Kei’ichiro Iguchi

The influence of rearing density on stress response and disease susceptibility of ayu (Plecoglossus altivelis)

Abstract The impact of coldwater disease caused by Flavobacterium psychrophilum is a recent important problem in the farming of ayu (Plecoglossus altivelis). We hypothesized that stress from high-rearing densities predisposes fish to disease by reducing their immunocompetence, and we sought to reduce fish losses by introducing an alternative culture regime. Fish enclosed at three densities (1250 fish or 8.0 kg, 400 fish or 2.6 kg and 100 fish or 0.6 kg/m3) were examined. Fish in the high-density treatment exhibited more elevated serum cortisol concentrations and more suppressed serum immunoglobulin M (IgM) concentrations accompanied by higher mortality, possibly caused by coldwater disease, than those in the medium- and low-density treatments. These results indicate that high-rearing densities stress fish and evoke subsequent physiological responses which have maladaptive effects. We propose that coldwater disease may be prevented by improving the disease resistance of fish by using moderate stocking densities.

Biogeography and evolution of the Carassius auratus-complex in East Asia

BackgroundCarassius auratus is a primary freshwater fish with bisexual diploid and unisexual gynogenetic triploid lineages. It is distributed widely in Eurasia and is especially common in East Asia. Although several genetic studies have been conducted on C. auratus, they have not provided clear phylogenetic and evolutionary descriptions of this fish, probably due to selection bias in sampling sites and the DNA regions analysed. As the first step in clarifying the evolutionary entity of the worlds Carassius fishes, we attempted to clarify the phylogeny of C. auratus populations distributed in East Asia.ResultsWe conducted a detailed analysis of a large dataset of mitochondrial gene sequences [CR, 323 bp, 672 sequences (528 sequenced + 144 downloaded); CR + ND4 + ND5 + cyt b, 4669 bp in total, 53 sequences] obtained from C. auratus in East Asia. Our phylogeographic analysis revealed two superlineages, one distributed mainly among the Japanese main islands and the other in various regions in and around the Eurasian continent, including the Ryukyus and Taiwan. The two superlineages include seven lineages with high regional specificity that are composed of endemic populations indigenous to each region. The divergence time of the seven lineages was estimated to be 0.2 million years ago (Mya) by a fossil-based method and 1.0-1.9 Mya by the molecular clock method. The antiquity and endemism of these lineages suggest that they are native to their respective regions, although some seem to have been affected by the artificial introduction of C. auratus belonging to other lineages. Triploids of C. auratus did not form a monophyletic lineage but were clustered mostly with sympatric diploids.ConclusionsThe results of the present study revealed the existence of two superlineages of C. auratus in East Asia that include seven lineages endemic to each of the seven regions examined. The lack of substantial genetic separation between triploids and diploids indicates that triploids are not composed of a single independent lineage. The ancient origins and evolutionary uniqueness of the seven lineages warrant their conservation. An overall phylogenetic framework obtained from the present study will be of use for estimating the phylogenetic relationships of Carassius fishes on the Eurasian continent.

Effect of fish in rice-fish culture on the rice yield

Rice-fish culture, which means the simultaneous culture of rice and fish, is one of the best options to increase food production from limited land and is practiced in many countries in the world. Although many researchers and farmers believe that the rice yield is increased by fish farming in paddy fields, this hypothesis has never been fully tested. Here, we report ecological processes leading to higher rice yields in the rice-fish culture using crucian carp (Carassius complex), which have adapted to the paddy field ecosystem in Japan. We compare the rice-fish and rice-only plots in the experimental paddy field for biota, water quality, and rice yield. Coverage of duckweed and densities of zooplankton and benthic invertebrates in the rice-fish plots were lower than those in the rice-only plots, indicating that fish utilized them as food. NO3–N concentration in the rice-fish plots was higher than that in the rice-only plots, indicating that the increase in NO3–N concentration results from excretion of unutilized food nutrients by the fish. Consequently, rice yield in the rice-fish plots was 20% higher than that in the rice-only plots. The fertilizing effect of the fish excrement probably increased rice yield.

