Pavel Jurajda

The reproductive ecology of the European bitterling (Rhodeus sericeus)

The European bitterlingRhodeussericeus Pallas is a freshwater fish belonging to the Acheilognathinae, a subfamily that has an unusual spawning symbiosis with freshwater mussels. Female bitterling possess long ovipositors that they use to place their eggs onto the gills of a mussel through the mussels’ exhalant siphon. Males fertilize the eggs by releasing sperm into the inhalant siphon of the mussel. The embryos develop inside the mussel for approximately am onth, eventually leaving the mussel as actively swimming larvae. Because they use a discrete spawning site that can be readily manipulated they have recently been used in studies to understand oviposition choice with respect to mate and spawning site quality, host–parasite coevolution, alternative male mating tactics, and linking behavioural decisions with population dynamics. The current state of knowledge of bitterling reproductive ecology is reviewed and synthesized, and future directions for research on this species are proposed.

Male-male interference competition decreases spawning rate in the European bitterling (Rhodeus sericeus)

We investigated the consequences of male-male interference competition associated with alternative male mating tactics in a freshwater fish, the European bitterling (Rhodeus sericeus). Male bitterling defend territories around living mussels and attract females to lay their eggs in the gill cavities of mussels. We experimentally manipulated spawning-site abundance and male density at two spatial scales. We showed that the total number of eggs spawned by females was constrained by the number of mussels available for oviposition. The effect was mediated by behavioral interactions among competing males because of variation in the Operational Sex Ratio (OSR) in close proximity to a mussel and not by a direct limitation in mussel capacity to accommodate the eggs. Both total and local male densities affected spawning behavior, and interacted in their effect on female spawning rate. Territorial male aggression caused courtship interruptions that prolonged the time until successful spawning and increased with male density. However, territoriality broke down at the highest male density, with a consequent stabilizing effect on spawning rate.

Adaptive host choice and avoidance of superparasitism in the spawning decisions of bitterling (Rhodeus sericeus).

Abstract Choice of a site for oviposition can have fitness consequences. We investigated the consequences of female oviposition decisions for offspring survival using the bitterling, Rhodeus sericeus, a freshwater fish that spawns inside living unionid mussels. A field survey of nine bitterling populations in the Czech Republic revealed a significantly lower rate of release of juvenile bitterling from Anodonta cygnea compared to three other mussel species. A field experiment demonstrated that female bitterling show highly significant preferences for spawning in A. anatina, Unio pictorum, and U. tumidus. Within a species, female bitterling avoided mussels containing high numbers of bitterling embryos. Mortality rates of bitterling embryos in mussels were strongly density dependent and the strength of density dependence varied significantly among mussel species. Female preferences for mussels matched survival rates of embryos within mussels and females distributed their eggs among mussels such that embryo mortalities conformed to the predictions of an ideal free distribution model. Thus, female oviposition choice is adaptive and minimizes individual embryo mortality.

Assessment of sperm competition by European bitterling, Rhodeus sericeus

We investigated male assessment of sperm competition in the bitterling, Rhodeus sericeus, a freshwater fish that spawns on the gills of living unionid mussels. Field experiments showed males increased their inspection rate of mussels into which a testis solution containing sperm had been experimentally released. Males avoided leading females to mussels that contained high numbers of embryos, but did not alter their leading behavior in response to the presence of sperm. In laboratory experiments males also increased their inspection rate of mussels into which a testis solution had been released and also failed to alter their leading behavior in response to the presence of sperm in mussels. However, males avoided leading females to mussels in close proximity to other males, and thereby may avoid sperm competition. In a second field study, territorial males were shown to ejaculate into mussels at a low rate in the absence of competitors, increase the frequency of ejaculations in competition with a rival, then decrease relative ejaculate expenditure as the number of competing males increased. Observed data were shown to be significantly correlated with predicted estimates of ejaculate expenditure for a model of sperm competition intensity. We discuss our results in the context of adaptive responses of males to sperm competition.

