Comparison of digenean faunas of sillaginid fishes from inshore and offshore waters of Visakhapatnam Coast, Bay of Bengal

The sillaginid fishes of Visakhapatnam Coast, Bay of Bengal, are found to be hosts for 16 species of digeneans of which seven are adult digeneans belonging to families Opecoelidae, Zoogonidae and Hemiuridae and nine are larval digeneans representing families Bucephalidae, Heterophyidae, Cryptogonimidae, Acanthocolpidae, Hemiuridae and Didymozoidae. The digenean fauna of sillaginid fishes from shallow coastal waters (< 10 m depth) and deeper offshore waters (30–50 m depth) are compared and results are discussed in the light of existing information on the migratory movements of different species of sillaginids and their occurrence in coastal or offshore waters. The usefulness of metacercariae of these fishes as 'biological tags' is considered.     

Gelatinous zooplankton of the Arctic Ocean: in situ observations under the ice

The gelatinous zooplankton of the Canada Basin were observed with a deep-diving remotely operated vehicle (ROV) during August–September, 2002. Taxa observed fell into four main groups: cnidarians, ctenophores, chaetognaths, and pelagic tunicates. We provide detailed data on the vertical distributions of many taxa from three sites which span the Canada Basin. The most common gelatinous organisms in the surface waters were the ctenophores Mertensia ovum and Bolinopsis infundibulum. These two species were found in very large numbers in the near-surface mixed layer. In the mesopelagic zone, below the transition from the Pacific water layer and the Atlantic water layer, the most common species was Sminthea arctica. Surprising numbers of the scyphomedusa Atolla tenella were found in the deep waters of the basin, along with an undescribed species of narcomedusae. The vertical distributions of the gelatinous zooplankton observed with the ROV show several trends related to the physical properties of the water and geographic location within the basin.     

Gelatinous zooplankton (ctenophores,salps and medusae): an important food resource of fishes in the temperate SW Atlantic Ocean

This study quantifies the occurrence of gelatinous zooplankton in the stomach contents of fishes from the southwest Atlantic Ocean (33°–55°S). More than 69,000 fish stomachs belonging to 107 species were examined. A total of 39 fishes were documented as consumers of gelatinous zooplankton, 23 of which were newly discovered. Three gelatinous organism consumption categories are recognized: (1) very frequent consumers (10 species, six of which were exclusive); frequent consumers (five species); and occasional consumers (26 species). Three types of gelatinous prey (ctenophores, salps and medusae) were found in the stomach contents of fishes. Ctenophores were consumed at high levels across almost the entire continental shelves of Argentina and Uruguay. Salps were frequent prey on the slope and southern shelf. In contrast, medusae were consumed in coastal areas, slopes and the southern shelf. Classification methods (group average sorting of the Bray–Curtis similarity measures based on log (X?+?1)-transformed percentage data) determined six areas where fishes predated on gelatinous organisms. SIMPER (similarity percentages) analysis determined which fishes contributed more to the consumption of gelatinous organisms. Results revealed that two fish species (Stromateus brasiliensis and Squalus acanthias) had high gelatinous zooplankton predation rates throughout the entire study area, while another six species (Patagonotothen ramsayi, Helicolenus dactylopterus lahillei, Macrourus holotrachys, Merluccius hubbsi, Schroederichthys bivius, and Macruronus magellanicus), while widely distributed, seemed to have specific areas where consumption occurred. This study not only provides new knowledge about the importance of gelatinous zooplankton in the diet of numerous fishes, but might also be valuable for planning and managing local fisheries.     

