Host Fish Species Research Articles

Distribution and host range of a highly invasive parasitic copepod

AbstractThe parasitic copepod Neoergasilus japonicus (Ergasilidae), native to East Asia, has spread rapidly around the world over the past half-century and now represents a clear example of successful parasitic invader. The species is now found in western Asia, Europe, America and Africa, with aquaculture and fish introductions identified as the primary vectors of dispersal. Regional field investigations have revealed surprisingly high number of affected localities, indicating potentially wider distribution than currently recognised. Neoergasilus japonicus exhibits low host specificity, parasitising a diverse range of freshwater fishes. This study updates the global fish host species list to 132, spanning 27 families across 15 orders, with Cypriniformes identified as the most susceptible host species. Under experimental conditions in this study, however, N. japonicus avoided its natural host, topmouth gudgeon Pseudorasbora parva, suggesting a level of resistance to its native parasite. Piscivorous and demersal fish were less infected by copepods than planktivorous, benthivorous, pelagic and benthopelagic species, reflecting the ecology of both the parasite and its hosts. An ability to re-attach to another host, though limited, was confirmed under experimental conditions. Recognising the ecological impacts and potential consequences associated with the introduction of non-native parasites emphasises the need for continuous monitoring and research globally.

Morphological and molecular characterisation of Cymothoa eremita (Brunnich, 1783) (Isopoda: Cymothoidae) from the South China Sea

A morphological description and molecular characterisation of the parasitic isopod Cymothoa eremita Brünnich, 1783, collected off the fish host Nemipterus furcosus (Valenciennes, 1830) from the South China Sea, is presented here. Since most cymothoid descriptions are primarily based on females, this paper highlights the morphological characteristics from 20 ovigerous female and 20 male C. eremita cymothoids, isolated from 300 fish individuals. Furthermore, our findings indicated significant variations in the morphological features of C. eremita, including: 1) average length and width dimensions, 2) the presence or absence of eyes, 3) the length of pleotelson, 4) the configuration of its posterior and lateral margins, and 5) the structure of the cephalon. These characteristics were compared among diverse fish host species reported in different geographical regions. Twenty one of 300 N. furcosus were infested with C. eremita, with a prevalence of 7%, and mean intensity of 1.9 (two cymothoids per individual fish) respectively. A total of 687 base pairs (bp) from the mitochondrial cytochrome oxidase subunit I (COI) gene were sequenced for C. eremita as a complementary measure to the morphological analysis undertaken in this study. These sequences were utilized to reconstruct a phylogenetic tree, incorporating our sequenced specimens and relevant sequences sourced from the GenBank database. The Maximum Likelihood (ML) phylogenetic tree comprised 45 cymothoid species, with partial COI gene sequences showing monophyletic clustering of most species within the same genus, which is in congruence with their morphological divergence. Continued efforts to conduct studies on the biology, taxonomy, ecology and genetics of both cymothoid and fish host are crucial to better understand the patterns of host-parasite specificity and co-evolutionary changes, which may indirectly contribute towards cymothoid speciation and diversification.

Thermal tolerance and survival are modulated by a natural gradient of infection in differentially acclimated hosts.

Wild ectotherms are exposed to multiple stressors, including parasites, that can affect their responses to environmental change. Simultaneously, unprecedented warm temperatures are being recorded worldwide, increasing both the average and maximum temperatures experienced in nature. Understanding how ectotherms, such as fishes, will react to the combined stress of parasites and higher average temperatures can help predict the impact of extreme events such as heat waves on populations. The critical thermal method (CTM), which assesses upper (CTmax) and lower (CTmin) thermal tolerance, is often used in acclimated ectotherms to help predict their tolerance to various temperature scenarios. Despite the widespread use of the CTM across taxa, few studies have characterized the response of naturally infected fish to extreme temperature events or how acute thermal stress affects subsequent survival. We acclimated naturally infected pumpkinseed sunfish (Lepomis gibbosus) to four ecologically relevant temperatures (10, 15, 20 and 25°C) and one future warming scenario (30°C) for 3 weeks before measuring CTmax and CTmin. We also assessed individual survival the week following CTmax. Parasites were counted and identified following trials to relate infection intensity to thermal tolerance and survival. Interestingly, trematode parasites causing black spot disease were negatively related to CTmax, suggesting that heavily infected fish are less tolerant to acute warming. Moreover, fish infected with yellow grub parasites showed decreased survival in the days following CTmax implying that the infection load has negative survival consequences on sunfish during extreme warming events. Our findings indicate that, when combined, parasite infection and high prolonged average temperatures can affect fish thermal tolerance and survival, emphasizing the need to better understand the concomitant effects of stressors on health outcomes in wild populations. This is especially true given that some parasite species are expected to thrive in warming waters making host fish species especially at risk.

