The reproductive ecology of the European bitterling (Rhodeus sericeus)

Reproductive Strategies and Embryonic Development of Autumn-Spawning Bitterling (Acheilognathus rhombeus) within the Mussel Host

We investigated the reproductive strategies and embryonic development of Acheilognathus rhombeus (a bitterling species that spawns in autumn) within its freshwater mussel host in the Bongseo Stream, South Korea. By focusing on survival mechanisms during critical stages of embryonic development, the selective use of mussel gill demibranchs by the bitterlings and associated adaptive traits were observed over 1 year. A significant diapause phase occurs at developmental stage D, which lasts for approximately 7 months, allowing embryos to survive winter. Development resumes when the temperature exceeds 10 °C. Minute tubercles on the embryos (crucial for anchoring within the host gill demibranchs and preventing premature ejection) exhibit the largest height during diapause, and the height decreases when developmental stage E is reached, when growth resumes. Acheilognathus rhombeus embryos were observed in 30.5% of the mussels, mostly within the inner gills, thereby maximizing spatial use and oxygen access to enhance survival. These results highlight the intricate relationship between A. rhombeus and its mussel hosts, demonstrating the evolutionary adaptations that enhance reproductive success and survival. This study provides valuable insights into the ecological dynamics and conservation requirements of such symbiotic relationships.

Description of three new species of Gyrodactylus von Nordmann, 1832 (Monogenea: Gyrodactylidae) on bitterling fishes (Acheilognathinae) from China

Three new species of Gyrodactylus are described from three species of bitterling in Donghu Lake, China: Gyrodactylus ocellorhodei n. sp. from Rhodeus ocellatus; G. sinenorhodei n. sp. from Rhodeus sinensis; and G. acheilorhodei n. sp. from Acheilognathus macropterus. All the three new species showed similar opisthaptor morphology, especially the marginal hooks: all had a slender and perpendicular sickle shaft, and flat sickle base with distinct heel and inner arch which was different from the G. rhodei-group species parasitic on bitterling. Multivariate analyses based on hamulus and marginal hooks suggested that these three new species cannot be completely distinguished, despite some morphology divergence observed in certain less reliable morphometric features, such as hamulus root length, ventral bar total length and process shape. These three new species shared an identical 18S ribosomal RNA gene sequence, while the variation in the Internal Transcribed Spacers (ITS1-ITS2) sequence among them (8.4-11.2%, K2P) far exceeded the 1% ITS sequence difference that had been suggested as a threshold for species delimitation of Gyrodactylus. Phylogenetic analysis based on ITS1-ITS2 showed that all these sequenced Gyrodactylus spp. parasitic on the subfamily Acheilognathinae host formed a monophyletic group. However, a clear differentiation (18.9-20.9%, K2P of ITS1-ITS2) could be found between the subgroup from China (G. ocellorhodei n. sp., G. sinenorhodei n. sp. and G. acheilorhodei n. sp.) and that from Europe (G. rhodei).

Impact of habitat engineering by invasive Corbicula clams on native European unionid mussels

Biological invasions cause biodiversity erosion on a global scale. Invasive species spreading beyond their natural range compete with native fauna for food and space, push native species to suboptimal habitats, impairing their behaviour and thus limiting their occurrence. Freshwater ecosystems are especially vulnerable to biological invasions and their ecological and economic impacts. The invasive Asian clams (Corbicula spp.), due to their opportunistic life style, can occur at densities of thousands ind. m-2. They act as ecosystem engineers transforming bottom substrata through accumulation of shells. Our goal was to determine the effect of substratum modification by living Corbicula and their shells on substratum choice and behaviour of Unio tumidus and Anodonta anatina, two European freshwater mussel species of the highly imperilled Unionidae family. We assessed their substratum selection in pairwise choice tests (pure sand vs. sand modified by living Corbicula or their shells, sand modified by shells vs. living Corbicula). Next, we tested locomotion and burrowing of unionids on pure substratum and substrata modified by Corbicula. Unionids avoided sand modified by living Corbicula and their empty shells, not distinguishing between these two types of substratum modification. In the presence of Corbicula, their burrowing was shallower or it took them longer to obtain the same depth as in the pure sand. Additionally, on sand modified by Corbicula shells, we observed a locomotion increase (U. tumidus) or slowing down (A. anatina). Our research showed a novel mechanism of negative impact of Corbicula on unionids, consisting in pushing them away from their optimal habitats. This may contribute to their habitat loss and future declines in invaded ecosystems.

Host size matters for reproduction: Evolution of spawning preference and female reproductive phenotypes in mussel-symbiotic freshwater bitterling fishes

Bitterling fishes evolve an idiosyncratic symbiosis with freshwater mussels, in which they are obligated to spawn in the gills of mussels for reproduction. In recent years, freshwater mussel populations have been drastically diminishing, due to accelerating anthropogenic impacts, which can be large threats to the risk of bitterling's extinction cascade (i.e. 'coextinction'). The host mussel size may be an important factor driving the adaptation and evolution of bitterling's reproductive phenotypes. Here we examined the host size preference and morphological adaptation of female bitterling to the host size from 17 localities at the Han River in Korea. Using our developed molecular-based species identification for bitterling's eggs/larvae inside the mussels, we further determined the spawning patterns of seven bitterling species. Mean length of spawned mussels (N = 453) was significantly larger than that of unspawned mussels (N = 1814), suggesting that bitterling prefers to use larger hosts as a spawning ground. Spawning probability was clearly greater as mussel size increases. Results of our reciprocal transplant experiments do provide some evidence supporting the 'bitterling's larger host preference' hypothesis. Interspecific competition appeared to be intense as two fish species often spawned eggs in the same mussel individuals simultaneously. Longer ovipositor and more elongated egg may evolve in females of Tanakia signifer in response to larger host environments. The observed bitterling's spawning preference for large-sized mussels may evolve perhaps because of the fitness advantage in relation to the offspring survival. Our findings further inform on the development of effective conservation and management strategy for the endangered bitterling fishes.