Infection Status of Zoonotic Trematode Metacercariae in Fishes from Vientiane Municipality and Champasak Province in Lao PDR

The infection status of fishborne zoonotic trematode (FZT) metacercariae was investigated in fishes from 2 localities of Lao PDR. Total 157 freshwater fishes (17 species) were collected in local markets of Vientiane Municipality and Champasak Province in December 2010 and July 2011, and each fish was examined by the artificial digestion method. Total 6 species of FZT metacercariae, i.e., Opisthorchis viverrini, Haplorchis taichui, Haplorchis yokogawai, Haplorchis pumilio, Centrocestus formosanus, and Procerovum varium, were detected in fishes from Vientiane Municipality. The metacercariae of O. viverrini were detected in 50 (49.5%) out of 101 fishes (6 species), and their average number was 154 per fish infected. The remaining 5 species of heterophyid metacercariae were detected in 36.8%, 65.8%, 9.4%, 23.9%, and 5.1% fishes examined, and their average densities were 12, 1,038, 4, 15, and 13 per infected fish, respectively. In fishes from Champasak Province, 3 species of FZT metacercariae, i.e., O. viverrini, H. taichui, and H. yokogawai, were detected. Only 2 O. viverrini metacercariae were found in only 1 Barbonymus schwanefeldi. The metacercariae of H. taichui and H. yokogawai were detected in 60.0% and 50.0% of fishes examined, and their average densities were 47 and 28 per fish infected. By the present study, it has been confirmed that several species of FZT metacercariae are prevalent in fishes from Vientiane Municipality, with P. varium being a new member of FZT in Lao PDR. In comparison, FZT metacercariae are less prevalent in fishes from Champasak Province.     

Does a facultative precocious life cycle predispose the marine trematode Proctoeces cf. lintoni to inbreeding and genetic differentiation among host species?

Intraspecific variability in parasite life cycle complexity (number of hosts and species of hosts in the life cycle) may have an impact how parasite genetic variation is partitioned among individual parasites, host individuals or host species within a given area. Among digenean trematodes, a three-host life cycle is common. However, a few species are precocious and may reach sexual maturity in what is typically regarded as the second intermediate host. The objective of this study was to determine whether a precocious life cycle predisposes digeneans to possible inbreeding or genetic subdivision among host species. As a study system, we used the digenean Proctoeces cf. lintoni whose metacercariae precociously mature (facultative) without a cyst wall in the gonads of multiple sympatric species of keyhole limpets (Fissurella spp.), typically regarded as the second intermediate hosts. Genotyped parasites were collected from four species of limpets and the clingfish Sicyases sanguineus, the third and final host where sexual maturity occurs. We found very high microsatellite diversity, Hardy–Weinberg equilibrium over all genotyped individuals, and little to no genetic structuring among parasites collected from the different host species. The fact that metacercariae do not encyst in the keyhole limpets, coupled with the high mixing potential of an aquatic environment, likely promote panmixia in local populations of P. cf. lintoni.     

Patterns of jellyfish abundance in the North Atlantic

A number of explanations have been advanced to account for the increased frequency and intensity at which jellyfish (pelagic cnidarians and ctenophores) blooms are being observed, most of which have been locally directed. Here, we investigate seasonal and inter-annual patterns in abundance and distribution of jellyfish in the North Atlantic Ocean to determine if there have been any system-wide changes over the period 1946–2005, by analysing records of the presence of coelenterates from the Continuous Plankton Recorder (CPR) survey. Peaks in jellyfish abundance are strongly seasonal in both oceanic and shelf areas: oceanic populations have a mid-year peak that is more closely related to peaks in phyto- and zooplankton, whilst the later peak of shelf populations mirrors changes in SST and reflects processes of advection and aggregation. There have been large amplitude cycles in the abundance of oceanic and shelf jellyfish (although not synchronous) over the last 60 years, with a pronounced synchronous increase in abundance in both areas over the last 10 years. Inter-annual variations in jellyfish abundance in oceanic areas are related to zooplankton abundance and temperature changes, but not to the North Atlantic Oscillation or to a chlorophyll index. The long-term inter-annual abundance of jellyfish on the shelf could not be explained by any environmental variables investigated. As multi-decadal cycles and more recent increase in jellyfish were obvious in both oceanic and shelf areas, we conclude that these are likely to reflect an underlying climatic signal (and bottom-up control) rather than any change in fishing pressure (top-down control). Our results also highlight the role of the CPR data in investigating long-term changes in jellyfish, and suggest that the cnidarians sampled by the CPR are more likely to be holoplanktic hydrozoans and not the much larger meroplanktic scyphozoans as has been suggested previously. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Distribution patterns of marine bird digenean larvae in periwinkles along the southern coast of the Barents Sea.