Ectoparasite crustaceans of ten fish species from the upper Araguari River in northern Brazil.

Fish parasites are an important part of aquatic biodiversity and knowing these species and their interactions with their hosts helps in monitoring the aquatic biota. The present study investigated the ectoparasite crustacean fauna of ten fish species from the upper Araguari River, in the state of Amapá, northern Brazil. A total of 508 fish were collected and analyzed from July to November 2014, of which 82.6% (109) were parasitized by one or more crustacean ectoparasite species. In the ten host fish species, a total of 308 ectoparasite specimens were collected, from 12 taxa, such as Argulus multicolor Stekhoven, 1937, Argulus spinulosus Silva, 1980, Argulus sp.1, Argulus sp.2, Argulus sp.3, Dipteropeltis sp., Dipteropeltis hirundo Calman, 1912, Dolops bidentata Bouvier, 1899, Dolops striata Bouvier, 1899 (Argulidae), Braga fluviatilis Richardson, 1911, Braga amapaensis Thatcher, 1996 (Cymothoidae) and Excorallana berbicensis Boone, 1918 (Corallanidae). Higher levels of prevalence and abundance were recorded for Hoplias aimara (Valenciennes, 1847) and Tometes trilobatus Valenciennes, 1850, respectively. These ectoparasites were found in the fins, integument, mouth, and anus of the host fish. Argulus sp.2 and D. bidentata were the most abundant parasites (65.1%), and had the highest species richness. This study registered 36 novel host-parasite interactions, and thus represents a new record for all host species here examined.

Habitat suitability of Unionidae host fish in the Rhine Delta

Unionid populations are declining worldwide as they are threatened by anthropogenic habitat alterations, the introduction of invasive alien species and the effects of climate change. Several studies have focused on the habitat preferences of adult mussels but none to date have reported on the importance of the habitat preferences of host fish species. The aim of this study was to determine the habitat preferences of unionid host fish species in the Netherlands, to examine potential differences between native and alien host fish species, and to evaluate the effect of alien host fish species on unionid dispersal. Species sensitivity distributions (SSDs) were constructed for the range of occurrences and laboratory tolerance of host fish species in the Netherlands for water temperature, substrate type, flow velocity, and water depth. A case study was selected to assess the suitability to substrate type, flow velocity and water depth of a novel river habitat along longitudinal training dams in the river Waal. Unionid host fish species in the Netherlands preferred shallow littoral zones (0.1 m) with low flow velocity (0.05 m/s), an average water temperature of the Rhine delta (15°C), and most substrate types (ranging from silt to cobbles). Boulder substrate was the most limiting factor of the abiotic factors assessed for both native and alien host fish species. Management options for unionid populations should include host fish species in their assessments while focusing on creating and maintaining areas with limited boulders and implementing alien host fish species removal measures.

Fish morphological and parasitological traits as ecological indicators of habitat quality in a Mediterranean coastal lagoon

