nonparametric estimators of species richness, observing the behavior of each method and its ability to estimate the richness using different subsample sizes. In addition, estimated species richness was compared with the previously observed species richness in the parasite component communities of both snail hosts, and the minimum sample size, necessary to include all the observed species of larval digeneans, was estimated using the nonparametric estimators. The bootstrap method had the best overall performance in the two communities. Although this estimator required large subsamples to estimate sample richness, it was in general the least biased and most accurate, and it presented the most similar curve shapes with lack of erratic behavior through the seasons. The results obtained from the comparison between the minimum sample sizes estimated in previous studies of both snail hosts and those suggested by the estimators also support the use of species richness estimators as methods to calculate the minimum sample size in studies of diversity.

The estuarine-dependent marine fish includes marine species that inhabit, at some stage of their life cycle, the Mar Chiquita coastal lagoon, Buenos Aires Province, Argentina. In the present study, we examined for parasites a total of 209 fishes belonging to following the estuarine-dependent marine species: Brevoortia aurea, Odontesthes argentinensis, Mugil platanus, Paralichthys orbignyanus, Micropogonias furnieri, and Pogonias cromis. The parasite fauna in the six host species comprised 43 parasite species, the majority of which represented new locality and new host records. A high percentage of fish was parasitized with at least one parasite species. The greatest values of prevalence were registered for M. furnieri, B. aurea, and P. cromis, at the component community level, followed by P. orbignyanus, O. argentinensis, and M. platanus. On the other hand, B. aurea, O. argentinensis, M. furnieri, and P. cromis showed similar percentages of both larval and adult stages of parasites. In the case of M. platanus, adult stages dominated the total number of parasites, whereas P. orbignyanus harbored mainly larval stages. Out of the six fish species herein studied, M. platanus seems to generally act only as definitive host in the local parasite's life cycle. From a parasitological point of view, the expression "estuarine-dependent marine fishes" remains valid, although the contribution of the fish species in the lagoon to the maintenance of parasite populations is relatively minor.

The use of parasites as biological tags allowed the identification of two stocks of Brazilian sandperch Pinguipes brasilianus, in the Bonaerense region of the Argentine Sea. A total of 156 adult specimens of P. brasilianus were examined for parasites. Temporal variability in parasite burdens was assessed from fish caught seasonally in coastal waters of two zones, south Bonaerense (during autumn and winter) and north Bonaerense (during summer). Additional data from a previous study, comprising P. brasilianus caught during spring in these two zones, as well as in two populations from north Patagonian gulfs were used for comparative analyses of spatial variability. A total of 14 193 metazoan parasites belonging to 19 species were found. Comparisons of seasonal variability in pooled samples and within each locality showed that locality effects exceeded seasonal ones, suggesting the possible existence of two discrete stocks in the Bonaerense region. These findings were strongly supported by discriminant analyses and comparisons of prevalence and abundance between zones, after pooling seasonal samples within each zone. Further evidence of the discreteness of both stocks was assessed by inclusion of samples from Patagonian gulfs in the discriminant analysis, confirming that their differences were at a inter-population level. The parasite species that contributed most to the separation of the samples were generally those identified as biological markers in previous studies. Differing oceanographic conditions are discussed as potential causes of inter-population variation of parasite burdens.

A new species, Paracapillaria argentinensis n. sp., is described from the pinguipedid fish Pinguipes brasilianus Cuvier, 1829 from waters off Mar del Plata, Argentina (38 degrees 08'S, 57 degrees 32'W) (prevalence 22.2%; mean intensity +/- SD, 4.42 +/- 5.19). The new species is assigned to the subgenus Paracapillaria Moravec, 1987. Of the 10 species so far known in the subgenus, the new species more closely resembles P. (P.) plectroplites, from which it is distinguished by having a spicule with an expanded anterior end and a slender medial section. A similar spicular morphology is observed in P. (P.) epinephell; however, it shows shorter spicules and a highly reduced caudal bursa. This is the first record of Paracapillaria in the southwestern Atlantic Ocean.

A new species, Acaenodera spinosior, is described based on specimens collected from the intestine of Conger orbignyanus Valenciennes (Pisces: Congridae) caught in waters off Mar del Plata (38 degrees 08'S, 57 degrees 32'W), Argentina. Five of 36 congers examined were parasitized (prevalence: 13.9%, mean intensity: 2.6+/-1.52, range: 1-5). The new species is distinguished from its only congener, Acaenodera placophora Manter and Pritchard, 1960, a parasite of congers off Hawaii, by its larger size, lacking eyespots, and exhibiting a greater number of circumoral, mid-ventral and lateral spines. This is the first record of Acaenodera Manter and Pritchard, 1960 in Atlantic waters.