An important component of the parasite fauna of seabirds in arctic regions are the flukes (Digena). Different species of digeneans have life cycles which may consist of 1 intermediate host and no free-living larval stages, 2 intermediate hosts and 1 free-living stage, or 2 intermediate hosts and 2 free-living larval stages. This study examined the distribution of such parasites in the intertidal zones of the southern coast of the Barents Sea (northwestern Russia and northern Norway) by investigating 2 species of periwinkles (Littorina saxatilis and L. obtusata) which are intermediate hosts of many species of digeneans. A total of 26,020 snails from 134 sampling stations were collected. The study area was divided into 5 regions, and the number of species, frequency of occurrence and prevalence of different digenean species and groups of species (depending on life cycle complexity) were compared among these regions, statistically controlling for environmental exposure. We found 14 species of digeneans, of which 13 have marine birds as final hosts. The number of species per sampling station increased westwards, and was higher on the Norwegian coast than on the Russian coast. The frequency of occurrence of digeneans with more than 1 intermediate host increased westwards, making up a larger proportion of the digeneans among infected snails. This was significant in L. saxatilis. The prevalence of different species showed the same pattern, and significantly more snails of both species were infected with digeneans with complicated life cycles in the western regions. In L. saxatilis, environmental exposure had a statistically significant effect on the distribution of the most common digenean species. This was less obvious in L. obtusata. The causes of changing species composition between regions are probably (1) the harsh climate in the eastern part of the study area reducing the probability of successful transmission of digeneans with complicated life cycles, and (2) the distribution of different final hosts.     

Trematodes and nematodes parasitizing the benthic insect community of an Andean Patagonian stream,with emphasis on plagiorchiid metacercariae

In freshwater systems, parasitological studies have mainly been carried out on vertebrates and molluscs, but little is known about parasites of aquatic insects. We describe the trematodes and nematodes parasitizing the benthic insects of an Andean Patagonian stream and the presence of parasites in the terrestrial adult stages. Members of 3 of 20 insect taxa were found to be parasitized by larval nematodes, and members of six taxa harbored metacercariae of digeneans. In benthic samples, chironomids, simuliids (Order Diptera), and baetids (Order Ephemeroptera) harbored mermithid larvae (Nematoda). The stonefly Antarctoperla michaelseni (Order Plecoptera), the caddisfly Smicridea annulicornis (Order Trichoptera), a watersnipe fly (Order Diptera: Athericidae), and three species of leptophlebiid mayflies (Order Ephemeroptera) were parasitized by encysted plagiorchiid metacercariae (Order Plagiorchiida). Most metacercariae were found in the three species of mayflies with prevalences ranging 15–63% and mean intensities ranging 1.2–4.9. Prevalence declined from summer to early winter, probably because of the emergence of infected nymphs and the recruitment of uninfected new cohorts. The imagos had live metacercariae with higher prevalences and intensities of infection than nymphs. We suggest that these plagiorchiids have an allogenic life cycle, involving a terrestrial definitive host.     

Extension of methods for jellyfish and ctenophore trophic ecology to large-scale research

