Effects of melatonin feeding on smoltification in masu salmon (Oncorhynchus masou)

Influences of photoperiod on plasma melatonin profiles and effects of melatonin administration on long-day-induced smoltification in masu salmon (Oncorhynchus masou) were investigated in order to reveal the roles of melatonin in the regulation of smoltification in salmonids. Under light-dark (LD) cycles, plasma melatonin levels exhibited daily variation, with higher values during the dark phase than during the light phase. The duration of nocturnal elevation under short photoperiod (LD 8:16) was longer than that under long photoperiod (LD 16:8). Melatonin feeding (0.01, 0.1 and 1 mg/kg body weight) elevated plasma levels of melatonin in a dose-dependent manner for at least 7 h but not for 24 h. When masu salmon reared under short photoperiod were exposed to long photoperiod (LD 16:8) and fed melatonin (1 mg/kg body weight) 7 hours before the onset of darkness, a significantly smaller proportion of smolts appeared in the melatonin-fed group after 32 days than in the control group. However, after 59 days of the treatment, there was no difference in the proportion of smolts between the control and melatonin-treated groups. Thus, melatonin feeding mimicked the effects of short photoperiod, which delays but does not completely suppress smoltification. These results indicate that the day length is transduced into changes in the duration of nocturnal elevation in plasma melatonin levels, and that artificial modification of the plasma melatonin pattern possibly delays the physiological processes of smoltification induced by long-day photoperiodic treatment.     

Dispersal of wild and domestic masu salmon fry (Oncorhynchus masou) in an artificial channel

The day and night pattern of upstream and downstream dispersal of masu salmon fry of wild and domestic origin was compared in artificial channels (45 m long), for two ages of planting: unfed alevins and eyed eggs. Early dispersal was important for the wild stock (48–50%) compared with the domestic one (16–36%). More wild fry moved downstream than upstream, and more domestic fry dispersed upstream. Upstream movement in wild and domestic fry was more active by day than by night, except for wild fry planted as eyed eggs, where upstream migration was higher at night. In contrast, downstream movement in wild and domestic fry was more common by night than by day, but daylight catches were not negligible for the wild stock.     

Serum immunoglobulin M (IgM) during early development of masu salmon (Oncorhynchus masou).

1. The development of serum immunoglobulin M (IgM) was studied by measuring IgM concentration in early developmental stages of masu salmon (Oncorhynchus masou) using single radial immunodiffusion. 2. IgM concentration was measurable from 88 days after hatching (46.5 micrograms/ml) and was maintained at a relatively constant level (less than 100 micrograms/ml) until 235 days after hatching. 3. IgM concentration increased significantly to a level of 691 +/- 37 micrograms/ml (mean +/- SE, N = 59) during the period from 251 to 429 days after hatching. The highest level of serum IgM was maintained from 429 days to 489 days after hatching. 4. The increase of IgM concentration was not accompanied by parallel increases in total serum protein concentration.     

Ultrastructure of the retinal pigment epithelium in the young masu salmon Oncorhynchus masou

The ultrastructure of retinal pigment epithelium (RPE) cells in the masu salmon Oncorhynchus masou fry was studied. The physiological state of the pigment was dependent on the level of light/dark adaptation. Similar morphological properties were observed in the response of the RPE cells to light and a magnetic field.     

Henneguya cartilaginis n. sp. (Myxozoa: Myxosporea) in the head cartilage of masu salmon Oncorhynchus masou masou

Henneguya cartilaginis n. sp. (Myxozoa: Myxosporea) is described from wild masu salmon Oncorhynchus masou masou in Hokkaido, Japan. H. cartilaginis n. sp. produced white cysts, up to 3mm in size, in the head of masu salmon. Infected fish exhibited cranial protrusion due to the cysts. Spores (11.4 × 8.6μm) of H. cartilaginis n. sp. were egg-shaped with the posterior end more pointed and possessed two caudal appendages (34.2μm average length). Histological observations revealed that large plasmodia possessing fine fibrous pseudopodia on the surface developed in the head cartilage. H. cartilaginis n. sp. resembles H. cerebralis, which was described from the cranial cartilage of Kosogol grayling Thymallus nigrescens in Mongolia. However, they were distinguishable by spore morphology. Molecular analysis of the 18S rDNA sequences indicated that H. cartilaginis n. sp. was most closely related to Henneguya zschokkei, H. nuesslini and H. salminicola of salmonid fish, with genetic similarities of 95.3%, 95.1% and 93.9%, respectively. Based on these differences in spore morphology, molecular data, the site of infection and geographical distribution, the present species is considered to be a new species.     

