Masou Research Articles

17β-hydroxysteroid dehydrogenase type 12-like is associated with maturation-inducing steroid synthesis during induced oocyte maturation and ovulation in sturgeons

17α, 20β-dihydroxy-4-pregnen-3-one (DHP) has been identified as a maturation-inducing steroid (MIS) in several teleosts. DHP is converted from 17α-hydroxyprogesterone (17OHP) by 20β-hydroxysteroid dehydrogenase (20β-HSD). A recent study reported that 17β-hydroxysteroid dehydrogenase type 12-like (hsd17b12L) is responsible for DHP production, and its expression is upregulated by gonadotropin during oocyte maturation in masu salmon and Nile tilapia. However, whether the hsd17b12L gene encodes 20β-HSD in Actinopterygii remains unknown. Sturgeons belong to ancient fish and are suggested to have diverged 200 million years ago. Currently, the MIS of sturgeons and its associated mechanism remain unknown. This study aimed to understand the evolution of Actinopterygii hsd17b12L and determine its function during sturgeon oocyte maturation. In the Amur sturgeon and sterlet, two hsd17b12L types were repeatedly isolated. Mammalian expression vectors containing Amur sturgeon hsd17b12L type I and sterlet hsd17b12L type I were transfected into HEK293T cells, which then exhibited strong 20β-oxidoreductase activity, converting 17OHP to DHP. LC/MS analyses indicated that Amur sturgeon hsd17b12L did not convert DHP, androstenedione, testosterone, estrone, estradiol-17β, adrenosterone, and 11-ketotestosterone, confirming that Amur sturgeon hsd17b12L has 20β-oxidoreductase activity, but not 20β-dehydrogenase activity toward DHP neither17β-HSD activity. Masu salmon hsd17b12L and Nile tilapia hsd17b12 expression were limited to the follicles and testes at the maturation stage, while Amur sturgeon hsd17b12L was upregulated in all tissues examined. In individuals that reached ovulation, serum DHP concentrations during oocyte maturation increased, and the Amur sturgeon hsd17b12L mRNA levels of follicles during oocyte maturation were higher than prior to maturation. Finally, sterlet and Amur sturgeon ovarian follicles were incubated with chum salmon pituitary extract (SPE, 500 μg/mL) for different times. The DHP concentration in the medium was increased by SPE. However, hsd17b12L mRNA levels were not significantly different following SPE stimulation. The present study demonstrated that sturgeon hsd17b12L encodes 20β-HSD, suggesting that the hsd17b12L gene encodes 20β-HSD in related fish species that evolved after the sturgeon. An in vivo experiment revealed that sturgeon hsd17b12L was associated with oocyte maturation and DHP production, but its mRNA expression in vitro did not exhibit a rapid increase during oocyte maturation as previously observed in masu salmon, suggesting that hsd17b12L does not solely regulate sturgeon DHP production.

Hatch timing of two subarctic salmonids in a stream network estimated by otolith increments

AbstractHatch timing in autumn‐spawning stream salmonids is poorly understood in the subarctic region because snow cover prevents direct sampling of cryptic early life stages. Otolith micro‐increment analysis was used to infer hatch dates of white‐spotted charr Salvelinus leucomaenis (Pallas) and masu salmon Oncorhynchus masou (Brevoort) in a mainstem‐tributary network in northern Japan. Accuracy and precision were validated by ageing hatchery individuals with known hatch date ranges. In July 2018, 93 wild young‐of‐the‐year white‐spotted charr and 81 masu salmon were collected and aged. Masu salmon hatched, on average, 24 days earlier (mean = February 8) than white‐spotted charr (March 4), and hatch dates spanned a minimum of 2 months for each species. In masu salmon, hatch dates of individuals collected in the mainstem were nearly 3 weeks earlier than those in a tributary. This study provided knowledge on intra‐ and inter‐specific variation in hatch timing of native salmonids in a subarctic stream network.

Anadromous red-spotted masu salmon (Oncorhynchus masou ishikawae), a southernmost sea-migration form of salmonid, displays low variation in both age at seaward migration and sea age.