Abstract Coastal lagoons are key ‘transitional’ aquatic environments for biodiversity conservation. Ecological indicators are useful tools for the management of water resources in the European Union. Among different biological communities, fish are a very sensitive tool to assess environmental integrity. Indeed, their parasites can be used as complementary indicators of habitat quality. Yet there is still a deep lack of information on ecological assessment using fish (along with their parasites) for coastal lagoons, particularly for the Iberian Peninsula. The aim of the present study was to assess the use of fish morphology and their parasite communities as ecological indicators of anthropogenic impact within the Mar Menor coastal lagoon (SE Spain), a Mediterranean area of special conservation concern for European/Iberian biodiversity. Fish samples (black‐striped pipefish Syngnathus abaster and marbled goby Pomatoschistus marmoratus) were collected in August 2022 from the Littoral (high level of nutrient enrichment) and Barrier habitats. Physical status (both external and internal indices), asymmetry (level of developmental instability), parasite load, diversity and life‐cycle complexity were compared between habitat types. Body condition and mainly the health assessment index were better in the Barrier habitat. Except for pectoral fins in pipefish, the fluctuating asymmetry was statistically greater in the Littoral habitat (i.e. with eutrophication leading to poorer fish development). The parasite load was higher in the Littoral habitat for both host fish species. However, the diversity and life‐cycle complexity of parasite communities were statistically lower in the Barrier habitat (a structurally simpler environment) only for gobies. This study demonstrates an elevated potential for certain fish morphological and parasitological traits to be considered as good ecological indicators of environmental health. This could help environmental managers and policy makers to design effective monitoring programmes to detect impacts within valuable areas for conservation, such as the Mar Menor coastal lagoon.

Effect of Probiotic Yeast, Sacchromyces cerevisiae on Tissue Biochemical and Carotenoid Variation in Freshwater Ornamental Fish Carassius auratus (Linnaeus, 1758)

The present work aims at assessing Bakers’s yeast, Saccharomyces cerevisiae as an efficient and eco-friendly probiotic feed additives for ornamental fish, Carassias auratus. For this study an indoor culture experiment was carried out 300l plastic trough containing 200l water in triplicate. In each tank five fishes at fingerlings stage (1.58 ± 0.14g) were reared for a period of 30 days. An uniform water quality parameters such as temperature (28.0 ± 1.0 0C), pH (7.60 ± 0.12), dissolved oxygen (4.80 ± 0.82mg/l) and ammonia (< 0.08 mg/l) were maintained. The results inferred that, the percentage increase in the biochemical constituents in both skin and muscle tissue of C. auratus received 2% yeast added diet (EDY2) registered higher values when compared to control and other experimental diets fed fishes. The total carotenoid content in the acetone extracted skin tissues of C. auratus fed with 2% yeast supplemented diet varied from 0.748+ 0.042 in 0 day to 0.944 + 0.014 µg/g wet tissue on 30th day of experiment. Likewise, in the muscle tissue of C. auratus extracted with acetone, the total carotenoid varied from 0.376+0.014 (0 day) to 0.541 + 0.008 µg/g wet tissue (30 day). The carotenoid content of both the tested tissues on methanol extract was low, when compared with acetone extract. The overall results inferred that S. cerevisiae supplementation @ 2g/ 100g diet distingly altered the carcass composition of C. auratus and also the accumulation of cartenoids in skin and muscle tissues. S. cerevisiae contains bioactive lead molecules, which accelerated the vital physical and metabolic activities in host fish species.

An Investigation on Argulus foliaceus (Crustacea) Infestation of Fishes in the Kunduzlar Dam Lake

In this study, crustacean parasite species, Argulus foliaceus was investigated on the seven fish species (Alburnus escherichii, Barbus plebejus, Capoeta tinca, Carassius gibelio, Chondrostoma nasus, Cyprinus carpio, Squalius cephalus) from the Kunduzlar Dam Lake in Kırka-Eskişehir, Türkiye. The parasite was found on the gills, fins, and skin of all of the host fish species except Chondrostoma nasus. Its prevalence and mean intensity of the parasitic infection value for the infected six fish species are as follows: 3 out of 38 Alburnus escherichii (prevalence 7.9%, mean intensity 4.3±3.1), 9 out of 16 Barbus plebejus (56.2%, 47.9±28.5), 12 out of 47 Capoeta tinca (25.5%, 27.5±18.9), 7 out of 17 Carassius gibelio (41.1%, 6.9±5.3), 4 out of 4 Cyprinus carpio (100%, 4.3±4.2) and 6 out of 14 Squalius cephalus (42.8%, 2.3±1.8). Argulus foliaceus was recorded for A. escherichii among the six host fish species for the first time within the study area from Türkiye. Thus, a new host fish species and a new locality have been added to the geographical infestation zone of the A. foliaceus species from Türkiye, which is located in the southeast region of Eurasia. Parasitological findings were evaluated concerning seasonal, age, and gender groups of the host fishes. In conclusion, A. foliaceus infestation was observed higher in summer due to the increased in water temperature, which is significantly different from winter, spring, and autumn. The infestation of A. foliaceus was more common in larger fish than in smaller fish. Moreover, the similar rates of infestation were recorded between the male and female fish.