Morphological and mtDNA sequence studies on three crucian carps (Carassius: Cyprinidae) including a new stock from the Ob River system, Kazakhstan

Three morphologically and genetically distinct forms of the genus Carassius were collected from the Ob River system, Kazakhstan, Central Asia; Carassius carassius, Carassius gibelio gibelio and an unknown stock tentatively referred to as Carassius gibelio sub-species M. The last mentioned had 33-41 gill rakers, being intermediate between the other two forms (23-27 in C. carassius and 44-49 in C. g. gibelio), and five scales in the upper transverse series, less than in the others. It also had a relatively larger erythrocyte suggesting triploidy and an mtDNA haplotype distinct from all other known crucian carps. Comparative mtDNA phylogenetic analysis suggested that C. gibelio gibelio in the Ob River system was introduced from China and the Amur River, the same possibly being true for European C. gibelio gibelio based on published haplotypes. C. gibelio sub-species M is thought to be more widely distributed in central Asia, probably extending as far west as European Russia.

Genetic constitution and phylogenetic relationships of Japanese crucian carps ( Carassius )

The genetic constitution and phylogenetic relationships among the proposed species and subspecies of the crucian carp complex in Japan (Carassius cuvieri, C. auratus subspp. 1 and 2, C. a. grandoculis, C. a. buergeri, and C. a. langsdorfii) were investigated based on analyses of the partial nucleotide sequences of the mitochondrial DNA control region and amplified fragment length polymorphisms of nuclear DNA. Our results demonstrate that C. cuvieri and C. auratus are different entities. However, although several distinct lineages were observed for C. auratus, none corresponded to the proposed subspecies. Moreover, the five subspecies of C. auratus were not necessarily separated genetically from each other. Triploid fish, which are currently classified as a single subspecies (C. a. langsdorfii) but are sometimes treated as an independent species, fell into various clades along with diploid individuals of other subspecies that have the same or similar mitochondrial haplotypes. This suggests that gynogenetic triploid crucian carps distributed throughout the Japanese Archipelago have polyphyletic maternal origins. Our results indicate that Japanese crucian carps are a much more complex assemblage than previously believed.

Exposure of a herbivorous fish to 134Cs and 137Cs from the riverbed following the Fukushima disaster

Ayu Plecoglossus altivelis, a herbivorous fish, is an important fishery resource and key component of the foodweb in many Japanese streams. Radionuclide contamination of this species is likely transferred to higher trophic levels, include humans, in the food chain. After the Fukushima accident in March 2011, ayu were exposed to highly contaminated silt while feeding on algae attached to the riverbed stones. To understand the route by which herbivorous fish are exposed to radionuclides, the activity concentrations of sum of (134)Cs and (137)Cs (radiocesium) were analyzed in riverbed samples (algae and silt) and in the internal organs and the muscle of ayu in five river systems in the Fukushima Prefecture between summer 2011 and autumn 2013. Although there was a positive correlation between the radiocesium activity concentrations in the muscle and the internal organs of ayu, the median activity concentration in the muscle was much lower than those in the internal organs. The activity concentrations of radiocesium in the riverbed samples and the internal organs and the muscle of ayu were correlated with contamination levels in soil samples taken from the watershed upstream of the sample sites. The results of the generalized linear mixed models suggest that the activity concentrations in both the internal organs and the muscle of ayu declined over time. Additionally, the activity concentrations in the internal organs were correlated with those in the riverbed samples that were collected around the same time as the ayu. The activity concentrations in the muscle were correlated with ayu body size. Our results suggest that ayu ingest (134)Cs and (137)Cs while grazing silt and algae from the riverbed, and a part of the (134)Cs and (137)Cs is assimilated into the muscle of the fish.