We examined variations in age at seaward migration and sea age for the anadromous form of red-spotted masu salmon (Oncorhynchus masou ishikawae) in two Japanese rivers. The anadromous form of red-spotted masu salmon expressed only two sea migration patterns in the two rivers: (a) the majority of the salmon (95%, n=81) were of age-0, and age-1 migrants were rare (n=4); and (b) all the salmon examined (n=22) made a return migration within a year, with 23% of the salmon exhibiting potamodromy in the river. Owing to low variation in their sea migratory patterns, the anadromous form of red-spotted masu salmon is likely vulnerable to environmental fluctuations.

Morphological evolution reduces downstream displacement in juvenile landlocked salmon.

Severe flooding often leads to downstream displacement of aquatic animals. Despite this, many salmonid populations persist in habitats located upstream of tall barriers, such as artificial check dams and/or natural waterfalls, that completely block fishes from returning to the upstream areas after flooding. The evolution of such populations may be affected by spatial sorting due to differential rates of downstream displacement. This study examined whether a morphological trait (increased body depth) that allows individuals to better maintain their position during flooding has evolved in juvenile amago salmon Oncorhynchus masou ishikawae inhabiting above-barrier habitats in two rivers. In both rivers, juveniles collected at the stations with multiple downstream barriers had deeper bodies than those collected at other stations. Similar differences were found in juveniles reared in a common-garden experiment. Field experiments with natural flooding also indicated that deep bodies help juveniles resist downstream displacement. These results consistently suggest that juveniles in some above-barrier habitats have evolved deep bodies to resist downstream displacement due to flooding. Our study is the first to show the evolutionary outcomes of passive spatial sorting during severe climate events.

Testing local adaptations of affiliate freshwater pearl mussel, Margaritifera laevis, to its host fish, Oncorhynchus masou masou

AbstractUnderstanding the limiting factors of the reproduction process in host–affiliate relationships is a high priority. We examined the effects of habitat location on the reproductive process of freshwater pearl mussels Margaritifera laevis (Bivalvia, Unionida) as a parasite using sympatric and allopatric Oncorhynchus masou masou (Actinopterygii, Salmoniformes) as a host fish. Initial infection rates of parasitic larvae (glochidia) and transformation rates to cysts (encysted glochidia) were examined for all parasite‐host combinations from three habitat locations (a total of nine combinations) to test the hypothesis that sympatric pairs of mussels and fish result in the highest success rates of glochidia infection and encystment. Measurements of glochidia‐infected fish reared in flow‐through experimental indoor tanks were taken at the initial infection point as well as at encystment, 2 weeks after the infection. Results disagreed with our hypothesis. Instead, an unexpected heterogeneity in a pathological deformity in gills explained a greater amount of variance in these processes. This deformity was responsible for reducing the initial infection rate and increasing the metamorphosis rates of initially attached glochidia to cysts. The field‐measured prevalence of the gill deformity was low in all habitat locations, indicating that the deformity occurred during the acclimation period before infection for relatively small‐sized host fish more susceptible to infection. Our results did not show the local adaptation of parasitic freshwater mussels to host fish but shed light on one of the least studied factors, providing an empirical underpinning of the importance of pathologically diversified host conditions in the reproductive processes of unionid mussels.

The Relative Effects of Biotic and Abiotic Factors on the Recruitment of Freshwater Mussels (Margaritifera laevis)

Freshwater mussels, Unionoida, are endangered across the globe due to recruitment failure. In the present study, with general linear mixed models, we investigated the relative effects of biotic (host fish density) and abiotic (water depth, fine sediment, water temperature, and water quality) factors on the recruitment of Margaritifera laevis in 10 streams of Hokkaido, northern Japan. We additionally examined the factors regulating the density of the host fish Oncorhynchus masou masou with general linear models. The proportion of juvenile mussels had a unimodal relationship with the host density, which was the most influential factor among the others examined. The positive relationship between mussel recruitment and host density can be attributed to an increased host fish infection rate. The negative correlation between mussel recruitment and host density at high fish densities may be due to reduced larval growth on host fish that are in poor physical condition. We also found that host fish density was negatively affected by nutrient enrichment. Our results suggest that mitigating water quality degradation to recover host fish density should be prioritized to improve mussel recruitment. Although stock enhancement is effective for increasing the salmon population density, excess stocking can further disturb mussel recruitment.