Cardicola ambrosioi n. sp. (Digenea: Sanguinicolidae) is found in the blood vessels of liver and gills of the Brazilian flathead, Percophis brasiliensis Quoy and Gaimard, 1824 (Pisces: Percophidae), from Mar del Plata, Argentina. Among the 13 known species within Cardicola Short, 1953, the new species closely resembles Cardicola coridodacis Manter, 1954, from which it is distinguished by having a relatively shorter oesophagus, the vitellaria extending anteriorly to the nerve commissure, rather than to the end of anterior caeca, the female pore located closer to male pore, the latter situated medially instead of laterally and by possessing a larger Mehlis gland and a smaller seminal vesicle.

A new nematode species Dichelyne (Cucullanellus) mariajuliae n. sp., is described based on specimens collected from the black drum, Pogonias cromis (Linnaeus, 1766), from Mar Chiquita coastal lagoon, Buenos Aires Province, Argentina (37 degrees 32'S, 57 degrees 19'W) (prevalence 75%, mean intensity 3.75). Among Dichelyne (Cucullanellus), the new species is unique in having the papillae 2 situated at the level of ventral sucker in males and bearing subcuticular ornamentation in the female tail.

Argentinochondria patagonensis n. gen., n. sp. (Copepoda: Chondracanthidae) is described from adult specimens recovered from the smooth kingklip Genypterus brasiliensis Reagan, 1903, from San Jorge Gulf, Patagonia, Argentina. The new genus is placed in the Chondracanthinae. Females of the new genus differ from other members of this subfamily in having 2 pairs of modified legs, a bulbose head, a long neck region, and a trunk devoid of processes. Males have usual chondracanthid form, oral appendages as in female, except a longer paragnath, fewer teeth on maxilla, and 2 pairs of reduced legs.

A new species of Steganoderma (Steganoderma) (Digenea: Zoogonidae) was found in the digestive tract of the southernmost, endemic and scale-less characid, Gymnocharacinus bergi, inhabiting thermal headwaters of the Valcheta Creek in northern Patagonia, Argentina. Steganoderma valchetensis n. sp. is included in the subgenus Steganoderma because of having a sessile ventral sucker and caeca reaching to the testes. This new species can be distinguished from other species of this genus on the basis of body-length, sucker-ratio, the length of the cirrus-sac, the morphology of the seminal vesicle and gonads, and the location of the vitelline follicles and genital pore. The taxonomic status of the species of Steganoderma, parasitising the freshwater fishes of Patagonia, in southern Argentina, and Chile, is also discussed.

Recently collected copepod parasites on the narrownose smooth-hound shark Mustelus schmitti Springer revealed enough similarities with Perissopus galeorhini (Yamaguti, 1936) to be considered conspecific. Comparison of these specimens with the types of P. oblongus (Wilson, 1908) showed that P. galeorhini and P. oblongus are distinct species. Therefore, P. galeorhini is considered a valid species rather than a synonym of P. oblongus, as it has been treated since the revision of Cressey (1967). Descriptions of P. galeorhini from M. schmitti and type-material of P. oblongus are provided along with a discussion on their taxonomic status. P. galeorhini differs from P. oblongus in features relating to the anterior margin of cephalothorax, the dorsal plates of the third pedigerous segment and details of the segmentation and armature of the appendages. In addition, an amended diagnosis of Perissopus Steenstrup & and Lütken, 1861 is given, including characters not previously considered by Cressey (1967) such as: the dorsal plate of segment 2 can be small, widely separated, covered by the posterior end of the lateral processes of the cephalothorax and not always extending to the posterior border of the plates of segment 3; and the rami of legs 3 can be 2-segmented.

During a parasitological survey of aquatic birds in Mar Chiquita coastal lagoon (Buenos Aires province, Argentina), specimens of Contracaecum Raillet & Henry, 1912 sp. were found in the esophagus and stomach of the great egret Egretta alba (Gmelin, 1789) (Aves: Ardeidae). The nematodes were identified as Contracaecum multipapillatum (Drasche, 1882) on the basis of the following features: ratio of ventricular appendage length to esophagus length, ratio of intestinal caecum length to esophagus length, spicule lengths, number and arrangement of postcloacal papillae (3-4 pairs of adanal papillae, 1 double pair, and a subterminal group of 2 subventral and 3 sublateral pairs). A complete description of the adult specimens of C. multipapillatum is given in the present paper, together with a discussion regarding the validity of other species of Contracaecum reported from Argentina. Contracaecum philomultipapillatum Labriola and Suriano, 1996 is reduced to a junior synonym of C. multipapillatum, and Contracaecum longicaecum Schuurmans Sterkhoven, 1952 is considered a species inquirenda. A revision of Contracaecum spheniscus Boero and Led, 1970 based on appropriate material is necessary.