Parasitic fish embryos do a "front-flip" on the yolk to resist expulsion from the host

Embryonic development is often considered shielded from the effects of natural selection, being selected primarily for reliable development. However, embryos sometimes represent virulent parasites, triggering a coevolutionary "arms race" with their host. We have examined embryonic adaptations to a parasitic lifestyle in the bitterling fish. Bitterlings are brood parasites that lay their eggs in the gill chamber of host mussels. Bitterling eggs and embryos have adaptations to resist being flushed out by the mussel. These include a pair of projections from the yolk sac that act as an anchor. Furthermore, bitterling eggs all adopt a head-down position in the mussel gills which further increases their chances of survival. To examine these adaptations in detail, we have studied development in the rosy bitterling (Rhodeus ocellatus) using molecular markers, X-ray tomography, and time-lapse imaging. We describe a suite of developmental adaptations to brood parasitism in this species. We show that the mechanism underlying these adaptions is a modified pattern of blastokinesis-a process unique, among fish, to bitterlings. Tissue movements during blastokinesis cause the embryo to do an extraordinary "front-flip" on the yolk. We suggest that this movement determines the spatial orientation of the other developmental adaptations to parasitism, ensuring that they are optimally positioned to help resist the ejection of the embryo from the mussel. Our study supports the notion that natural selection can drive the evolution of a suite of adaptations, both embryonic and extra-embryonic, via modifications in early development.

Feeding of mussel-associated leeches Hemiclepsis kasmiana on bitterling embryos: Novel interaction between parasites in a shared host

We investigated an interaction between bitterlings and a parasitic leech Hemiclepsis kasmiana in freshwater mussel hosts. We found that leeches fed on bitterling eggs and embryos; this may exert a considerable negative effect on bitterling fitness. Host choices by females of three bitterling species may be differently affected by the presence of leeches within mussels; Tanakia limbata apparently avoided laying eggs in infested mussels while T. lanceolata and Acheilognathus rhombeus did not. Our novel findings suggest that relationships between the parasitic leech and the host mussel may be context dependent.

The volume and shape of bitterling eggs are more strongly influenced by germ cell autonomy than by the surrounding somatic cells

There is great variation in the size and shape of teleost eggs from species to species. The size of the teleost egg depends on the amount of yolk accumulated in the egg, which is an important factor directly affecting the survival of hatchlings. Egg shape also contributes significantly to spawning ecology and survival during the prehatching stage. In this study, we used bitterlings, which show a wide variety of egg volumes and shapes, to elucidate whether these factors are determined by germ cells or somatic cells. Reciprocal transplantations of germ cells between two bitterling species revealed that the egg volume was identical to that of the germ cell donor species in both combinations. The egg shape was also very similar to that of the species providing the germ cells. These results suggest that the volume and shape of teleost eggs are greatly influenced by germ cell autonomy.

Parasites and their impact on thick-shelled river mussels Unio crassus from two populations in Luxembourg

The thick-shelled river mussel Unio crassus Philipsson, 1788 is a species native to many European habitats, with declining populations. The impact of parasite communities on health status of this species is poorly understood. In this study, parasites of 30 U. crassus specimens from the Our and Sauer Rivers in Luxembourg were identified morphologically and, in some cases, using molecular genetic methods. The findings were correlated to selected parameters (total length, visceral weight, shell lesions, gonadal stage). The 2 populations did not differ in shell length, visceral weight, number of males and females, gonadal scoring, shell lesions, and the occurrence of glochidia. The prevalence and infestation intensities of detected Trichodina sp., Conchophthirus sp., and freshwater mite larvae did not differ between the 2 populations, whereas the prevalence and infestation intensities of mite eggs, nymphs, and adults were significantly higher in the Sauer River. Rhipidocotyle campanula and European bitterling Rhodeus amarus larvae were only present in the Sauer. Histopathology revealed the destruction of the gonads by R. campanula and tissue damage by the mites. The only significant correlation of the selected parameters was a positive correlation between R. amarus occurrence and total length as well as a negative correlation between R. amarus occurrence and gonadal stage. In the Sauer River, 2 mussels were found to be hermaphrodites.

Changes in the height of minute tubercles on the skin of Korean bitterling embryos (Acheilognathus signifer) and embryo movement in the host mussels

Bitterlings are freshwater fish that have developed morphological adaptations to improve the survival of their embryos in host mussels. The most well-known adaptation is the development of minute tubercles, which develop in the early embryonic stage when the embryos have poor swimming ability, and disappear when the embryos reach the free-swimming stage and leave the host mussels. In this study, the embryonic developmental stages of Acheilognathus signifer were analysed to elucidate the relationship between the changes in the height of the minute tubercles and their movement. The height changes in the minute tubercles in the embryos can be divided into five stages, i.e., formation, growth, peak, reduction and disappearance. The authors found that the embryos lived in the gill demibranch of the host mussel until day 6 after hatching. The movement of embryos to the suprabranchial cavity in the gill demibranch was firstly observed on day 7. At this point, the embryos showed a heartbeat and movement. From day 13, the minute tubercles had almost disappeared, and the hatchlings started swimming outside the host mussels from day 16. These observations highlight the different adaptations of minute tubercles among bitterling groups without wing-like projections.