Seasonal changes of immunocompetence and parasitism in chub (Leuciscus cephalus), a freshwater cyprinid fish

Seasonal variation of water characteristics, predominantly temperature, is considered to strongly affect fish physiology and immunology. In nature, this variation directly influences the life cycle especially in fish parasites, but the infection of parasites is also altered by the host immune response. This study is aimed to analyze the seasonal changes in selected physiological and immunological parameters, the latter a potential measure of fish immunocompetence. Moreover, the seasonal changes in metazoan parasite infection were investigated, and the potential associations between fish physiology, immunocompetence, and parasitism were estimated. No differences in gender were recognized for immunological parameters. The significant differences in the spleen–somatic index were found among fish samples of different seasons indicating higher investment in spleen size in April (after winterizing) and August (post-breeding with low gonado-somatic index [GSI]). The significant seasonal differences in erythrocyte and leukocyte cell counts, as well as in phagocyte count affecting respiratory burst, were recorded. The general trend of leukocyte composition was similar in all seasons investigated; however, the changes in proportion of different neutrophilic cells were reported. The parasite diversity and the infection parameters in parasite communities were highest in spring and early summer. When comparing parasite abundance in infracommunities among seasons, the statistically highest values were observed in April and June for Monogenea, in April and November for Acanthocephala, and in April for Cestoda. The positive correlations between GSI and the parasite abundance of groups with higher infection parameters were found in males. Moreover, the positive association between Monogenea as the dominant parasite group and respiratory burst was observed. The higher investment in condition and the seasonal variation in GSI were associated with a decrease immune function measured by spleen size or leukocyte cell count especially for males suggesting the seasonal energy allocation between immune function and somatic or reproductive investment.

Sexual conflict, sexual selection and sperm competition in the spawning decisions of bitterling, Rhodeus sericeus

Abstract. Sexual selection may operate through either direct selection on preference genes or indirect genetic benefits. However, in many species both direct and indirect selection may operate and can influence female mate and oviposition choice. Adaptations by males for sperm competition can also affect female mate and oviposition choice and can lead to sexual conflict. We investigated the role of direct and indirect benefits and male adaptation to sperm competition for female oviposition decisions in the bitterling, Rhodeussericeus, a freshwater fish that spawns on the gills of living unionid mussels. Field experiments showed that females chose mussels for spawning on the basis of the number of embryos already in mussels and the body size and/or extent of red coloration of the eyes of territorial males. There was also a significant interaction between these direct and indirect benefits of oviposition choice. Territorial male bitterling avoided leading females to mussels into which rivals had already released sperm. Males also increased their rate of sperm release into mussels in response to sperm release by sneaking males. We discuss the possibility that sexual conflict in oviposition choice occurs in this species.

Size-structured diel use of river banks by fish

Abstract:We examined the role of body size in the diel use of river banks by fish and the abundance of piscivorous fishes to determine if predation pressure influences diel habitat use. We hypothesised that the mean standard length (SL) of non-piscivorous fish would decrease at night. Fish were sampled via point sampling by electrofishing every 3-4 h over three 24-h periods (late August 1992, early September 1993, late May 1994) along two adjacent stretches of bank (shallow sand, steep boulder) of the River Morava, Czech Republic. Gill netting in mid-channel adjacent to the sand bank was also used in 1993 to determine predator abundance. Analysis of variance and size-class ordination revealed higher numbers of smaller fish were captured at night along the banks than during the day, being in most cases significant. In almost all cases (all years and along all bank types), the standard length (SL) of non-piscivorous fishes was significantly lower during hours of darkness, in particular near midnight. Conversely, the SL of the non-piscivorous fishes in August 1993 was significantly higher in the gill nets during hours of darkness. Size class ordination and electivity indices revealed that bitterling Rhodeus sericeus in August 1992 were distributed according to size along the sand bank (smaller size classes) and the boulder bank (larger size classes); also, equivalent or adjacent size classes of Rutilus rutilus (41-50, 51-60, 61-70, 71-80, 81-105 mm SL), a large bodied species, and Gobio albipinnatus (26-50, 51-60, 61-70, 71-80, 81-90mm SL), a small-bodied species, co-exploited both types of river bank, moving to the sand bank at night, a possible example of recruitment bottleneck. In May 1994, smaller size classes of Leuciscus leuciscus (78-84, 85-90, 90-108 mm SL) and L. cephalus (63-99, 100-149, ≥ 150 mm SL) preferred the sand and boulder banks, respectively, with the largest sizes of the two species occurring together more often than expected along both banks. The results generally support our hypothesis, suggesting that shallow river shorelines represent important night-time refuges from predation for the smaller size classes of fish, thus constituting crucial life-history habitat important for the recruitment of many riverine fishes.