Effect of estradiol on gonads development for resident form males of cherry salmon Oncorhynchus masou

Effect of estradiol on development of gonads in resident form males of cherry salmon is investigated. The fish aged 0+ with initial weight of 3.48-10.70 g were treated with the hormone for 30 days, then reared for 8 months. Estradiol was given with pelleted food in concentration of 100 mg/kg. The gonads were examined histologically during entire period of the experiment. The control group included 184 specimens (90 females, 59 anadromous males and 35 males of resident form). The experimental group of 50 females and 48 males was taken randomly in 3.5 months after the end of hormonal exposure. The fund of germ cells in gonads of the experimental group consisted only of gonias. Males of resident form were identified in 6 and 8 months after the end of hormonal exposure, they were distinguished by both size of the testes and presence of germ cells at different stages. Portion of the males of resident form was similar in the control and experimental groups — 37.0 % and 40.7 % of total number of males, respectively.

Species-specific vulnerability to angling and its size-selectivity in sympatric stream salmonids

In mixed fisheries where multiple species are caught, to manage resources sustainably, knowledge about the species-specific vulnerability to fishing is equally or even more important than knowledge of size selectivity of the gear. We compared the vulnerability to bait recreational angling in four salmonid species in natural streams in Japan. The ranking of species-specific angling vulnerability was as follows (from highest to lowest): rainbow trout (Oncorhynchus mykiss), masu salmon (Oncorhynchus masou), white-spotted char (Salvelinus leucomaenis), and southern Asian Dolly Varden (Salvelinus curilus). In all species, larger individuals were more vulnerable to angling, but there were differences in the size dependence between species. In rainbow trout and Dolly Varden (which have a nonanadromous life history in the study area), the probability of being caught monotonically increased with body size, while the vulnerability to angling in masu salmon and white-spotted char (which have an anadromous life history in the study area) showed a domed-shaped pattern. We found that across the species the catch per unit effort showed a hyperstable relationship with population density. Therefore, diminishing local populations are prone to collapse, and this collapse would be hard to foresee based on catch rate data alone.

Comparing redd distributions of chum and masu salmon in the Kotonihassam River, Hokkaido.

Comparing redd distributions of chum and masu salmon in the Kotonihassam River, Hokkaido.

Eco-evolutionary dynamics may show an irreversible regime shift, illustrated by salmonids facing climate change

The enhanced or reduced growth of juvenile masu salmon (Oncorhynchus masou masou) may result from climate changes to their environment and thus impact on the eco-evolutionary dynamics of their life-history choices. Male juveniles with status, i.e., if their body size is larger than a threshold, stay in the stream and become resident males reproducing for multiple years, while those with smaller status, i.e., their body size is below the threshold, migrate to the ocean and return to the stream one year later to reproduce only once. Since juvenile growth is suppressed by the density of resident males, the fraction of resident males may stay in equilibrium or fluctuate wildly over a 2-year period. When the threshold value evolves, the convergence stable strategy may generate either an equilibrium or large fluctuations of male residents. If environmental changes occur faster than the rate of evolutionary adaptation, the eco-evolutionary dynamics exhibit a qualitative shift in the population dynamics. We also investigated the relative assessment models, in which individual life-history choices are made based on the individual’s relative status within the juvenile population. The eco-evolutionary dynamics are very different from the absolute assessment model, demonstrating the importance of understanding the mechanisms of life history choices when predicting the impacts of climate change.