Reproductive ecology and adaptive host choice correlated with body size in an autumn-spawning bitterling Acheilognathus typus

The life history and reproductive ecology of an autumn-spawning bitterling Acheilognathus typus were studied under natural and experimental conditions. In the study pond, the embryos of A. typus emerged from mussels in May and grew rapidly until August, whereas overwintered age-1 fish grew slowly. Adult A. typus in the pond was smaller (32-47 mm in standard length) than they were in other habitats and mainly spawned in smaller mussels. The number of A. typus embryos in mussels was negatively correlated with the shell length of the mussel, and a lower number of embryos were observed in larger mussels (over 110 mm in shell length). In the mussel size-choice experiment conducted in an enclosure, smaller A. typus selected smaller mussels, and larger A. typus selected larger mussels for spawning. In some cases, smaller A. typus spawned in larger mussels and the number of spawned eggs ejected increased by over four times compared with other cases. These results of the enclosure experiment explained the lower number of embryos in larger mussels in the study pond. In addition, reproductive traits such as ovipositor length and the number of ovulated eggs of female A. typus, which are considered to contribute to their size-dependent host utilization, were positively correlated with their standard length. Because A. typus is geologically or seasonally isolated from other bitterling species, this size-dependent host utilization contributes to a reduction in intraspecies rather than interspecies competition.

Developmental neuroanatomy of the rosy bitterling Rhodeus ocellatus (Teleostei: Cypriniformes)-A microCT study

Bitterlings are carp-like teleost fish (Cypriniformes: Acheilanathidae) known for their specialized brood parasitic lifestyle. Bitterling embryos, in fact, develop inside the gill chamber of their freshwater mussel hosts. However, little is known about how their parasitic lifestyle affects brain development in comparison to nonparasitic species. Here, we document the development of the brain of the rosy bitterling, Rhodeus ocellatus, at four embryonic stages of 165, 185, 210, 235 hours postfertilization (hpf) using micro-computed tomography (microCT). Focusing on developmental regionalization and brain ventricular organization, we relate the development of the brain divisions to those described for zebrafish using the prosomeric model as a reference paradigm. Segmentation and three-dimensional visualization of the ventricular system allowed us to identify changes in the longitudinal brain axis as a result of cephalic flexure during development. The results show that during early embryonic and larval development, histological differentiation, tissue boundaries, periventricular proliferation zones, and ventricular spaces are all detectable by microCT. The results of this study visualized with differential CT profiles are broadly consistent with comparable histological studies, and with the genoarchitecture of teleosts like the zebrafish. Compared to the zebrafish, our study identifies distinct developmental heterochronies in the rosy bitterling, such as a precocious development of the inferior lobe.

Complete mitochondrial genome of Rhodeuscyanorostris (Teleostei, Cyprinidae): characterization and phylogenetic analysis

Rhodeuscyanorostris Li, Liao & Arai, 2020 is a freshwater fish that is endemic to China and restricted to Chengdu City in Sichuan Province. This study is the first to sequence and characterize the complete mitochondrial genome of R.cyanorostris. The mitogenome of R.cyanorostris is 16580 bp in length, including 13 protein-coding genes, two rRNA genes, 22 tRNA genes, and a control region (D-loop). The base composition of the sequence is 28.5% A, 27.6% C, 26.4% T, and 17.5% G, with a bias toward A+T. The genome structure, nucleotide composition, and codon usage of the mitogenome of R.cyanorostris are consistent with those of other species of Rhodeus. To verify the molecular phylogeny of the genus Rhodeus, we provide new insights to better understand the taxonomic status of R.cyanorostris. The phylogenetic trees present four major clades based on 19 mitogenomic sequences from 16 Rhodeus species. Rhodeuscyanorostris exhibits the closest phylogenetic relationship with R.pseudosericeus, R.amarus, and R.sericeus. This study discloses the complete mitochondrial genome sequence of R.cyanorostris for the first time and provides the most comprehensive phylogenetic reconstruction of the genus Rhodeus based on whole mitochondrial genome sequences. The information obtained in this study will provide new insights for conservation, phylogenetic analysis, and evolutionary biology research.

Individual copy number variation and extensive diversity between major MHC-DAB1 allelic lineages in the European bitterling

Polymorphism of the major histocompatibility complex (MHC), DAB1 gene was characterized for the first time in the European bitterling (Rhodeus amarus), a freshwater fish employed in studies of host-parasite coevolution and mate choice, taking advantage of newly designed primers coupled with high-throughput amplicon sequencing. Across 221 genotyped individuals, we detected 1–4 variants per fish, with 28% individuals possessing 3–4 variants. We identified 36 DAB1 variants, and they showed high sequence diversity mostly located within predicted antigen-binding sites, and both global and codon-specific excess of non-synonymous mutations. Despite deep divergence between two major allelic lineages, functional diversity was surprisingly low (3 supertypes). Overall, these findings suggest the role of positive and balancing selection in promotion and long-time maintenance of DAB1 polymorphism. Further investigations will clarify the role of pathogen-mediated selection to drive the evolution of DAB1 variation.

The first record of egg masses in tunicates deposited by the snubnose sculpin, Orthonopias triacis, from the Northeastern Pacific: evidence for convergent evolution of an unusual reproductive strategy

In this study, the authors report the first record of egg masses deposited in solitary tunicates by the snubnose sculpin, Orthonopias triacis, from the Northeastern Pacific. Four egg masses were discovered in the tunicate Ascidia ceratodes that were genetically determined to be O. triacis. Female O. triacis had long ovipositors that allow deposition of their eggs inside the atrium of the tunicates. A comparison of host-tunicate size with ovipositor length of sculpins from the Northwestern Pacific, including the genera Furcina and Pseudoblennius, revealed that O. triacis had shorter ovipositors and spawned in the atrium of smaller species of tunicates. Ancestral state reconstruction of egg deposition in solitary tunicates using 1.86Mbp RNAseq data of 20 sculpin species from Northeastern and Northwestern Pacific revealed that this unusual spawning behaviour may have evolved convergently in different species occurring in the Northeastern vs. the Northwestern Pacific.