New record of Tylodelphys metacercariae (Diplostomidae) from Perccottus glenii (Odontobutidae) and their phylogenetic assessment.

Metacercariae of Tylodelphys sp. were found in the abdominal cavity of the Chinese sleeper (Perccottus glenii) collected in Liaoning Province and Inner Mongolia Autonomous Region of China. The sequences of the mitochondrial cox1 gene and ribosomal ITS1-5.8S rDNA-ITS2 region were obtained and used for molecular identification and phylogenetic assessment of this parasite species. Results of phylogenetic analyses based on ITS and cox1 markers showed that the metacercariae of Tylodelphys sp.ex P. glenii from China were conspecific with specimens of Tylodelphys sp. collected by Sokolov etal. (2013) from the same fish-host species captured earlier in West Siberia, Russia. The examined Tylodelphys sp. ex. P. glenii is the only member of the genus whose metacercariae parasitise the abdominal cavity of fish in northern Eurasia. Tylodelphys sp. ex P. glenii clustered with T. darbyi, T. immer, T.podicipina, and Tylodelphys sp. of Soldánová etal., 2017 based on mitochondrial DNA markers, and with T. darbyi, T. immer, T. kuerepus, and T. schreuringi using nuclear DNA markers.

Redescription of Procamallanus (Spirocamallanus) mysti (Karve, 1952) Infecting Freshwater Fishes from Muzaffarnagar, India

During a survey of parasitic nematodes of freshwater fishes in Muzaffarnagar region, a knownspecies of family Camallinidae belonging to the genus Procamallanus (Spirocamallanus)was isolated from two different fish host species- Mystus cavasius and Ompok bimaculatus.Morphological and morphometrical description supported by li

Host Fish Suitability for Freshwater Bivalve Anodonta woodiana Breeding Programs

The freshwater bivalve Anodonta woodiana is native to China and widely distributed in Asia, Europe, and North America. However, natural populations of A. woodiana in China have dramatically declined recently. Several fish species have been used as potential hosts in breeding programs; nonetheless, the optimal host species is yet to be identified. In this study, we examined the suitability of five potential host fish species (bighead carp, common carp, crucian carp, yellow catfish, and tilapia) for A. woodiana under laboratory conditions. No significant difference was found in the number of glochidia attached to the five fish species; however, tilapia hosted more transformed juveniles than bighead carp, common carp, or yellow catfish (p < 0.05), with bighead carp hosting the least (p < 0.05). Yellow catfish had the highest host capacity index (the number of successfully transformed juveniles/the weight of the host fish × the survival rate of the host fish), 133 times higher than bighead carp and 1.3–2.9 times higher than the other species. The shell length and height of freshly transformed juveniles from yellow catfish were significantly larger than those of other host fish (p < 0.05). The juvenile index (shell length × shell height) of yellow catfish was significantly higher than that of other fish species (p < 0.05). In conclusion, yellow catfish appears to be the most suitable host fish out of the five species tested here.

Different hosts in different lakes: prevalence and population genetic structure of plerocercoids of Ligula intestinalis (Cestoda) in Czech water bodies.

Ligula intestinalis (Linnaeus, 1758) is a tapeworm parasite with a worldwide distribution that uses a wide variety of fish species as its second intermediate host. In the present study, we investigated the prevalence and population genetic structure of plerocercoids of L. intestinalis in five common cyprinoid species, roach Rutilus rutilus (Linnaeus), freshwater bream Abramis brama (Linnaeus), white bream Blicca bjoerkna (Linnaeus), bleak Alburnus alburnus (Linnaeus), and rudd Scardinius erythrophthalmus (Linnaeus), collected in six water bodies of the Czech Republic (Milada, Most, Medard, Jordán, Římov and Lipno). Of the six study sites, the highest frequency of parasitism was recorded in Lake Medard (15%). The overall prevalence rate among the species was as follows: roach > rudd ≥ freshwater bream > bleak > white bream. Two mitochondrial genes (cytb and COI) were used to compare the population genetic structure of parasite populations using selected samples from the five fish species. The results of the phylogenetic analysis indicated that all populations of L. intestinalis were placed in Clade A, previously identified as the most common in Europe. At a finer scale, haplotype network and PCoA analyses indicated the possible emergence of host specificity of several mtDNA haplotypes to the freshwater bream. Moreover, pairwise Fixation indices (FST) revealed a significant genetic structure between the parasite population in freshwater bream and other host species. Parasite populations in roach not only showed the highest rate of prevalence but also depicted a maximum number of shared haplotypes with populations from bleak and rudd. Our results suggest that recent ecological differentiation might have influenced tapeworm populations at a fine evolutionary scale. Thus, the differences in prevalence between fish host species in different lakes might be influenced not only by the parasite's ecology, but also by its genetic diversity.