Artificially increased habitat complexity reduces predation-induced mortality for juvenile salmon in simplified environments

Many river systems have been artificially modified for various reasons worldwide. Although simplified riverbeds due to artificial modifications often alter the species compositions and interactions, methods for mitigating habitat degradation in such rivers are underdeveloped. In this study, we evaluated the effects of tree branches on the survival of juvenile masu salmon Oncorhynchus masou under predation risk by piscivorous fish (White-spotted charr Salvelinus leucomaenis) in artificially simplified environments. We found that 0.4 m2 or more of tree branches per 1.0 m2 of artificial concrete pond doubled the survival of juvenile salmon for 20 days. In addition, body sizes of predator and prey did not have a significant effect on salmon survival, whereas the number of branches did in this experiment. These results indicate that adding structural complexity can mitigate predation pressure by piscivorous fish in artificially simplified environments. This study provides important guidelines for the management and restoration of suitable habitats for stream salmonids in artificially simplified environments where the main predators are piscivorous fish.

Comparing impacts of metal contamination on macroinvertebrate and fish assemblages in a northern Japanese river.

Researchers have long assessed the ecological impacts of metals in running waters, but few such studies investigated multiple biological groups. Our goals in this study were to assess the ecological impacts of metal contamination on macroinvertebrates and fishes in a northern Japanese river receiving treated mine discharge and to evaluate whether there was any difference between the metrics based on macroinvertebrates and those based on fishes in assessing these impacts. Macroinvertebrate communities and fish populations were little affected at the downstream contaminated sites where concentrations of Cu, Zn, Pb, and Cd were 0.1–1.5 times higher than water-quality criteria established by the U.S. Environmental Protection Agency. We detected a significant reduction in a few macroinvertebrate metrics such as mayfly abundance and the abundance of heptageniid mayflies at the two most upstream contaminated sites with metal concentrations 0.8–3.7 times higher than the water-quality criteria. There were, however, no remarkable effects on the abundance or condition factor of the four dominant fishes, including masu salmon (Oncorhynchus masou). These results suggest that the richness and abundance of macroinvertebrates are more sensitive to metal contamination than abundance and condition factor of fishes in the studied river. Because the sensitivity to metal contamination can depend on the biological metrics used, and fish-based metrics in this study were limited, it would be valuable to accumulate empirical evidence for ecological indicators sensitive to metal contamination within and among biological groups to help in choosing which groups to survey for general environmental impact assessments in metal-contaminated rivers.

Comparative Analysis of Nutritional Components in Muscles between Wild and Cultured Masu Salmon Oncorhynchus masou

Comparative Analysis of Nutritional Components in Muscles between Wild and Cultured Masu Salmon Oncorhynchus masou

Small biased body size of salmon fry preyed upon by piscivorous fish in riverine and marine habitats

Predator–prey interactions must be considered when aiming to enhance populations by releasing artificially reared individuals into natural environments. Released individuals create an abundance of prey for predators, and a basic hypothesis of predator–prey interactions suggests that predators select large prey due to the high caloric content. An alternative hypothesis is that small individuals are vulnerable to predation due to their poor predator avoidance. This study tested these hypotheses using stocked chum (Oncorhynchus keta) and masu (Oncorhynchus masou) salmon fry and piscivorous salmonids in marine and riverine habitats in Hokkaido, Japan. Stomach contents were sampled from predators, and fork length of prey fry was measured. Then, their fork length was compared with whole stocked fry (range of mean fork length (±SD): 45.8 ± 2.55 to 49.2 ± 2.76 mm) for each habitat. As a result, prey fry were ∼3%–6% smaller than whole stocked fry, even under a prey-abundant condition (i.e., just after hatchery reared salmon fry were stocked). Piscivorous salmonids pursue schooling fry, and small fry may be easily caught due to their slow speed in avoiding predators.