Rearing of Bitterling (Rhodeus amarus) Larvae and Fry under Controlled Conditions for the Restitution of Endangered Populations

Simple Summary

Among the many species threatened with extinction and covered by protection of species is bitterling Rhodeus amarus. It belongs to ostracophilic fish that place spawn inside live mussels. Bitterlings, such as mussels, belongs to indicator species that testify to the good state of the natural environment. Supporting the populations of these organisms is a necessity in order to preserve the biodiversity of inland waters, which are subjected to severe anthropopression. The development in advance of a method of reproduction and breeding of bitterling under controlled conditions can ensure their survival in the event of an imbalance in the natural environment. These were the first studies of this type, where after 6.5 months of rearing, sexually mature individuals were obtained. In this way, a suitable stocking material of the bitterling was obtained in order to save the endangered populations.

Abstract

Among the several dozen European freshwater fish species, only European bitterling (Rhodeus amarus Bloch) and Rhodeus meridionalis belong to the group of ostrakophilous fish. The embryonic and larval development of the fish in this reproductive group until the time of the yolk sac resorption takes place in the gill cavity of river mussels (Anodonta sp. or Unio sp.). This paper presents the results of the European bitterling Rhodeus amarus being reared under controlled conditions. Bitterling larvae were caught together with river mussels in the natural environment and subsequently placed in a tank for behavioural observations. Bitterling larvae were seen swimming in the water within a week of placing the bivalves under controlled conditions. The bitterling larvae were 8.6 ± 0.11 mm long when they started to swim actively. The rearing was conducted in water at 20 and 26 ± 0.5 °C and lasted for 6.5 months (200 days) in both variants. Initially, the larvae were fed with live nauplii of Artemia salina and subsequently with fodder. The bitterlings in tanks with water at 26 ± 0.5 °C were 66.2 ± 3.0 mm long and weighed 3389 ± 548 mg. For comparison, bitterlings kept in water at 20 ± 0.5 °C were 64.48 ± 3.4 mm long and weighed 3242 ± 427 mg. No larval malformities or mortality were observed during the larvae and fry rearing. The bitterlings had well-developed secondary sexual characteristics and exhibited pre-spawning behaviour at the end of the rearing. This produced suitable bitterling stocking material to be used in the conservation of small or endangered populations.

Normal stages of embryonic development of a brood parasite, the rosy bitterling Rhodeus ocellatus (Teleostei: Cypriniformes)

Bitterlings, a group of freshwater teleosts, provide a fascinating example among vertebrates of the evolution of brood parasitism. Their eggs are laid inside the gill chamber of their freshwater mussel hosts where they develop as brood parasites. Studies of the embryonic development of bitterlings are crucial in deciphering the evolution of their distinct early life-history. Here, we have studied 255 embryos and larvae of the rosy bitterling (Rhodeus ocellatus) using in vitro fertilization and X-ray microtomography (microCT). We describe 11 pre-hatching and 13 post-hatching developmental stages spanning the first 14 days of development, from fertilization to the free-swimming stage. In contrast to previous developmental studies of various bitterling species, the staging system we describe is character-based and therefore more compatible with the widely-used stages described for zebrafish. Our bitterling data provide new insights into to the polarity of the chorion, and into notochord vacuolization and yolk sac extension in relation to body straightening. This study represents the first application of microCT scanning to bitterling development and provides one of the most detailed systematic descriptions of development in any teleost. Our staging series will be an important tool for heterochrony analysis and other comparative studies of teleost development, and may provide insight into the co-evolution of brood parasitism.

Production of Chinese rosy bitterling offspring derived from frozen and vitrified whole testis by spermatogonial transplantation

Bitterling is a small cyprinid fish facing an increasing risk of extinction owing to habitat destruction and decreasing freshwater mussel population that are used as their spawning substrates. Owing to their large size and high yolk contents, methods for cryopreservation of their eggs or embryos, which is a promising method for long-term preservation of their genetic resources, are still not available. We conducted this study to evaluate the feasibility of gamete production by transplanting cryopreserved testicular cells into germ cell-less recipients that were produced by knockdown of dead end gene. Immature testes isolated from recessive albino Chinese rosy bitterlings were cryopreserved by slow freezing or vitrification. Approximately 3000 slow-frozen or vitrified cells were transplanted into the peritoneal cavity of 4-day-old germ cell-less wild-type Chinese rosy bitterlings. We observed no significant differences in the incorporation rates of the slow-frozen and vitrified cells into the genital ridges of recipients compared with those of freshly prepared cells. When the recipients matured, almost half of the male or female recipients that received freshly prepared, slow-frozen, or vitrified cells produced gametes derived from donor cells, with no significant differences in their fecundity among the 3 groups. Moreover, fertilization of the resulting eggs and sperm produced donor-derived offspring exhibiting the albino phenotype. Therefore, the abovementioned methods could be used as a powerful and practical method for long-term preservation of bitterling genetic resources for biotic conservation.