Lipid classes and fatty acid composition in two parasitic copepods Peroderma cylindricum and Lernaeocera lusci and their respective fish hosts Sardina pilchardus and Merluccius merluccius from the Tunisian waters

The present study investigates the detailed lipid classes and their fatty acid (FA) compositions from two parasitic copepods Lernaeocera lusci and Peroderma cylindricum and their respective fish host species Merluccius merluccius and Sardina pilchardus. The lipid classes, including phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), triacylglycerol (TAG), wax ester/cholesterol ester (WE/CE), mono-diacylglycerol (MDG), and free fatty acids (FFA) were separated by thin layer chromatography. The results revealed that TAG and PC were the major lipid classes in parasites; while WE/CE and PS were the most abundant in hosts. As for FA composition, C16:0, C18:0, C18:1n-9, C20:5n-3, and C22:6n-3 were recurrently found to be dominant in all lipid classes of the different organisms studied. However, some differences concerning the abundance and the distribution of several FAs were observed. Overall, the obtained results highlighted that despite the quite strong trophic connection between the parasites and their respective hosts, the parasites could be distinguished by specific lipid profiles.

Four carangid fish species as new host records for Kudoa trachuri Matsukane, Sato, Tanaka, Kamata et Sugita-Konishi, 2011 (Myxozoa: Multivalvulida), and description of a new species, Kudoa longichorda sp. n., forming pseudocysts in the muscle of Decapterus tabl Berry.

Multivalvulid myxosporeans of the genera Kudoa Meglitsch, 1947 and Unicapsula Davis, 1924 (Cnidaria: Myxozoa) are often the cause of unsightly cyst formation or postmortem myoliquefaction in the trunk muscle of commercial marine fish, which reduces the market value of infected individuals. Twenty species (18 Kudoa spp. and two Unicapsula spp.) have been recorded from carangid fish, although the majority of them, excluding polyxenous species, such as K. amamiensis Egusa et Nakajima, 1980, K. iwatai Egusa et Shiomitsu, 1983, K. nova Naidenova, 1975, K. quadratum (Thélohan, 1895) and K. yasunagai (Hsieh et Chen, 1984), are limited to a single or a few fish species. We report the occurrence of macroscopic cysts of Kudoa trachuri Matsukane, Sato, Tanaka, Kamata et Sugita-Konishi, 2011 in the trunk muscle of four new host fish species, i.e., Pseudocaranx dentex (Bloch et Schneider), Decapterus akaadsi Abe, D. muroadsi (Temminck et Schlegel) and Decapterus tabl Berry, fished from the Philippine Sea (Northwest Pacific Ocean), off southwestern of Japan. Myxospore morphology and genetic characteristics of the ribosomal RNA gene (rDNA) of these isolates were consistent with previous records of K. trachuri from Trachurus japonicus (Temminck et Schlegel) from around Japan. In addition, a new species of Kudoa that forms long filamentous pseudocysts in trunk myofibres was found in four of the six D. tabl collected in this study. We describe Kudoa longichorda sp. n. for this new isolate, based on its morphology of subquadrate myxospores with four shell valves and polar capsules and with small dimensions (length 4.3-5.5 µm, width 6.0-6.8 µm, thickness 4.8-6.3 µm, polar capsule length 2.3-3.1 µm, polar capsule width 1.1-1.7 µm), as well as 18S and 28S rDNA sequences distinct from those of known species.