Ancyracanthopsis winegardi Wong & Anderson, 1990 (Nematoda: Acuarioidea) is described from Larus dominicanus Lichtenstein (Aves: Laridae) on the Southwest Atlantic coast (38 degrees 42 minutes S, 59 degrees 47 minutes W). The main character used to distinguish species of Ancyracanthopsis is the morphology of the ptilina. Thus, although the specimens described here have some differences in the morphology and size of the spicules and in the female genitalia, they were referred to A. winegardi because they have a very similar ptilina. This is the first record of a member of Ancyracanthopsis from larid birds and for A. winegardi in the Southwest Atlantic coast. We have also studied acuariid larvae found inhabiting the gizzard alongside adult specimens of A. winegardi. Among those larvae, two morphological groups were clearly distinguished. The first group was characterised by the absence of ptilina and the presence of spicular primordia and rectal cells (third-stage larvae). The second group could be distinguished by the presence of ptilina and partly-developed genitalia (fourth-stage larvae). In order to identify the larvae, a Principal Component Analysis was applied to morphometric data taken from the third-stage larvae. These results and the morphology of the partly-developed ptilina of the fourth-stage larvae indicated that the larval stages found in L. dominicanus appear to belong to Sciadiocara haematopodi, Cremonte, Navone & Etchegoin, 1999.

In the present paper we describe a new species of the genus Sciadiocara from Haematopus palliatus from the Mar Chiquita coastal lagoon (37 degrees 46' S, 57 degrees 27' W), Argentina. We provide two host records (H. palliatus and Larus dominicanus) for the genus, both from Buenos Aires province, Argentina. S. haematopodi n. sp. closely resembles S. legendrei Petter, 1967. The new species can be distinguished by the morphology of the distal end of the left spicule, the shorter length of the right spicule, the number of pre-anal papillae and the ratio of the length of the vagina vera to the length of the vagina uterina.

Two species of mazocraeid monogeneans are reported for the first time from Argentinean-Uruguayan engraulid fish hosts, Engraulis anchoita and Anchoa marini. Mazocraes australis n. sp. collected from the gills of E. anchoita is described: this new species can be distinguished from the other congenerics by the number of genital hooks in the copulatory organ, the relative size of clamps, and the number of hooks in the terminal lappet. Pseudanthocotyloides heterocotyle found in the gills of E. anchoita and A. marini is redescribed. The diagnosis of both mazocraeid genera are also emended.

Adults of Maritrema bonaerensis n. sp. (Digenea:Microphallidae) were collected from the intestine of the gulls Larus maculipennis and Larus atlanticus from Mar Chiquita coastal lagoon, Buenos Aires province, Argentina. The new species differs from M. pulcherrima Travassos, 1928 in having smaller sucker ratio, longer intestinal ceca, ovary not lobed, and larger eggs, and from Maritrema prosthometra Deblock and Heard, 1969 it differs in having larger suckers, smaller vitelline follicles, and in the absence of pars prostatica. It differs from Maritrema acadiae (Swales, 1933) in having larger body and pharynx and longer cirrus. It differs from Maritrema paracadiae Ching, 1974 in the larger pharynx, longer cirrus, and in the absence of prominent prostatic cells. From Maritrema majestova Ke, 1976, it differs in having a larger body, smaller eggs, and a long and protusible cirrus. Finally, M. bonaerensis differs from Maritrema chiricae Deblock, 1975 in the larger body, longer cirrus, and in the morphology of the seminal vesicle. Sporocysts and metacercarial stages isolated from the snail Heleobia australis australis and from the crabs Cyrtograpsus angulatus and Chasmagnatus granulata, respectively, suggest that those invertebrates could act as intermediate host in the life cycle of the new species in the lagoon. This assumption is supported by evidence derived from natural and experimental infections, from studies on biology of the actual and presumed hosts, as well as from previous reports on life cycles of species of Maritrema with similar transmission patterns.

In an attempt to clarify the number and identity of copepod species of the genus Chondracanthus Delaroche, 1811 parasitizing common hake Merluccius hubbsi Marini, 1933 from Argentinean waters, a total of 4,569 fish were examined. All of the 995 copepods of the genus Chondracanthus collected from hake showed identical morphology and were identified as Chondracanthus australis Ho, 1991. This species was detected on fish longer than 15 cm of total length and showed an increase in both prevalence and mean intensity in relation to the increase in the size of the host. The highest prevalence and mean intensity were 53.55% and 2.51, respectively. According to our results that coincide with those of other investigators, C. australis seem to be the only species of this genus harbored by M. hubbsi from Argentinean waters.

A total of 2164 common hake Merluccius hubbsi captured in the Argentine-Uruguayan Common Fishing Zone was examined for parasitic copepods. The fish were infested with Chondracanthus palpifer and Neobrachiella insidiosa f. lageniformis, the former parasitizing the buccal cavity and the latter the branchial arches. C. palpifer showed an increase in both prevalence and intensity in relation to the host size; infestation with N. insidiosa f. lageniformis decreased with increasing length of hake. Attachment site preferences and variations in the parasite distribution patterns as related to the host biology were observed. Evidence of negative association between copepod species and seasonal changes in the parasite composition was also found.