Rose bitterling (Rhodeus ocellatus) embryos parasitize freshwater mussels by competing for nutrients and oxygen

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.

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.

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.

Nest-site selection in a fish species with paternal care

Fish that perform paternal care may increase their fitness by choosing nest sites that enhance survival and development of embryos. We studied nest-site choice with respect to dissolved oxygen concentration and water temperature in males of the three-spined stickleback (Gasterosteus aculeatus), a small fish species with exclusive male parental care that usually breeds in the littoral zone of freshwaters of the Northern hemisphere. Fathers oxygenate the embryos by fanning movements of their pectoral fins. We expected choice for conditions at potential nest sites that would benefit offspring development, i.e., higher temperature and higher dissolved oxygen concentration. In the laboratory, we offered males a choice between two potential nest sites that differed in dissolved oxygen concentration or water temperature. Males preferred to build a nest at sites with a higher dissolved oxygen level or higher temperature and thus chose sites that would promote embryo development.

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.

Environmental behavior of methamphetamine and ketamine in aquatic ecosystem: Degradation, bioaccumulation, distribution, and associated shift in toxicity and bacterial community

Ketamine (KET) and methamphetamine (METH) have been recognized as emerging contaminants in aquatic ecosystems. This paper aimed to investigate the environmental behaviour, including the degradation, distribution, and bioaccumulation, of METH, KET, and their main metabolites (amphetamine (AMP) and norketamine (NorKET)). The changes in acute toxicity in the aqueous phase and in the bacterial community in sediment were determined to assess the associated eco-risk of the drug exposure. Five types of lab-scale aquatic ecosystems were established and exposed to KET or METH for 40 days: a water- sediment- organisms- KET system (K), a water- sediment- organisms- METH system (M), a water- sediment- organism- METH- KET system (M + K), a water-sediment- KET- METH system (control), and a water- sediment- organisms system (biocontrol). The results demonstrated that much faster degradation occurred for both METH (t_1/2 = 3.89 and 2.37 days in the M and M + K group, respectively) and KET(t_1/2 = 5.69 days 5.39 days in the K group and M + K group, respectively) than in the control group (t_1/2 = 7.83 and 86.71days for METH and KET, respectively). Rapid adsorption of KET, METH, and their metabolites was observed in the sediment, which had clay and silt as the main particle sizes. KET was observed to be absorbed by shallow-water fish (Chinese medaka, rosy bitterling and mosquito fish), while METH was dominantly ingested by bottom-dwellers (loach). Duckweed might play a crucial role in the dissipation process of METH and KET, which were mainly adsorbed by duckweed roots. During incubation, the acute toxic levels in the K and M + K groups changed from non-toxic to medium toxicity levels, and the toxicity in the M and control groups changed from non-toxic to low toxicity levels. Moreover, marked changes in the bacterial community in the sediment induced by METH or KET exposure were observed, and the most significant change in the bacterial community was observed in the group spiked with both METH and KET. This work for the first time elucidated the environmental behaviors of METH and KET in aquatic ecosystem and associated the impact on ecological system equilibrium.

Intensity-dependent energetic costs in a reciprocal parasitic relationship

Parasitic infections elicit host defences that pose energetic trade-offs with other fitness-related traits. Bitterling fishes and unionid mussels are involved in a two-way parasitic interaction. Bitterling exploit mussels by ovipositing into their gills. In turn, mussel larvae (glochidia) develop on the epidermis and gills of fish. Hosts have evolved behavioural responses to reduce parasite load, suggesting that glochidia and bitterling parasitism are costly. We examined the energetic cost of parasitism on both sides of this relationship. We used intermittent flow-through respirometry to measure (1) standard metabolic rate (SMR) of individual duck mussels Anodonta anatina (a common bitterling host) before and during infection by embryos of the European bitterling Rhodeus amarus, and (2) SMR and maximum oxygen uptake (MO₂max) of individual R. amarus before and during infection with glochidia of the Chinese pond mussel Sinanodonta woodiana (a mussel species that successfully infects bitterling). As predicted, we observed an increase in mussel SMR when infected by bitterling embryos and an increased SMR in glochidia-infected bitterling, though this was significantly mediated by the time post-infection. Contrary to our predictions, glochidia infection did not impair MO₂max and the number of glochidia attached to gills positively (rather than negatively) correlated with MO₂max. The results suggest that tolerance is the prevailing coping mechanism for both fish and mussels when infected, while resistance mechanisms appear to be confined to the behavioural level.

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.

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.

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.

A sperm competition model for the European bitterling (Rhodeus amarus)

Sperm competition occurs when the spermatozoa of one male coincide with those of another to fertilise the same eggs. In some taxa males perform multiple ejaculations, which may function in sperm competition or in maintaining a baseline density of spermatozoa in the female reproductive tract to ensure fertilisation, a process that has been termed ‘topping up’. We investigated multiple ejaculations in the European bitterling (Rhodeus amarus), a freshwater fish that oviposits in freshwater mussels. We quantified spermatozoa in the mussel mantle cavity following ejaculation, and measured sperm motility parameters of males adopting different mating tactics. Following ejaculation spermatozoa density in the mussel increased linearly, peaked after 30 s, and then declined exponentially. Spermatozoa motility parameters did not differ between male mating tactics. We parameterised a model of sperm competition forR. amarus, which accurately predicted male fertilisation probability. We discuss these results in the context of multiple ejaculations and male mating tactics.