Minute tubercles in bitterling larvae: Developmental dynamic structures to prevent premature ejection by host mussels

Bitterlings are small freshwater fish that use long ovipositors to lay eggs in host mussels, and they have morphological adaptations to increase larval survival. The most well-known adaptation is the minute tubercles on the skin surface of larvae; they are developed in early-stage larvae with weak swimming ability and disappear in free-swimming larvae before they leave the host mussel.In the present study, I comprehensively analyzed the developmental stages of Rhodeus pseudosericeus larvae, their morphological and physiological characteristics, their migration inside mussels, and the development of minute tubercle in order to elucidate the morphological function of the minute tubercles. These tubercles began to develop 1 day after hatching (formation stage), grew for 2-5 days (growth stage), reached the peak height after 6-7 days (peak stage), abruptly reduced in height after 8-10 days (abrupt reduction stage), and gradually reduced in height (reduction stage) until completely disappearing 27 days after hatching (disappearance stage).The larvae remained in the mussels' interlamellar space of the gill demibranchs until 10 days after hatching and began to migrate to the suprabranchial cavity 11 days after hatching. At this time, the larvae had a heart rate and the caudal fin began to develop. At 24 days after hatching, the minute tubercles had almost disappeared, and some individuals were observed swimming out of the mussels.The results presented herein elucidate that the minute tubercles are the developmental dynamic structures that the bitterling larvae have morphologically adapted to prevent premature ejection from the mussel.

Limited impacts of chronic eye fluke infection on the reproductive success of a fish host

Parasitic infections may affect the reproductive success of the host either directly, through behavioural modification, or indirectly, by altering their reproductive investment in response to infection. We determined the effects of infection with the eye fluke Diplostomum pseudospathaceum (Trematoda) on the reproductive traits of European bitterling (Rhodeus amarus, Cyprinidae), an intermediate fish host with a resource-based mating system. Male bitterling infected by Diplostomum exhibited a larger but less pronounced red eye spot (sexually selected signal) than control males, suggesting that infected males were less preferred by females. The frequency of female ovulation and number of offspring were comparable between the infected and the control group, although there was a 1–2 week delay in the peak of ovulation and offspring production in infected fish, which is known to coincide with higher juvenile mortality. Chronic eye fluke infection had minimal metabolic costs (measured as oxygen consumption) and, consistent with these results, reproductive activity did not differ between infected and control fish in an experimental test of intersexual selection. Overall, the impact of eye fluke infection on the reproduction of European bitterling was limited. We consider the potential effect of favourable conditions during experiments (abundant food, access to spawning substrate and lack of predators and co-infections) on experimental outcomes and recognize that the effects of chronic eye fluke infection in natural conditions might be more pronounced.

Hybridization between an endangered freshwater fish and an introduced congeneric species and consequent genetic introgression

Artificial transplantation of organisms and consequent invasive hybridization can lead to the extinction of native species. In Matsuyama, Japan, a native bitterling fish, Tanakia lanceolata, is known to form hybrids with another bitterling species, T. limbata, which was recently introduced from western Kyushu, Japan. These bitterlings spawn in the gills of two freshwater unionid species, Pronodularia japanensis and Nodularia douglasiae nipponensis, which have rapidly declined on the Matsuyama Plain in the past 30 years. To gauge the effect of invasive hybridization, we determined the genetic introgression between T. lanceolata and T. limbata and analyzed the morphology of these species and their hybrids to infer their niche overlap. We collected adult individuals of Tanakia spp. and genotyped them based on six microsatellite loci and mitochondrial cytochrome b sequences. We analyzed their meristic characters and body shapes by geometric morphometrics. We found that 10.9% of all individuals collected were hybrids. Whereas T. lanceolata were more densely distributed downstream and T. limbata were distributed upstream, their hybrids were widely distributed, covering the entire range of native T. lanceolata. The body height and anal fin length of T. limbata were greater than those of T. lanceolata, but their hybrids were highly morphologically variable, covering both parental morphs, and were widely distributed in the habitats of both parental species. Hybridization has occurred in both directions, but introduced T. limbata females and native T. lanceolata males are more likely to have crossed. This study shows that invasive hybridization with the introduced T. limbata is a potential threat to the native population of T. lanceolata via genetic introgression and replacement of its niche in streams.

High cryptic diversity of bitterling fish in the southern West Palearctic

South-east Europe, along with the adjacent region of south-west Asia, is an important biodiversity hotspot with high local endemism largely contributed by contemporary continental lineages that retreated to southern refugia during colder Quaternary periods. We investigated the genetic diversity of the European bitterling fish (Rhodeus amarus) species complex (Cyprinidae) across its range in the western Palearctic, but with a particular emphasis in the region of Balkan, Pontic and Caspian refugia. We genotyped 12 polymorphic microsatellite loci and a partial sequence of mitochondrial gene cytochrome b (CYTB) for a set of 1,038 individuals from 60 populations. We used mtDNA sequences to infer phylogenetic relationships and historical demography, and microsatellite markers to describe fine-scale genetic variability and structure. Our mtDNA analysis revealed six well-supported lineages, with limited local co-occurrence. Two lineages are distributed throughout central and western Europe (lineages "A" and "B"), with two zones of secondary contact. Another two lineages were restricted to the Ponto-Aegean region of Greece (lineages "C" and "D") and the final two lineages were restricted south of the Caucasus mountains (lineage "E" from the Black Sea watershed and lineage "F" from the Caspian watershed). A signal of recent expansion was revealed in the two widespread lineages and the Ponto-Aegean lineage "C". The geographic distribution of clusters detected by nuclear microsatellites corresponded well with mitochondrial lineages and demonstrated finely sub-structured populations. A profound population structure suggested a significant role of genetic drift in differentiation among lineages. Lineage divergence in the Ponto-Aegean and Caspian regions are substantial, supporting the validity of two described endemic species (Rhodeus meridionalis as lineage "D" and Rhodeus colchicus as lineage "E") and invite taxonomic evaluation of the other two southern lineages (Thracean "C" and Caspian "F").