Early dispersal of ayu during marine stages as inferred from geographic variation in the number of vertebrae

Marine environments often allow interbreeding of individuals over the species range, and analyses using neutral molecular markers may lose extant genetic boundaries laid between geographic majorities. Ayu Plecoglossus altivelis has a typical amphidromous life history, migrating between rivers and the sea. In order to clarify reproductive elements of the species, migrants from 64 rivers and streams sampled over a wide latitudinal range were examined for their vertebral number (VN) and dorsal pterygiophore number (DPN) as morphological markers to estimate the temperature history. The irregular variability in VN without a geographic cline suggests that the ascending schools of fish are composed of conspecifics sharing incubation temperature or hatching site. The intersample difference in DPN indicates that larvae and juveniles of ayu spend their marine life inside a water body with a distinct temperature. Site tenacity during the marine stage may be hhelpful to enhance the opportunity to return to the river where they hatched. Each assemblage of fish ascending to rivers and streams is deemed to mostly represent a reproductive element, and therefore, each acts as an evolutionarily significant unit within a metapopulational structure.

Mitochondrial DNA Population Structure of the White-Spotted Charr (Salvelinus leucomaenis) in the Lake Biwa Water System

Abstract A phylogeographic analysis of mitochondrial DNA sequences was performed in order to elucidate the origin, dispersal process, and genetic structure of white-spotted charr in the Lake Biwa water system. Two haplotypes were most common in the Lake Biwa water system, and were also common in the adjacent inlet rivers of the Sea of Japan. These results suggest that in the glacial periods of the Pleistocene, white-spotted charr dispersed into the northern inlet rivers of Lake Biwa from adjacent inlet rivers of the Sea of Japan by watershed exchanges, colonizing the whole of the Lake Biwa water system. Mitochondrial DNA diversity contrasted sharply between the western and eastern parts of the system, suggesting that the populations in the western part might be more reduced than those in the eastern part in relation to the smaller habitat size. The high overall FST estimate (0.50), together with pairwise comparisons of FST, indicated significant genetic divergence between populations due to isolation and small population size. Hierarchical analysis (AMOVA) also showed that genetic variation was more pronounced among regions (28.39%) and among populations within regions (47.24%) than within populations (24.37%). This suggests that each population in and around the Lake Biwa water system should be treated as a significant unit for conservation and management.

Effect of saline water on early success of amphidromous fish

Organisms that migrate between rivers and the sea inevitably pass through estuarine habitats. Despite the potential importance of salinity and temperature fluctuations for metabolic adaptation, little is known about the impact of environmental changes in estuaries on the survival of residents. Ayu (Plecoglossus altivelis) is a migratory fish that inhabits estuarine brackish water in its early life stages. The recent decline in the abundance of populations ascending into rivers is of concern for local biodiversity. The present study aims to elucidate the ecological processes that determine the early success of Ayu larvae under variable environmental conditions. The effects of salinity and water temperature on the endogenous growth of newly hatched larvae from the same brood were examined experimentally based on morphological and metabolic characteristics. High salinity and high water temperature together appeared to require more energy for larval osmoregulation, resulting in the acceleration of yolk depletion and reduced growth of the notochord. Increasing the osmoregulative cost during the yolk-sac stage resulted in the faster induction of a state of starvation. Seawater is considered to have an adverse effect on the survival of newly hatched larvae, as it lowers the efficiency of foraging and predator avoidance due to an energetic tradeoff. More attention should be paid to the significance of estuarine environments involving brackish waters to ensure the early survival of amphidromous fish such as Ayu.

Habitat Use of the Grazing Goby (Sicyopterus japonicus) in Response to Spatial Heterogeneity in Riparian Shade

ABSTRACT The local density of the grazing goby (Sicyopterus japonicus) and benthic algal production were investigated in a 720 m reach of the Sate River in the northern part of Okinawa Island, Japan. The reach was spatially heterogenous in riparian shade and was divided into three segments (open-canopy, shaded and patchy segment) based on vegetation cover. In the reach, the goby was more abundant in the open-canopy segment and open-canopy patches than in the adjacent shaded segment and shaded patches. Between the open-canopy and shaded segments there was no significant difference in the algal biomass; however, the algal growth rate was greater in the open-canopy segment than in the shaded segment. These results indicate that the spatial heterogeneity in riparian shade controls the habitat use of the goby, with open-canopy areas with high algal renewal rates being preferred.