Effect of parental phenotype on dispersal, growth and maturation of offspring in wild masu salmon (Oncorhynchus masou)

Offspring traits are influenced by complex interactions among parent genotypes and phenotypes. However, ecological studies of these effects have tended to focus on one of either maternal or paternal influences. In this study, we investigated the effects of both parental phenotypes on offspring dispersal, growth and early sexual maturity in masu salmon. We used wild-caught parental fish in a half-sib mating design that allowed comparison of offspring from males of two different life-history types (precocious males that had lived only in fresh water and anadromous males that had migrated to the sea). These males were mated with anadromous females and the eggs planted in natural streams in three different years. Natal dispersal distance depended on offspring sex, body size and paternal life history: female offspring moved further downstream than males, whilst daughters of precocious fathers tended to move further downstream than those of anadromous sires. There was a maternal effect on offspring growth, with larger eggs resulting in larger offspring at least until these became precociously mature at the end of the first summer. However, while faster growing male offspring were more likely to become precociously mature, there was no evidence that the probability of precocious maturation was influenced by parental life history. We conclude that, although body size of young salmon was significantly influenced by egg size and there were sex differences in dispersal, their growth rate and divergence in life history types were mainly environmentally driven.

РАЗМЕРНО-ВЕСОВАЯ СТРУКТУРА И ПЛОДОВИТОСТЬ АНАДРОМНОЙ СИМЫ (ONCORHYNCHUS MASOU) НА ЗАПАДНОЙ КАМЧАТКЕ

Masu salmon Oncorhynchus masou is the most thermophilic species among the Pacific salmon, and Kamchat-ka peninsula is the northern border of its distribution. Due to small abundance, the biology of this species on the peninsula is poorly studied. Regular scientific researches of Masu salmon in Kamchatka have been con-ducted since 2009. The length-weight structures and fecundity of mature Masu salmon have been analyzed in several streams on western Kamchatka over a 10-year period (from 2009 to 2018) and present time. It has been established that the length, weight and fecundity of mature fish are different in the rivers of the peninsula. A tendency of increasing the biological characteristic of the species in recent years has been noted.

Captive-bred populations of a partially migratory salmonid fish are unlikely to maintain migratory polymorphism in natural habitats.

Variation in life history is fundamental to the long-term persistence of populations and species. Partial migration, in which both migratory and resident individuals are maintained in a population, is commonly found across animal taxa. However, human-induced habitat fragmentation continues to cause a rapid decline in the migratory phenotype in many natural populations. Using field and hatchery experiments, we demonstrated that despite both migrants and residents being maintained in captive environments, few individuals of the red-spotted masu salmon, Oncorhynchus masou ishikawae, became migrants in natural streams when released prior to the migration decision. Released fish rarely reached the threshold body size necessary to become migrants in natural streams, presumably owing to lower growth rates in natural than in captive environments. The decision to migrate is often considered a threshold trait in salmonids and other animal taxa. Our findings highlight the need for management programmes that acknowledge the effects of the environment on the determination of the migratory phenotypes of partially migratory species when releasing captive-bred individuals prior to their migratory decisions.

Hydrogen Sulfide Modulates Adult and Reparative Neurogenesis in the Cerebellum of Juvenile Masu Salmon, Oncorhynchus masou.