Modelling the invasion history of Sinanodonta woodiana in Europe: Tracking the routes of a sedentary aquatic invader with mobile parasitic larvae

Understanding the invasive potential of species outside their native range is one of the most pressing questions in applied evolutionary and ecological research. Admixture of genotypes of invasive species from multiple sources has been implicated in successful invasions, by generating novel genetic combinations that facilitate rapid adaptation to new environments. Alternatively, adaptive evolution on standing genetic variation, exposed by phenotypic plasticity and selected by genetic accommodation, can facilitate invasion success. We investigated the population genetic structure of an Asian freshwater mussel with a parasitic dispersal stage, Sinanodonta woodiana, which has been present in Europe since 1979 but which has expanded rapidly in the last decade. Data from a mitochondrial marker and nuclear microsatellites have suggested that all European populations of S. woodiana originate from the River Yangtze basin in China. Only a single haplotype was detected in Europe, in contrast to substantial mitochondrial diversity in native Asian populations. Analysis of microsatellite markers indicated intensive gene flow and confirmed a lower genetic diversity of European populations compared to those from the Yangtze basin, though that difference was not large. Using an Approximate Bayesian Modelling approach, we identified two areas as the probable source of the spread of S. woodiana in Europe, which matched historical records for its establishment. Their populations originated from a single colonization event. Our data do not support alternative explanations for the rapid recent spread of S. woodiana; recent arrival of a novel (cold-tolerant) genotype or continuous propagule pressure. Instead, in situ adaptation, facilitated by repeated admixture, appears to drive the ongoing expansion of S. woodiana. We discuss management consequences of our results.

Hybridization between two bitterling fish species in their sympatric range and a river where one species is native and the other is introduced

The distributions of two bitterling fish (subfamily: Acheilognathinae), Tanakia lanceolata and T. limbata, overlap in western Japan. Acheilognathinae fish lay their eggs in the gills of freshwater bivalves, and the early juvenile stage develops in the gills. Populations of freshwater bivalves are declining worldwide, which has limited the number of spawning substrate for bitterlings. T. limbata has been artificially introduced to some rivers in Ehime, Japan, where it coexists with native T. lanceolata, and some hybrids have been observed. We collected both species from several sites in western Japan, and from the Kunichi River system in Ehime, and analyzed genetic population structure based on six microsatellite loci and sequences of the mitochondrial cytochrome b gene. Structure analysis identified three genetically distinct populations: T. lanceolata, T. limbata “West Kyushu”, and T. limbata “Setouchi”. Two clades of T. limbata were also supported by molecular phylogenetic analyses based on cytochrome b. Hybrids in Ehime originated mostly from interbreeding between male T. lanceolata and female T. limbata “West Kyushu”, and made up 10.2% of all collected fish, suggesting that hybrids occurred frequently between females of colonizing species and males of native species. On the other hand, interspecific hybrids were detected at rates of 40.0%, 20.0%, and 17.6% in the Ima River (Fukuoka), Midori River (Kumamoto), and Kase River (Saga), respectively, which are naturally sympatric regions. We found a few T. limbata “Setouchi” in the Midori and Kase Rivers, which were supposed to be introduced from other regions, coexisting with native T. limbata “West Kyushu”, and this cryptic invasion may have triggered the interspecific hybridization. These results suggest that artificial introduction of a fish species, a decline in the unionid population, and degradation of habitat have caused broad hybridization of bitterlings in western Japan.

Sperm is a sexual ornament in rose bitterling

In many taxa, odour cues mediate mating decisions. A key question is what these odours comprise, where they are produced, and what they signal. Using rose bitterling, fish that spawn in the gills of freshwater mussels, we investigated the role of sperm cues on female oviposition decisions using individuals of known MHC genotype. Male bitterling frequently released sperm prior to female oviposition and females responded with an increased probability of oviposition and released a greater number of eggs, particularly if males had a dissimilar MHC genotype. These mating preferences by females were shown to be adaptive, with MHC dissimilarity of males and females correlated positively with embryo survival. These results support a role for indirect benefits to rose bitterling mate choice, and we propose that sperm acts as a releaser pheromone in bitterling, functioning as a sexual ornament signalling male quality as a mate.

Fine-scale genetic structure of the European bitterling at the intersection of three major European watersheds

BACKGROUND

Anthropogenic factors can have a major impact on the contemporary distribution of intraspecific genetic diversity. Many freshwater fishes have finely structured and locally adapted populations, but their natural genetic structure can be affected by river engineering schemes across river basins, fish transfers in aquaculture industry and conservation management. The European bitterling (Rhodeus amarus) is a small fish that is a brood parasite of freshwater mussels and is widespread across continental Europe. Its range recently expanded, following sharp declines in the 1970s and 1980s. We investigated its genetic variability and spatial structure at the centre of its distribution at the boundary of three watersheds, testing the role of natural and anthropogenic factors in its genetic structure.

RESULTS

Sequences of mitochondrial cytochrome B (CYTB) revealed that bitterling colonised central Europe from two Ponto-Caspian refugia, which partly defines its contemporary genetic structure. Twelve polymorphic microsatellite loci revealed pronounced interpopulation differentiation, with significant small-scale differentiation within the same river basins. At a large scale, populations from the Baltic Sea watershed (middle Oder and Vistula basins) were distinct from those from the Black Sea watershed (Danube basin), while populations from rivers of the North Sea watershed (Rhine, Elbe) originated from the admixture of both original sources. Notable exceptions demonstrated the potential role of human translocations across watersheds, with the upper River Oder (Baltic watershed) inhabited by fish from the Danube basin (Black Sea watershed) and a population in the southern part of the River Elbe (North Sea watershed) basin possessing a signal of admixture from the Danube basin.