Anisakid nematodes in Trichiurus lepturus and Saurida undosquamis (Teleostea) from the South-West Indian Ocean: Genetic evidence for the existence of sister species within Anisakis typica (s.l.), and food-safety considerations

Nematode parasites of the genus Anisakis (Nematoda, Anisakidae) are considered among the most important biological hazards in seafood products worldwide. In temperate and tropical waters, the most common species appears to be Anisakis typica, generally found around the viscera and sporadically in the flesh of various fish host species. This study investigated the infection sites and genetic diversity of A. typica infecting commercial fishes from the South-West Indian Ocean. Largehead hairtail (N = 20) and brushtooth lizardfish (N = 72) fished off Tanzania were inspected for anisakid nematodes by UV-press. A subsample of 168 nematodes were identified by sequence analyses of the cox2 mtDNA gene and ITS region of rDNA. The species A. typica (s.l.) (N = 166), Pseudoterranova ceticola (N = 1) and Anisakis paggiae (N = 1) were molecularly identified. Phylogenetic analysis of A. typica (s.l.) sequences based on both genes, indicated the existence of two distinct phylogenetic lineages forming two well-supported clades. The first clade comprised 12 A. typica specimens including individuals from its type locality (central Atlantic Ocean). The second clade comprising 154 specimens, clustered with reference sequences retrieved from GenBank including one apparently undescribed taxon, i.e., Anisakis sp. 1, and A. typica var. indonesiensis. The two reciprocally monophyletic clades are closely related and correspond to two distinct sister species within A. typica (s.l.), presently indicated as A. typica sp. A and A. typica sp. B. Two and four fixed alternative nucleotide substitutions (SNPs), i.e., diagnostic positions, between the two taxa, respectively, were found at the mtDNA cox2 and the ITS region of rDNA. The genetic data, as well as their occurrence in sympatry, strengthens the hypothesis that the actual specimens represent two distinct gene pools. The occurrence of both A. typica sp. A and A. typica sp. B in the musculature of freshly examined T. lepturus and S. undosquamis, suggests that both species can migrate intra-vitam into the flesh. Although the zoonotic potential of A. typica s.l. is still unclear, the presence of these parasites in the musculature, edible part of the fish, raises health concerns for consumers.

Prevalence and Molecular Characterization of Ascaridoid Parasites of Philippine Decapterus Species.

There are relatively few studies on parasite fauna of marine fishes in Philippine waters. This study aimed to determine the prevalence of marine ascaridoid infection in Decapterus species in Balayan Bay and Tayabas Bay. A total of 371 fishes belonging to three different species of Decapterus (D. tabl [n = 130], D. macrosoma [n = 121], and D. maruadsi [n = 120]) were collected. Ascaridoid parasite larvae were found in all fish host species, with an overall fish infection rate of 22%. The highest infection rate was observed in D. tabl (27.69%), followed by D. macrosoma (19%), and then D. maruadsi (17.50%). Moreover, a higher prevalence of infection was detected in Tayabas Bay (27.57%) than in Balayan Bay (15.59%). Molecular analyses based on the ITS2 and 18S rRNA gene supported the identification of the larvae into two species: Anisakis typica and Raphidascaris (Ichthyascaris) lophii. This is the first report of the genetic identification of these two helminth parasites in Decapterus fish species in the Philippines. Paucity in the database of Philippine marine fish parasites warrants more research efforts, especially concerning economically important fish species with implications to food safety and food security.

First record of trace element accumulation in a freshwater ectoparasite, Paradiplozoon ichthyoxanthon (Monogenea; Diplozoidae), infecting the gills of two yellowfish species, Labeobarbus aeneus and Labeobarbus kimberleyensis