Science has rapidly expanded its frontiers with new technologies in the 20th Century. Oceanography now is studied routinely by satellite. Predictive models are on global scales. At the same time, blooms of jellyfish and ctenophores have become problematic, especially after 1980. Although we have learned a great deal about gelatinous zooplankton ecology in the 20th Century on local scales, we generally have not scaled-up to estimate the extent, the causes, or effects of large blooms. In this age of global science, research on gelatinous zooplankton needs to utilize large-scale approaches and predictive equations. Some current techniques enable jellyfish populations (aerial, towed cameras), feeding (metabolic rates, stable isotopes), and dynamics (predictive modeling) to be studied over large spatial and temporal scales. I use examples of scyphomedusae (Aurelia spp., Cyanea capillata, Chrysaora quinquecirrha) and Mnemiopsis leidyi ctenophores, for which considerable data exist, to explore expanding from local to global scales of jellyfish trophic ecology. Regression analyses showed that feeding rates of Aurelia spp. (FR in copepods eaten medusa⁻¹ d⁻¹) generally could be estimated ±50% from in situ data on medusa wet weight (WW) and copepod density; temperature was not a significant factor. FR of C. capillata and C. quinquecirrha were similar to those of Aurelia spp.; the combined scyphomedusa regression underestimated measured FR of C. quinquecirrha and Aurelia spp. by 50% and 180%, respectively, and overestimated measured FR of C. capillata by 25%. Clearance rates (CR in liters cleared of copepods ctenophore⁻¹ d⁻¹) of M. leidyi were reduced in small containers (≤20 l), and a ratio of container-volume to ctenophore-volume of at least 2,500:1 is recommended for feeding experiments. Clearance rates were significantly related to ctenophore WW, but not to prey density or temperature, and estimated rates within 10–159%. Respiration rates of medusae and ctenophores were similar across habitats with greatly ambient different temperatures (10–30°C), and can be predicted from regressions using only mass. These regressions may permit estimation of feeding effects of gelatinous predators without exhaustive collection of feeding data in situ. I recommend that data on feeding and metabolism of jellyfish and ctenophores be entered in a database to allow generalized predictive relationships to be developed to promote inclusion of these important predators in ecosystem studies and models. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Survey of Zoonotic Trematode Metacercariae in Fish from Water systems of Geum-gang (River) in Republic of Korea

The infection status of zoonotic trematode metacercariae (ZTM) was surveyed in freshwater fishes from the water systems of Geum-gang (River) in the Republic of Korea (Korea). A total of 1,161 freshwater fishes from 6 local sites of Geum-gang were examined with the artificial digestion method for 4 years (2012–2015). Clonorchis sinensis metacercariae were detected in 122 (37.2%) out of 328 fishes in the positive fish species from 4 surveyed areas, and their mean intensity was 43 per fish infected. Metagonimus spp. metacercariae were found in 432 (51.7%) out of 835 fishes in the positive fish species from all 6 surveyed areas, and their mean intensity was 30 per fish infected. Centrocestus armatus metacercariae were detected in 285 (75.0%) out of 380 fishes in the positive fish species from 6 surveyed areas, and their mean intensity was 2,100 per fish infected. Echinostoma spp. metacercariae were found in 56 (19.7%) out of 284 fishes in the positive fish species from 5 surveyed areas, and their mean intensity was 10 per fish infected. Clinostomum complanatum metacercariae were detected in 98 (57.3%) out of 171 fishes in the positive fish species from only 2 surveyed areas, and their mean intensity was 11 per fish infected. Conclusively, the endemicity of ZTM is not so high in fishes from water systems of Geum-gang in Korea although it is more or less different by fish species, surveyed areas and ZTM species.     

Use of rDNA polymorphism for identification of Heterophyidae infecting freshwater fishes

Infections by trematodes are among the most common fish-borne zoonoses. Metacercariae of the Family Heterophyidae in marine and freshwater fishes are nonfastidious in their choice of definitive hosts, and therefore, cause infections in human and domestic animals. In the present study, species-specific polymerase chain reaction (PCR) assays were developed for identifying and differentiating the various species examined. Sequencing and aligning the 18S (SSU) rDNA revealed interspecific variation for which species-specific DNA oligonucleotides were designed and used for the identification of 6 heterophyid species recovered from piscivorous birds. The oligonucleotides were further used to evaluate the various stages (cercariae recovered from snails, metacercariae recovered from fish and adult trematodes) of the digeneans. By applying this method we elucidated for the first time the life cycle of Pygidiopsis genata. The phylogenetic interrelationship among the newly sequenced species of Heterophyidae is outlined.     