CONCLUSIONS

Hydrography and physical barriers to dispersal are only partly reflected in the genetic structure of the European bitterling at the intersection of three major watersheds in central Europe. Drainage boundaries have been obscured by human-mediated translocations, likely related to common carp, Cyprinus carpio, cultivation and game-fish management. Despite these translocations, populations of bitterling are significantly structured by genetic drift, possibly reinforced by its low dispersal ability. Overall, the impact of anthropogenic factors on the genetic structure of the bitterling populations in central Europe is limited.

Bayesian inference supports the host selection hypothesis in explaining adaptive host specificity by European bitterling

Generalist parasites have the capacity to infect multiple hosts. The temporal pattern of host specificity by generalist parasites is rarely studied, but is critical to understanding what variables underpin infection and thereby the impact of parasites on host species and the way they impose selection on hosts. Here, the temporal dynamics of infection of four species of freshwater mussel by European bitterling fish (Rhodeus amarus) was investigated over three spawning seasons. Bitterling lay their eggs in the gills of freshwater mussels, which suffer reduced growth, oxygen stress, gill damage and elevated mortality as a result of parasitism. The temporal pattern of infection of mussels by European bitterling in multiple populations was examined. Using a Bernoulli Generalized Additive Mixed Model with Bayesian inference it was demonstrated that one mussel species, Unio pictorum, was exploited over the entire bitterling spawning season. As the season progressed, bitterling showed a preference for other mussel species, which were inferior hosts. Temporal changes in host use reflected elevated density-dependent mortality in preferred hosts that were already infected. Plasticity in host specificity by bitterling conformed with the predictions of the host selection hypothesis. The relationship between bitterling and their host mussels differs qualitatively from that of avian brood parasites.

Population-specific responses to an invasive species

Predicting the impacts of non-native species remains a challenge. As populations of a species are genetically and phenotypically variable, the impact of non-native species on local taxa could crucially depend on population-specific traits and adaptations of both native and non-native species. Bitterling fishes are brood parasites of unionid mussels and unionid mussels produce larvae that parasitize fishes. We used common garden experiments to measure three key elements in the bitterling-mussel association among two populations of an invasive mussel (Anodonta woodiana) and four populations of European bitterling (Rhodeus amarus). The impact of the invasive mussel varied between geographically distinct R. amarus lineages and between local populations within lineages. The capacity of parasitic larvae of the invasive mussel to exploit R. amarus was higher in a Danubian than in a Baltic R. amarus lineage and in allopatric than in sympatric R. amarus populations. Maladaptive oviposition by R. amarus into A. woodiana varied among populations, with significant population-specific consequences for R. amarus recruitment. We suggest that variation in coevolutionary states may predispose different populations to divergent responses. Given that coevolutionary relationships are ubiquitous, population-specific attributes of invasive and native populations may play a critical role in the outcome of invasion. We argue for a shift from a species-centred to population-centred perspective of the impacts of invasions.

Cognitive ability is heritable and predicts the success of an alternative mating tactic

The ability to attract mates, acquire resources for reproduction, and successfully outcompete rivals for fertilizations may make demands on cognitive traits--the mechanisms by which an animal acquires, processes, stores and acts upon information from its environment. Consequently, cognitive traits potentially undergo sexual selection in some mating systems. We investigated the role of cognitive traits on the reproductive performance of male rose bitterling (Rhodeus ocellatus), a freshwater fish with a complex mating system and alternative mating tactics. We quantified the learning accuracy of males and females in a spatial learning task and scored them for learning accuracy. Males were subsequently allowed to play the roles of a guarder and a sneaker in competitive mating trials, with reproductive success measured using paternity analysis. We detected a significant interaction between male mating role and learning accuracy on reproductive success, with the best-performing males in maze trials showing greater reproductive success in a sneaker role than as a guarder. Using a cross-classified breeding design, learning accuracy was demonstrated to be heritable, with significant additive maternal and paternal effects. Our results imply that male cognitive traits may undergo intra-sexual selection.

Life history and reproductive ecology of the endangered Itasenpara bitterling Acheilognathus longipinnis (Cyprinidae) in the Himi region, central Japan

The life history, reproductive ecology and habitat utilization of the Itasenpara (deepbody) bitterling Acheilognathus longipinnis were investigated in a lowland segment of the Moo River in Toyama Prefecture, central Honshu, Japan. Analysis of 1285 individuals revealed that the study population comprised a single size class, an age at maturation of 3 months and a life span of 1 year. On the basis of the growth pattern, the life cycle was divided into two stages: the juvenile stage, characterized by rapid growth, and the adult stage at which growth ceased. Spawning by A. longipinnis was recorded between early September and late October. Female A. longipinnis in the 0+ year age class began to mature when they reached a standard length (LS ) of 56·4 mm. Mature females had a large clutch size (maximum 273 eggs) and deposited highly adhesive and relatively large eggs (2·55 mm(3) ; major axis, 3·12 mm; minor axis, 1·22 mm) via a short ovipositor (mean length, 21·5 mm) into freshwater mussels. The embryos remained in the gill cavities of the freshwater mussels (used as a spawning substratum) and emerged as juveniles (LS , 9 mm). Habitat utilization during spawning was analysed using a generalized linear model. The best-fit model showed that three environmental factors (freshwater mussel availability, water depth and vegetation cover) were important variables for habitat utilization by A. longipinnis. Shallow areas (water depth, 250-330 mm) created for rice paddy management and areas with an abundance of cover were particularly effective for predator avoidance. These results suggest that maintenance of water level fluctuations corresponding with rice cultivation and the abundance of vegetation on the river bank (particularly avoidance of concrete revetments) is essential for conservation of this species under current practices for rice cultivation in Japan.