Interpopulation variation in reproductive traits of female masu salmon, Oncorhynchus masou

Several life history models predict that larger eggs and lower fecundity should be favored in a low-growth environment. We applied the model of Sibly et al. to seven Japanese populations of masu salmon ( Oncorhynchus masou ) in order to test the predictions that populations in which individual growth rate is low are characterized by larger eggs and lower fecundity. Two populations, Shumarinai and Shikaribetsu, had the lowest growth rates of individuals, largest egg sizes and lowest fecundities of all populations examined. In contrast, the Shiribetsu, Chitose and Uono populations had highest growth rates of individuals, smallest egg sizes and highest fecundities. The Shibetsu and Toya populations were intermediate between these two groups. The gonadsomatic index (GSI) of the Shumarinai and Shikaribetsu populations was smaller than that of all other populations. Correlation analysis indicated that populations with lower individual growth rates had larger eggs and lower fecundities. The results were consistent with the predictions of the Sibly et al. model: increased egg size which results in decreased fecundity is probably an adaptation to low-growth environments. Therefore, in masu salmon, growth differences among populations may explain interpopulation variation in egg size and fecundity.     

Latitudinal variation in egg size and number in anadromous masu salmon Oncorhynchus masou

Latitudinal variation in egg size and number in anadromous masu salmon Oncorhynchus masou was examined. Relatively greater variation in egg size occurred among rivers than among females within rivers or within females. Egg size was generally greater and egg number generally lower at more northerly latitudes.     

Ultrastructural study of spermatogenesis in silver salmon Oncorhynchus kisutch and cherry salmon O. masou (Salmonidae, Teleostei)

Spermatogenesis in silver salmon Oncorhynchus kisutch and cherry salmon O. masou was studied by transmission electron microscopy. The spermatogenic cells of both species have been shown to have no considerable differences in their dimensional and ultrastructural parameters. The characteristic peculiarity of spermatogonia was the presence of a germ determinant substance, which is the cytoplasmic marker of sex line cells. The primary and secondary spermatocytes, as well as early spermatids, were arranged in gonads as cell clusters at the same stage of development. In spermiogenesis, the regular appearance of the electron-lucent vesicle was observed, which forms a structural complex with the apical condensation of the nuclear membrane. This complex is morphologically similar to the not-yet-formed spermatid acrosomes in many Metazoa and is referred to by the authors as an acrosome-like structure (AS). At the final stage of spermiogenesis, the disappearance of an AS allows one to considering it to be a temporarily existing structure that recapitulates the ancestral morphotype of salmonid sperm. Spermatozoa of the both species are typical, primitive, acrosome-lacking cells, which are characteristic of many representatives of Teleostei that have external insemination.     

Ascending speed and nocturnal activity of adult masu salmon (Oncorhynchus masou Brevoort) during upstream migration

We released five adult masu salmon (Oncorhynchus masou) tagged with external transmitters to track their ascending behaviour. The signals of all specimens were recorded in the upper area of the river system. Two patterns of ascending behaviour were recognized: ascending upward immediately after release and ascending during increased river discharge. The fastest ascending speed was about 1000 m h^–1. Active movements were detected at night. The signal recording duration at each receiver for each fish was generally brief. Most fish did not stay at the pools where the receivers were installed.     

Purification of serum precursor proteins to vitelline envelope (choriogenins) in masu salmon,Oncorhynchus masou

Three vitelline envelope-related proteins (VERPs), very-high-molecular-weight VERP (vhVERP), high-molecular-weight VERP (hVERP) and low-molecular-weight VERP (lVERP) were purified from female masu salmon serum. The apparent molecular weights of vhVERP, hVERP and lVERP, in their native state, were 520, 88 and 54 kDa, respectively, by gel-filtration chromatography. Very-high-molecular-weight VERP comprises two subunits, corresponding to 175 and 126 kDa. On SDS-PAGE, hVERP and lVERP migrate at 53 and 47 kDa, respectively. Amino acid analysis of vhVERP and hVERP showed that they share a high content of glutamic acid and proline. By contrast, lVERP is rich in glutamic acid and asparatic acid. These features are in good agreement with the amino acid composition of the vitelline envelope. Immuno-biochemical analysis suggested that vhVERP is derived from hVERP by polymerization and/or aggregation. Antibodies against hVERP and lVERP specifically immunostained the vitelline envelope and liver of female masu salmon. In addition, both hVERP and lVERP were induced in the serum of estrogen-treated male fish. Taken together, it is suggested that hVERP and lVERP are homologous molecules with choriogenin H and choriogenin L in medaka, respectively. These results indicate that hVERP and lVERP are precursor proteins to the vitelline envelope (choriogenins) in masu salmon.     