Parasitism (Trematoda,Digenea) in medusae from the southwestern Atlantic Ocean: medusa hosts,parasite prevalences,and ecological implications

Digenean are important endoparasites of fish with complex life cycles; some genera include medusae as secondary hosts. Their transmission to fish occurs when fish prey on these jelly hosts. Fish predation on jellyfish is a widespread phenomenon, even though predation by fish on jellyfish has not been determined through parasitism yet. We hypothesized that medusae with high prevalences of digeneans could be important for their transmission to fish. A total of 48,900 specimens of 50 medusa species were analyzed; 2,181 harbored digeneans. Opechona sp. and Monascus filiformis were the most frequent and abundant parasites with the widest range of hosts. Hemiuridae gen. sp. and Bacciger sp. were found in few specimens of some medusa species. Prevalences were unevenly distributed in the region. Three groups with high prevalence values were identified mainly related to frontal areas: Río de la Plata, Bahía Blanca, and North Patagonian tidal front. Eucheilota ventricularis, Clytia hemisphaerica, Proboscidactyla mutabilis, Liriope tetraphylla, and Aequoerea spp. were the medusae that contributed the most as secondary hosts to M. filiformis and Opechona sp. The high prevalences found in these medusae suggest that may be a fundamental part of the life cycles of both parasites in these areas.     

Epibiosis and parasitism of coquina clam Donax spp.: Location,location, location!

Cryptic species of coquina clams Donax fossor and D. variabilis carry the hydroid epibiont Lovenella gracilis and are infected with metacercariae of the monorchiid parasites Lasiotocus trachinoti and L. choanura. The associations among this host–epibiont–parasite system were investigated. Fifty clams were collected at low tide over 3 days in June 2020 in South Carolina from each of three groups: clams with no hydroid from the upper intertidal zone and clams with and without hydroids from the swash zone. Clams were measured, identified using a newly developed PCR-RFLP, and examined for infection by metacercariae. Parasites were identified based on cercarial morphology and on metacercariae habitat in the clams. D. fossor was most often found in the swash zone and D. variabilis in the upper intertidal zone. The hydroid was most often associated with D. fossor, which was more infected by both digeneans than D. variabilis. Mean abundance of metacercariae of L. choanura was higher than that of L. trachinoti in both clams and increased over time for both parasites, because higher infection was correlated with larger clams. Greater time spent in the water by individuals of D. fossor appears to best explain these results, with the presence of the hydroids also being associated with higher infection by metacercariae in this coquina. Integration of D. variabilis in both digenean life cycles appears to lead to a positive outcome for the parasites as prevalence and abundance of infection were high; however, because D. variabilis is most frequent in the upper intertidal, more emersed, zone, it is likely deleterious to the epibiont to establish on this clam.     

From gelatinous to muscle food chain: rock cod Patagonotothen ramsayi recycles coelenterate and tunicate resources on the Patagonian Shelf

Stomach contents of 4808 fishes of 20 species caught in the eastern part of the Patagonian Shelf between 1999 and 2012 were analysed to assess dietary contributions of gelatinous plankton resources. Gelatinous plankton occurred in diets of seven species with two species, Patagonotothen ramsayi and Squalus acanthias, having >10% ctenophores in their diet. Consumption of gelatinous plankton was important in P. ramsayi and was strikingly seasonal, with maximum occurrence (up to 46% of non‐empty stomachs) in late summer to autumn. Ctenophores were most abundant in P. ramsayi of 25–34 cm total length, L_T, whereas salps were more frequent in larger >35 cm L_T individuals. In winter to spring, occurrence of gelatinous plankton in diets was minimal, reflecting their overall seasonal abundance in the ocean. The recent increase in abundance of P. ramsayi has enabled the species to recycle a significant proportion of the ecosystem production from gelatinous dead end to the main muscular food chain via seasonal reliance on ctenophores, jellyfish and tunicates. This additional influx of production that has been diverted from the gelatinous food chain favours the increase in abundance of several piscivorous top predators and affects the trophic web structure of the Patagonian Shelf ecosystem.     