Spatial distribution of drifting cyprinid fishes in a shallow lowland river

We investigated downstream drift of the early developmental stages of cyp- rinid fishes in a shallow lowland river to ascertain the spatial dynamics of this mode of transport. Drift density and mean body size were compared among four sampling sta- tions positioned at different distances from the riverbank through a series of diel sam- pling periods. Common bream ( Abramis brama ) was the dominant species (76%), fol- lowed by silver bream ( Abramis bjoerkna ) and roach (Rutilus rutilus ). Ninety-five per- cent of drifting fish were larvae. Most fish drifted 1 to 3m from the shore, at a max- imum density of 131 fish 100m -3 . Relatively few fish drifted in midchannel and most fish drifted at night. Body size of drifting fishes was positively correlated with distance from the bank. Consistent results were obtained from analysis of developmental sta- ges, with younger larvae drifting near to the bank and older larvae and early juveniles drifting in midchannel. We propose that the spatial distribution of drifting fish is linked to their swimming ability and fish larvae may enter the river current as a means of ac- tive transport.

Parasitism, life history traits and immune defence in cyprinid fish from Central Europe

BackgroundThe main prediction of life-history theory is that optimal energy allocated among the traits is related to the growth, maintenance and survival. It is hypothesized that the optimal resource allocated to immune function, which generates resistance towards parasites and reduce the fitness losses caused by parasitism, is depending on other requirements for energetic resource and the benefits associated with them. The aims of this study are to investigate in a comparative way (1) how parasitism is related to fish life history traits (fecundity, longevity, mortality), (2) whether there is a trade-off between reproduction and immune investments in fish females (i.e. energetic hypothesis) and in males (i.e. immunohandicap hypothesis), (3) whether parasitism influences host immunity (spleen size) and reproduction (gonad size) in females and males.ResultsData on metazoan parasites of 23 cyprinid fish species from Central Europe were used for the analyses as well as new data collected from a field study. Ectoparasite species richness was negatively correlated with the fish mortality estimated by the k-value and positively correlated with fish body size, suggesting that parasite diversity increases with fish longevity. A negative relationship between spleen size and gonad size, controlling for fish body size, was found in females but not in males. Moreover, parasite abundance was positively correlated with fish spleen size and negatively with fish gonad size in females.ConclusionThe comparative analyses using cyprinid fish species demonstrated that natural mortality could be considered as a factor contributing to the variability of parasite species richness and moreover, parasite species benefit from long-lived fish. The results obtained from the analyses investigating the potential trade-off between reproduction and immunity could be interpreted as an energetic trade-off between female reproduction and immune function. The lack of negative relationship between gonad size and spleen size in males did not support our prediction based on the immunohandicap hypothesis.

The interaction between an introduced fish host and local parasite fauna: Neogobius kessleri in the middle Danube River

Parasite communities of introduced fish Neogobius kessleri Günther (Gobiidae) were studied at five localities in the Slovak section of the Danube River during 2002–2005. Thirty-three metazoan parasite species were identified. All fish were infected with at least two parasite species; most of the parasite species were generalists. At all sampling sites, high susceptibility to local parasites was observed. The parasite community was dominated by three parasite species: glochidia of Anodonta anatina, larval or subadult acanthocephalan Pomphorhynchus laevis, and larval nematode Raphidascaris acus. The infection of both A. anatina and P. laevis was affected by season and habitat type, with higher abundance in spring and more frequent occurrence of A. anatina in side channels and P. laevis in main river channels. At both the component and infracommunity levels, a more diverse parasite community was found in side channels. This habitat was dominated by actively transmitted parasites, whilst endoparasites were more abundant in fish from the main river channel. Larval stages of parasites dominated the endoparasite community at all sampling sites. The introduced N. kessleri was used as intermediate host for most of the recorded parasites, in some cases also as a paratenic host. Finally, the importance of gobies as suitable hosts for local non-native parasite species (Anguillicoloides crassus, Anodonta woodina, Hydrozetes lacustris) is discussed.