A visceral mycosis in cultured masu salmon (Oncorhynchus masou) caused by a species of Ochroconis

Ochronconis sp. infection was found in masu salmon (Oncorhynchus masou) with visceral mycosis in Japan. The external and internal clinical signs were reddening of the anal area, swelling of the abdomen due to accumulation of ascitic fluid in the abdominal cavity and extensive swelling of the posterior kidney. Many pale brown, septate hyphae were found in the kidney by direct microscopical examination; these were usually not found in the other organs of infected fish. Histopathological examination of the kidney revealed large granulomas with the fungal hyphae and giant cells. The isolated fungus was identified as a species of the genus Ochroconis and was compared with O. tshawytschae, a known fish pathogen. Based on morphological and growth characteristics, we believe that these cases resulted from infection with a different species.     

Genetic diversity and differentiation of masu salmon (Oncorhynchus masou masou) between and within cultured populations inferred from microsatellite DNA analysis

Masu salmon, Oncorhynchus masou masou, is one of the most valuable fishery species that has been introduced to China, though to date no studies on the genetic diversity and genetic relationship among hatchery populations has been performed with molecular markers. We undertook such a study and sampled 120 individuals from three hatchery stocks and analyzed 20 microsatellite loci. All loci were polymorphic and a total of 91 alleles were detected. A relatively low level of genetic diversity was revealed with effective number of allele of 3.1094, 3.3299 and 3.1894 and expected heterozygosity of 0.6600, 0.6648 and 0.6638 in the three stocks, respectively. Deviations from Hardy-Weinberg equilibrium were found due to heterozygote deficit. Accordingly, evidence of genetic bottlenecks were found in the three stocks. An individual assignment test demonstrated that 85% of individuals were correctly assigned into their original stocks. Pairwise Fst revealed that significant differentiation occurred between these three stocks. The results of the study indicated that disequilibrium of genetic structure and differentiation has occurred in all three stocks. This information collectively provides a basis for measures to avoid of loss of genetic diversity and introgression in Chinese aquaculture.     

New polymorphic microsatellite markers for the masu salmon (Oncorhynchus masou masou) from Korea and their application to wild and hatchery populations

Masu salmon, Oncorhynchus masou masou, is an economically important fish species in the Far East and occurs in two life history forms: sea-run migratory (anadromous) and freshwater resident (non-anadromous). The non-anadromous form has recently become a popular freshwater food and game fish during a well-known Korean winter festival. However, the genetic background of this species remains largely unknown, partly due to a lack of molecular genetic markers. In this study, we developed new polymorphic microsatellite markers for masu salmon using next-generation sequencing technology. From 40 primer sets, 11 primer sets (27.5% of the primer sets selected) were successfully amplified with 106 alleles (range 2–9) in 64 individuals from different populations: two wild and one hatchery. Observed and expected heterozygosities ranged from 0.304 to 0.947 and 0.278 to 0.865, respectively. Significant departures from the Hardy–Weinberg equilibrium were detected for four markers (OMM11, OMM17, OMM28, and OMM33) in a single population. All pair-wise F_ST values were highly significant between the wild and hatchery populations (range 0.084–0.183, P < 0.0001). We identified a set of robust microsatellite markers that worked well even in formalin-fixed samples, which will be suitable for biogeographical and population structure analyses of the masu salmon.     

Genetic diversity and differentiation of masu salmon (Oncorhynchus masou masou) between and within cultured populations inferred from microsatellite DNA analysis

Masu salmon,Oncorhynchus masou masou,is one of the most valuable fishery species that has been introduced to China,though to date no studies on the genetic diversity and genetic relationship among hatchery populations has been performed with molecular markers.We undertook such a study and sampled 120 individuals from three hatchery stocks and analyzed 20 microsatellite loci.All loci were polymorphic and a total of 91 alleles were detected.A relatively low level of genetic diversity was revealed with effective number of allele of 3.1094,3.3299 and 3.1894 and expected heterozygosity of 0.6600,0.6648 and 0.6638 in the three stocks,respectively.Deviations from Hardy-Weinberg equilibrium were found due to heterozygote deficit.Accordingly,evidence of genetic bottlenecks were found in the three stocks.An individual assignment test demonstrated that 85% of individuals were correctly assigned into their original stocks.Pairwise Fst revealed that significant differentiation occurred between these three stocks.The results of the study indicated that disequilibrium of genetic structure and differentiation has occurred in all three stocks.This information collectively provides a basis for measures to avoid of loss of genetic diversity and introgression in Chinese aquaculture.     