Physiological and ecological hosts of Popenaias popeii (Bivalvia: Unionidae): laboratory studies identify more hosts than field studies

1. Freshwater mussels are critically endangered in North America, making it important to understand their environmental requirements at all life stages. As glochidia (larvae), they attach to fish hosts where they undergo substantial mortality, making this transition important in their life cycle. Larval host fish requirements have typically been described using data from laboratory infestations to determine suitable hosts. 2. Laboratory infestations circumvent many natural barriers that prevent infestation of physiologically compatible fishes by mussel larvae. While such methods are invaluable for identifying ‘physiological hosts,’ they cannot fully describe realised ‘ecological hosts’ in the field. 3. We studied Popenaias popeii in the Black River in New Mexico, because it is of conservation concern and it is the only mussel species present, facilitating identification of glochidial infestation. To explore the difference between physiological hosts and ecological hosts, we conducted a 3‐year field study of fishes infested by P. popeii glochidia. 4. Substantially fewer fish species were infested by P. popeii in the wild (10 of 20 observed) than had been identified as physiological hosts in laboratory trials (24 of 31). We combined data on fish abundance, proportion of fish hosts infested (prevalence) and the number of glochidia per fish (intensity) and identified three fish species that probably contributed substantially more to mussel recruitment by carrying more glochidia than other host species. 5. Similarities in behaviour among these fishes allowed us to hypothesise routes of infestation, such as benthos‐feeding by catostomids, that allow glochidia to infest these hosts at higher rates than other suitable hosts. Overall, this approach provides a method of quantifying the relative importance of different species of host fish in the mussel lifecycle.     

A comparison of carbon-specific respiration rates in gelatinous and non-gelatinous zooplankton: A search for general rules in zooplankton metabolism

Using 470 data from the literature the dry weight-specific respiration rates of gelatinous zooplankton (cnidarians, ctenophores and salps) and non-gelatinous zooplankton (mainly crustacea) were converted to carbon-specific values. The resulting carbon-specific respiration rates showed no significant differences between the two groups of zooplankton, indicating similar oxygen requirements per gram of carbon biomass. From this finding, it can be suggested that the differences in the rates of oxygen consumption measured in the two types of zooplankton in the sea can be explained by the carbon biomass ratio between gelatinous and non-gelatinous zooplankton. Furthermore, the low rate of metabolism of gelatinous species compared with that of non-gelatinous animals of the same volume can be attributed predominantly to the relatively low organic matter content in the former. It is recommended that all weight-specific metabolism rates be expressed using carbon as body mass unit (e.g. mg O₂ gC⁻¹ d⁻¹) which enables more accurate comparisons between individuals exhibiting different dry weight/carbon ratios.     

Redescription and life cycle of the monorchiid Postmonorcheides maclovini Szidat, 1950 (Digenea) from the Southwestern Atlantic Ocean: Morphological and molecular data

The adult monorchiid, Postmonorcheides maclovini Szidat, 1950, digenean parasite of the Patagonian blennie Eleginops maclovinus (Cuvier) (Eleginopidae) from Puerto Deseado (47° 45′ S, 65° 55′ W), Argentina, was characterized and its life cycle elucidated. P. maclovinus is the only species of the genus Postmonorcheides, proposed by Szidat (1950) from Tierra del Fuego province (~ 54° S), Argentina. This digenean uses the Patagonian blennie as definitive host, and the intertidal bivalve Lasaea adansoni (Gmelin) (Lasaeidae) as both first and second intermediate hosts (metacercariae encyst inside sporocysts), being the first record of this clam as intermediate host of trematode parasites. The cercaria may, in addition to encysting in the sporocyst, emerge and presumably infect other intermediate hosts. This is the second report of a monorchiid species with metacercariae encysting inside the sporocyst. Adults were found parasitizing the fish stomach, pyloric caeca and intestine with a prevalence of 100%; sporocysts with cercariae and/or metacercariae were found parasitizing the gonad of the bivalve with a prevalence of 2.78%. The cercariae possess a well-developed tail and eye-spots are absent. The ITS1 sequence from the adult digeneans found in the Patagonian blennie, identified as P. maclovini, was found to be identical to the ITS1 sequences obtained both from sporocysts containing cercariae and encysted metacercariae found in L. adansoni.     