Fish are a convenient model for the study of reparative and post-traumatic processes of central nervous system (CNS) recovery, because the formation of new cells in their CNS continues throughout life. After a traumatic injury to the cerebellum of juvenile masu salmon, Oncorhynchus masou, the cell composition of the neurogenic zones containing neural stem cells (NSCs)/neural progenitor cells (NPCs) in the acute period (two days post-injury) changes. The presence of neuroepithelial (NE) and radial glial (RG) neuronal precursors located in the dorsal, lateral, and basal zones of the cerebellar body was shown by the immunohistochemical (IHC) labeling of glutamine synthetase (GS). Progenitors of both types are sources of neurons in the cerebellum of juvenile O. masou during constitutive growth, thus, playing an important role in CNS homeostasis and neuronal plasticity during ontogenesis. Precursors with the RG phenotype were found in the same regions of the molecular layer as part of heterogeneous constitutive neurogenic niches. The presence of neuroepithelial and radial glia GS+ cells indicates a certain proportion of embryonic and adult progenitors and, obviously, different contributions of these cells to constitutive and reparative neurogenesis in the acute post-traumatic period. Expression of nestin and vimentin was revealed in neuroepithelial cerebellar progenitors of juvenile O. masou. Patterns of granular expression of these markers were found in neurogenic niches and adjacent areas, which probably indicates the neurotrophic and proneurogenic effects of vimentin and nestin in constitutive and post-traumatic neurogenesis and a high level of constructive metabolism. No expression of vimentin and nestin was detected in the cerebellar RG of juvenile O. masou. Thus, the molecular markers of NSCs/NPCs in the cerebellum of juvenile O. masou are as follows: vimentin, nestin, and glutamine synthetase label NE cells in intact animals and in the post-traumatic period, while GS expression is present in the RG of intact animals and decreases in the acute post-traumatic period. A study of distribution of cystathionine β-synthase (CBS) in the cerebellum of intact young O. masou showed the expression of the marker mainly in type 1 cells, corresponding to NSCs/NCPs for other molecular markers. In the post-traumatic period, the number of CBS+ cells sharply increased, which indicates the involvement of H2S in the post-traumatic response. Induction of CBS in type 3 cells indicates the involvement of H2S in the metabolism of extracellular glutamate in the cerebellum, a decrease in the production of reactive oxygen species, and also arrest of the oxidative stress development, a weakening of the toxic effects of glutamate, and a reduction in excitotoxicity. The obtained results allow us to consider H2S as a biologically active substance, the numerous known effects of which can be supplemented by participation in the processes of constitutive neurogenesis and neuronal regeneration.

Production of juvenile masu salmon (Oncorhynchus masou) from spermatogonia-derived sperm and oogonia-derived eggs via intraperitoneal transplantation of immature germ cells

No effective cryopreservation technique exists for fish eggs and embryos; thus, the cryopreservation of germ cells (spermatogonia or oogonia) and subsequent generation of eggs and sperm would be an alternative solution for the long-term preservation of piscine genetic resources. Nevertheless, in our previous study using rainbow trout, we showed that recipients transplanted with XY spermatogonia or XX oogonia produced unnatural sex-biased F1 offspring. To overcome these obstacles, we transplanted immature germ cells (XX oogonia or XY spermatogonia; frozen for 33 days) into the body cavities of triploid hatchlings, and the transplanted germ cells possessed a high capacity for differentiating into eggs and sperm in the ovaries and testes of recipients. Approximately 30% of triploid recipients receiving frozen germ cells generated normal salmon that displayed the donor-derived black body color phenotype, although all triploid salmon not receiving transplants were functionally sterile. Furthermore, F1 offspring obtained from insemination of the oogonia-derived eggs and spermatogonia-derived sperm show a normal sex ratio of 1:1 (female:male). Thus, this method presented a critical technique for practical conservation projects for other teleost fish species and masu salmon.

Links between paternal life history and offspring metabolic rate and body size during early life in masu salmon Oncorhynchus masou

AbstractOffspring growth is greatly influenced by both the maternal and paternal genotypes. However, although the effects of maternal genotype on offspring growth are recognised widely, ecological studies of the paternal genotype have not focused on the links with physiological traits, especially at the embryo stage. In this study, we conducted three experiments to determine the effects of two contrasting paternal life history traits (anadromous males, which return to their natal rivers for spawning after migrating to the sea, and resident males, which mature in rivers without migrating to the sea) on offspring growth during early life based on the metabolic rate in masu salmon Oncorhynchus masou. In the first experiment, metabolic rates of fertilised eggs were compared between 24 half‐sibships. Results suggested that the metabolic rate of embryos was higher for the offspring of resident males than of anadromous males. In the second experiment, the body sizes of 133 half‐sibships at the yolk‐sac absorption stage were compared. The offspring from the resident males were larger, which could be related to metabolic rate differences at the embryo stage associated with paternal life history. In the third experiment, the differences in body size observed at the yolk‐sac absorption stage were positively correlated with growth after 60 days of fry rearing. Our findings, taken together with previous demonstrations that faster‐growing males tend to become younger resident males, are thus consistent with early growth and associated traits, such as metabolic rate, being heritable from the male parent to the offspring.