BackgroundElevated levels of trace elements in the aquatic environment poses risks to the health of biota and humans. Parasites are important components in ecosystems; responding to changes in the health of aquatic ecosystems and can accumulate trace elements in their tissues to higher levels than their hosts. Monogeneans are an important group of fish ectoparasites being directly exposed to the aquatic environment. MethodsIn this study concentrations of Ti, Fe, Cu, Zn, Rb, Sr and Ag were analysed in the monogenean parasite, Paradiplozoon ichthyoxanthon (by total reflection x-ray fluorescence spectrometry and graphite furnace atomic absorption spectrometry), and the muscle, liver and gills of two host fish species, Labeobarbus aeneus and Labeobarbus kimberleyensis (by inductively coupled plasma -– mass spectrometry). ResultsMost striking was the accumulation pattern for Zn in parasites; mean levels of Zn were as high as 1448 and 1652 mg kg−1 dw, respectively, with no significant difference between the two host-parasite groups, leading to bioconcentration factors of approximately 93 (parasite/fish muscle) and 15 (parasite/fish liver). In addition, Fe was accumulated in the parasite to a higher degree compared to the fish hosts’ tissues. Cu levels were higher in P. ichthyoxanthon than in the muscle tissue of both host fishes, but lower than liver tissue. ConclusionThese findings demonstrate the usefulness of this parasite species as a sentinel organism in aquatic ecosystems it inhabits for Fe and Zn. Other trace elements under investigation were not accumulated higher in the parasite compared to its fish host. Lower Rb levels in the parasite compared to its hosts indicate no biomagnification of this metal. Further investigations are required to determine if similar trends in trace element accumulation occur in other monogeneans. Data availabilityAll data generated in the analysis of host and parasite tissues are presented in the manuscript.

Genetic isolation and homogenization: Potential effects of landscape features on the population genetic structure of freshwater mussels

Dispersal, the movement of individuals from their natal population to their breeding population, has important ecological and evolutionary consequences. In lotic freshwater systems, landscape features such as barriers or connectors, can affect dispersal and thus gene flow. It is of special interest to characterize population genetic structure in the presence of impassable barriers because they can restrict dispersal and thus isolate populations. On the other hand, connectors such as man-made canals connect watersheds that would otherwise be isolated, thereby enhancing gene flow and potentially genetically homogenizing populations. Freshwater mussels (Order: Unionida), many of which are highly threatened, have a unique life history involving parasitism on host fish species for larval development and dispersal. Landscape features that affect fish passage will consequently affect mussel dispersal and the maintenance of populations and metapopulations. This manuscript explores how landscape features affect the population genetic structure and genetic diversity of the freshwater mussel species Lampsilis siliquoidea, Fatmucket, in the southwestern Lake Ontario drainage. Data from seven microsatellite loci revealed that 1) waterfalls act as barriers for gene flow between freshwater mussel populations within the same river, 2) populations above the barrier had lower genetic diversity, 3) there was no detectable effect of man-made canals on genetic diversity and, 4) populations that were not hydrologically connected by a canal were marginally more differentiated than the populations connected by a canal. Landscape features can alter the connectivity within and between drainages thus changing the scale at which management will be effective.

High prevalence of Ichthyophonus sp. infections in Northeast Atlantic mackerel (Scomber scombrus).

High prevalence of Ichthyophonus sp. infections in Northeast Atlantic mackerel (Scomber scombrus).

A Highly Unstable and Elusive Plasmid That Encodes the Type III Secretion System Is Necessary for Full Virulence in the Marine Fish Pathogen Photobacterium damselae subsp. piscicida.

The marine bacterium Photobacterium damselae subsp. piscicida (Pdp) causes photobacteriosis in fish and important financial losses in aquaculture, but knowledge of its virulence factors is still scarce. We here demonstrate that an unstable plasmid (pPHDPT3) that encodes a type III secretion system (T3SS) is highly prevalent in Pdp strains from different geographical origins and fish host species. We found that pPHDPT3 undergoes curing upon in vitro cultivation, and this instability constitutes a generalized feature of pPHDPT3-like plasmids in Pdp strains. pPHDPT3 markers were detected in tissues of naturally-infected moribund fish and in the Pdp colonies grown directly from the fish tissues but were undetectable in a fraction of the colonies produced upon the first passage of the primeval colonies on agar plates. Notably, cured strains exhibited a marked reduction in virulence for fish, demonstrating that pPHDPT3 is a major virulence factor of Pdp. The attempts to stabilize pPHDPT3 by insertion of antibiotic resistance markers by allelic exchange caused an even greater reduction in virulence. We hypothesize that the existence of a high pressure to shed pPHDPT3 plasmid in vitro caused the selection of clones with off-target mutations and gene rearrangements during the process of genetic modification. Collectively, these results show that pPHDPT3 constitutes a novel, hitherto unreported virulence factor of Pdp that shows a high instability in vitro and warn that the picture of Pdp virulence genes has been historically underestimated, since the loss of the T3SS and other plasmid-borne genes may have occurred systematically in laboratories for decades.