Phylogenetic relationships of Acheilognathidae (Cypriniformes: Cyprinoidea) as revealed from evidence of both nuclear and mitochondrial gene sequence variation: evidence for necessary taxonomic revision in the family and the identification of cryptic species

Bitterlings are relatively small cypriniform species and extremely interesting evolutionarily due to their unusual reproductive behaviors and their coevolutionary relationships with freshwater mussels. As a group, they have attracted a great deal of attention in biological studies. Understanding the origin and evolution of their mating system demands a well-corroborated hypothesis of their evolutionary relationships. In this study, we provide the most comprehensive phylogenetic reconstruction of species relationships of the group based on partitioned maximum likelihood and Bayesian methods using DNA sequence variation of nuclear and mitochondrial genes on 41 species, several subspecies and three undescribed species. Our findings support the monophyly of the Acheilognathidae. Two of the three currently recognized genera are not monophyletic and the family can be subdivided into six clades. These clades are further regarded as genera based on both their phylogenetic relationships and a reappraisal of morphological characters. We present a revised classification for the Acheilognathidae with five genera/lineages: Rhodeus, Acheilognathus (new constitution), Tanakia (new constitution), Paratanakia gen. nov., and Pseudorhodeus gen. nov. and an unnamed clade containing five species currently referred to as "Acheilognathus". Gene trees of several bitterling species indicate that the taxa are not monophyletic. This result highlights a potentially dramatic underestimation of species diversity in this family. Using our new phylogenetic framework, we discuss the evolution of the Acheilognathidae relative to classification, taxonomy and biogeography.

Phylogenetic relationships of bitterling fishes (Teleostei: Cypriniformes: Acheilognathinae), inferred from mitochondrial cytochrome B sequences

Bitterling (Teleostei: Acheilognathinae) are small cyprinid fishes with a discrete distribution in East Asia and Europe. We used a complete mitochondrial cytochrome b sequence (1141 bp) from 49 species or subspecies in three genera (Tanakia, Rhodeus, and Acheilognathus), sampled across the major part of their distribution, to elucidate their phylogeny and biogeography, focusing particularly on their origin and dispersal. Based on high support value, the monophyletic Acheilognathinae separated into two major clades, Acheilognathus and Tanakia-Rhodeus. In the latter clade, the monophyly of Rhodeus was poorly supported, though it was topologically nested in Tanakia. On the basis of molecular-clock calibration, both clades diverged in the middle Miocene, with Tanakia-Rhodeus diverging slightly earlier than Acheilognathus. The Tanakia-Rhodeus clade expanded its distribution westward from the Far East, eventually reaching Europe, while Acheilognathus dispersed in the temperate regions of East Asia. A feature common to both clades is that most extant species, including Japanese endemics, appeared by the end of the Pliocene, corresponding with the present delineation of the Japanese archipelago. Autumn-spawning species with an embryonic diapause, unique to bitterling among cyprinid fishes, formed two distinct lineages (barbatulusrhombeus and longipinnis-typus) within Acheilognathus. The estimated time of divergence of the two lineages was approximately from the late Pliocene, a period characterized by glaciations. The timing of divergence suggests that the shift of spawning from spring to autumn, coupled with embryonic diapause, convergently emerged twice in the evolution of bitterling, possibly as an adaptation to the climate of the late Pliocene.

The complete mitochondrial genome of Tanakia lanceolata (Cypriniformes: Cyprinidae)

Bitterling Tanakia lanceolata is a small sized freshwater fish species. The unique reproductive behavior makes bitterlings monophyletic. In this study, the complete mitochondrial genome of T. lanceolata is sequenced to be 16,607 bp in length, including 13 protein-coding genes, 2 ribosomal RNAs, 22 transfer RNAs, a control region and the origin of the light strand replication. The overall base composition of T. lanceolata in descending order is A 28.3%, C 28.0%, T 26.1%, and G 17.6%, with a slight A + T bias. The mitogenome sequence data may provide useful information to the population genetics analysis of T. lanceolata and the elucidation of evolutionary mechanisms in Cyprinidae.

Experimental evidence for parasite-induced over-winter mortality in juvenile Rhodeus amarus

In this study, the effects of the eye fluke Diplostomum pseudospathaceum (Trematoda) infection on over-winter survival of young-of-the-year (YOY) European bitterling Rhodeus amarus (Cyprinidae) were examined between September 2010 and April 2011. The fish were reared in semi-natural conditions to ensure that results were not confounded by other parasite infections. The cumulative mortality of R. amarus from November until April was significantly higher in D. pseudospathaceum-infected fish (57·3%) compared to controls (42·1%). Infection of the parental generation did not have any effect on the mortality of juveniles. The results indicate that D. pseudospathaceum infection increases over-winter mortality of YOY R. amarus. The possible mechanisms causing mortality are discussed.

Complete mitochondrial genome of the Chinese bitterling Acheilognathus macropterus (Cypriniformes: cyprinidae)

Acheilognathus macropterus belongs to the family Acheilognathinae. In the present study, we obtain the complete mitochondrial genome of the Chinese bitterling A. macropterus by PCR amplification and DNA sequencing. It is a circular double-stranded DNA molecule of 16,773 bp in length, containing 13 protein-coding genes, 22 transfer RNA genes, two ribosomal RNA genes, and two main non-coding regions (the control region and the origin of the light strand replication). The gene composition and order of which are similar to most other vertebrates. Compared with Acanthorhodeus macropterus, they share 98.90% nucleotide sequences similarity, and the biggest nucleotide sequence discrepancy between homologous genes are observed in ND5 gene. The molecular data we presented here could provide useful information for the studies on species identification, evolutionary relationships and population genetics of the Acheilognathinae.