Prevalence and Infection Intensity of Zoonotic Trematode Metacercariae in Fish from Soyang-cheon (Stream), in Wanju-gun,Jeollabuk-do,Korea

We surveyed on the infection status of zoonotic trematode metacercariae (ZTM) in freshwater fishes from Soyang-cheon (a branch stream of Mangyeong-gang) in Wanju-gun, Jeollabuk-do, the Republic of Korea. A total of 927 fishes were individually examined with the artificial digestion method during 2013–2015 (462 fish in 15 spp.) and 2018–2019 (465 fish in 25 spp.). Clonorchis sinensis metacercariae were detected in 207 (31.4%) out of 659 fishes in 14 positive fish species (PFS), and their mean intensity was 114 per fish infected (PFI). Metagonimus spp. metacercariae were found in 302 (37.4%) out of 808 fishes in 21 PFS, and their mean intensity was 12 PFI. Centrocestus armatus metacercariae were detected in 222 (59.0%) out of 376 fishes in 12 PFS, and their mean intensity was 383 PFI. Echinostoma spp. metacercariae were found in 139 (22.1%) out of 628 fishes in 10 PFS, and their mean intensity was 7 PFI. Clinostomum complanatum metacercariae were detected in 14 (6.5%) out of 214 fishes in 4 PFS, and their mean intensity was 2.4 PFI. Metorchis orientalis metacercariae were detected in 36 (13.5%) out of 267 fishes in 5 PFS, and their mean intensity was 4.3 PFI. Conclusively, the prevalence and infection intensity of ZTM is generally not so high in fishes from Soyang-cheon. However, those of C. sinensis metacercariae are more or less higher in 2 fish species, Pungtungia herzi and Sarcocheilichthys variegatus wakiyae.     

High Endemicity with Clonorchis sinensis Metacercariae in Fish from Yongjeon-cheon (Stream) in Cheongsong-gun,Gyeongsangbuk-do,Korea

The infection status with Clonorchis sinensis metacercariae (CsMc) was examined in freshwater fishes from Yongjeon-cheon (a branch of Nakdong-gang) located in Cheongsong-gun, Gyeongsangbuk-do, the Republic of Korea (Korea). A total of 750 fishes in 19 species were examined by the artificial digestion method for 2 years (2019 and 2020). CsMc were detected in 378 (51.4%) out of 735 fishes in 14 species (73.7%), and the infection intensity was 666 per fish infected. In 2019, CsMc were found in 172 (68.0%) out of 253 fishes in 10 species, and the infection intensity was 565 per fish infected. In 2020, CsMc were detected in 206 (62.2%) out of 331 fishes in 10 species, and the infection intensity was 751 per fish infected. The other zoonotic trematode, ie. Metagonimus spp., Centrocestus armatus, Echinostoma spp. and Clinostomum complanatum, metacercariae were also detected in fishes from the survey streams, but their endemicities were relatively low. Conclusively, it was first confirmed that CsMc are highly endemic in fishes from Yongjeon-cheon in Cheongsong-gun, Gyeongsangbuk-do, Korea.     

Nudibranch predation and dietary preference for the polyps of <Emphasis Type="Italic">Aurelia</Emphasis><Emphasis Type="Italic">labiata</Emphasis> (Cnidaria: Scyphozoa)

There is concern that jellyfish blooms may be increasing worldwide. Some factors controlling population size, such as temperature and food, often have been studied; however, the importance of predators is poorly known. Aeolid nudibranchs feed on cnidarians, but their predation on the benthic polyps of scyphozoan rarely has been documented. To understand the potential of nudibranchs to consume polyps, we tested several predation preference hypotheses with the generalist feeding nudibranch, Hermissenda crassicornis, and polyps of the common moon jellyfish, Aurelia labiata. Of the six prey species tested during feeding experiments, A. labiata polyps and the tunicate Distaplia occidentalis were significantly preferred. Nudibranch size, diurnal cycle, and ingestive conditioning did not significantly influence prey choice. Nudibranchs showed significant positive chemotaxis toward living polyps, hydroids, and tunicates, but not to sea anemones. Nudibranch chemotaxis was significantly more positive to polar extract of A. labiata than of D. occidentalis. Consumption of polyps was correlated with nudibranch size, with mean consumption by large nudibranchs (>0.92 g) of about 31 polyps h⁻¹. Three other nudibranch species also ate A. labiata polyps. Our results emphasize the potential importance of predation for controlling jellyfish benthic polyp populations and consequent jellyfish blooms.