Latitudinal variation in sexual size dimorphism of sea-run masu salmon, Oncorhynchus masou

Sexual size dimorphism (SSD), a difference in body size between the sexes, occurs in many animal species. Although the larger sex is often considered invariable within species, patterns of selection may result in interpopulation variation or even reversal of SSD. We evaluated correlations between latitude and female body size, male body size, and relative body size (male body size/female body size) in 22 populations (ranging from 37 degrees N to 49 degrees N) of sea-run masu salmon (Oncorhynchus masou) that spawn in rivers along the Sea of Japan coast. Male size and the relative body size increased with latitude, but female size did not correlate with latitude. In addition, increase in male size with latitude was sufficient to result in a reversal of SSD, the switch-point being around 45 degrees N. We suggest that the positive correlation between latitude and male size is due to increasing operational sex ratios or sexual selection on sea-run male body size that result from sex-biased patterns of anadromy. In conclusion, our study provides the first example of predictable geographic variation in SSD shaped by apparent patterns of sexual selection.     

Molecular cloning and characterization of three distinct choriogenins in masu salmon, Oncorhynchus masou

Three cDNAs, each encoding a different choriogenin (Chg), were isolated from a female masu salmon (Oncorhynchus masou) liver cDNA library. Two of the cDNA clones, Chg Halpha and Chg Hbeta, showed a close relationship and contained the typical domains of zona pellucida (ZP) B genes in fish, namely proline and glutamine rich repeats, a trefoil factor family domain, and a ZP domain. Specific antibodies against recombinant Chg H products (rmHalpha and rmHbeta) were generated to elucidate the relationship between the Chg H cDNAs and two types of serum Chg H protein, which were previously purified and characterized, and designated as very-high-molecular-weight vitelline envelope-related protein (vhVERP) and Chg H of masu salmon. The immunobiochemical analyses revealed that the Chg Halpha and Chg Hbeta clones encoded vhVERP and Chg H proteins, respectively. The third cDNA clone (Chg L) appeared to be a ZPC gene and, by mapping the N-terminal sequence of purified Chg L, was shown to encode serum Chg L protein. Various types of heteromultimer of the three Chgs were identified immunologically as high molecular weight chorion components, indicating the involvement of complex heterodimerization of multiple Chgs in the construction of chorion architecture in masu salmon.     

Functional analysis of thermo-sensitive TRPV1 in an aquatic vertebrate,masu salmon (Oncorhynchus masou ishikawae)

Transient receptor potential vanilloid 1 (TRPV1) is mainly expressed in nociceptive primary sensory neurons and acts as a sensor for heat and capsaicin. The functional properties of TRPV1 have been reported to vary among species and, in some cases, the species difference in its thermal sensitivity is likely to be associated with thermal habitat conditions. To clarify the functional properties and physiological roles of TRPV1 in aquatic vertebrates, we examined the temperature and chemical sensitivities of TRPV1 in masu salmon (Oncorhynchus masou ishikawae, Om) belonging to a family of salmonids that generally prefer cool environments. First, behavioral experiments were conducted using a video tracking system. Application of capsaicin, a TRPV1 agonist, induced locomotor activities in juvenile Om. Increasing the ambient temperature also elicited locomotor activity potentiated by capsaicin. RT-PCR revealed TRPV1 expression in gills as well as spinal cord. Next, electrophysiological analyses of OmTRPV1 were performed using a two-electrode voltage-clamp technique with a Xenopus oocyte expression system. Heat stimulation evoked an inward current in heterologously expressed OmTRPV1. In addition, capsaicin produced current responses in OmTRPV1-expressing oocytes, but higher concentrations were needed for its activation compared to the mammalian orthologues. These results indicate that Om senses environmental stimuli (heat and capsaicin) through the activation of TRPV1, and this channel may play important roles in avoiding environments disadvantageous for survival in aquatic vertebrates.