Yasunori Sakurai

Genetic population structure of Nibea albiflora in Yellow Sea and East China Sea

The population genetic structure and level of gene flow of Nibea albiflora from the Yellow Sea and the East China Sea were examined with a 479-bp segment of a mtDNA control region. In total, 65 samples were collected from three locations and 37 haplotypes were obtained. Mean haplotype diversity and nucleotide diversity for the three populations ranged from 0.9130±0.0308 (Zhoushan) to 0.9926±0.0230 (Xiamen), and from 0.0073±0.0043 (Qingdao) to 0.0099±0.0057 (Xiamen). Analysis of molecular variance and pairwise FST revealed little genetic structure between the Yellow Sea and the East China Sea in N. albiflora. But based on the exact test of differentiation, the null hypothesis that N. albiflora within the Yellow Sea and the, East China Sea constitutes a panmictic mtDNA gene pool was rejected. This might be caused by the broad spawning areas but not by the Yangtze River outflow. Mismatch distribution revealed that N. albiflora has undergone population expansion, possibly before the last 85000–170000 years. The existence of high gene flow between stocks in the studied area was supported by our results. Annual migrations, larval drift in the ocean currents, and recent range expansion could be the reasons for little genetic structure in the studied area.

Deep phylogeographic break among white croaker Pennahia argentata (Sciaenidae, Perciformes) populations in North-western Pacific

The Quaternary cold periods in the North-western Pacific are thought to have greatly influenced the genetic structures of marine species. One hundred and thirty-two individuals of white croaker Pennahia argentata were sampled from 12 localities throughout its distribution range to estimate the demographic history and genetic structure based on mitochondrial DNA cytochrome b gene and control region sequences. Two distinct clades with net genetic divergence of 3% in the control region were detected, one in Chinese coastal waters and the other in Japanese coastal waters. These clades may have been isolated and diverged during Pleistocene low sea levels. Nucleotide diversity was much higher in the Chinese clade than in the Japanese clade. The demographic history of the two clades was examined using neutrality tests and mismatch analyses, and the results indicated Pleistocene population expansion in both clades. Molecular variance and pairwise FST analyses revealed significant differentiation between two Japanese populations and lack of genetic structure in the Chinese populations. The significant geographic structure in white croaker suggests a low level of dispersal in this species. The lack of phylogeographic structure in Chinese coastal waters may reflect a recent range expansion after the last glacial maximum and insufficient time to attain migration-drift equilibrium.

Fertilizing capacity of squid (Todarodes pacificus) spermatozoa collected from various sperm storage sites, with special reference to the role of gelatinous substance from oviducal gland in fertilization and embryonic development

Summary Fertilizing capacity of spermatozoa collected from various storage sites in the male and female squids were examined by inseminated eggs in the presence or absence of gelatinous substances of the oviducal gland origin. Over 90% of the eggs were fertilized by the spermatozoa collected from the females seminal receptacle regardless of the presence or absence of the gelatinous substance. Elevation of the chorion and the formation of perivitelline space was observed only in the jellied eggs. All jellyless eggs without perivitelline space died in 4 days after insemination. Spermatozoa collected from the spermatophore in the males accessory gland could fertilize the majority of the eggs. On the other hand, those from the males vas deferens could fertilize only 30–50% of both the jellied and jellyless eggs. It may be concluded from these experiments that the gelatinous substance of the oviducal gland origin has nothing to do with fertilization. However, it is essential for the elevation of the chorion...

Remarkably low mtDNA control-region diversity and shallow population structure in Pacific cod Gadus macrocephalus

To investigate the genetic diversity and describe the population structure in Gadus macrocephalus, a 452 base pair (bp) fragment of the mitochondrial DNA control region was analysed in 259 individuals. The results showed remarkably low nucleotide diversity and a lack of genealogical structure. Small but significant genetic differentiations, however, were detected among north-western Pacific populations, but no large-scale regional differences were detected. These results indicate that populations of G. macrocephalus in the north-western Pacific are genetically subdivided and represent evolutionary lineages that should be managed individually.

Transport of the ommastrephid squid Todarodes pacificus under cold-water anesthesia

Abstract We describe a method for transporting live squid, Todarodes pacificus (Cephalopoda: Ommastrephidae), using cold-water anesthesia. Squid were placed individually in 30-l plastic bags with 5 l of 0–1°C seawater and 10–15 l of oxygen gas. Thirteen of 14 anesthetized squid survived 6.5 h transport by car and airplane with no signs of distress or shock. The longest survival under anesthesia was 10 h.

What maintains the central North Pacific genetic discontinuity in Pacific herring

Pacific herring show an abrupt genetic discontinuity in the central North Pacific that represents secondary contact between refuge populations previously isolated during Pleistocene glaciations. Paradoxically, high levels of gene flow produce genetic homogeneity among ocean-type populations within each group. Here, we surveyed variability in mtDNA control-region sequences (463 bp) and nine microsatellite loci in Pacific herring from sites across the North Pacific to further explore the nature of the genetic discontinuity around the Alaska Peninsula. Consistent with previous studies, little divergence (ΦST  = 0.011) was detected between ocean-type populations of Pacific herring in the North West Pacific, except for a population in the Yellow Sea (ΦST  = 0.065). A moderate reduction in genetic diversity for both mtDNA and microsatellites in the Yellow Sea likely reflects founder effects during the last colonization of this sea. Reciprocal monophyly between divergent mtDNA lineages (ΦST  = 0.391) across the Alaska Peninsula defines the discontinuity across the North Pacific. However, microsatellites did not show a strong break, as eastern Bering Sea (EBS) herring were more closely related to NE Pacific than to NW Pacific herring. This discordance between mtDNA and microsatellites may be due to microsatellite allelic convergence or to sex-biased dispersal across the secondary contact zone. The sharp discontinuity between Pacific herring populations may be maintained by high-density blocking, competitive exclusion or hybrid inferiority.

Feeding habits of the mesopelagic fish Gonostoma gracile in the Northwestern North Pacific

The diet of Gonostoma gracile, a numerically abundant mesopelagic fish in the Subtropical Region and the Transition Domain of the northwestern North Pacific, was examined using 520 specimens collected during June–July 1988, June 1995 and November 1995. The prey included mainly copepods, ostracods, amphipods and euphausiids. Copepods and ostracods were the most abundant, comprising approximately 70% of the total diet. There was little evidence of an ontogenetic dietary shift; Pleuromamma copepods were the most abundant prey for all size classes of fish ranging from 19 to 116 mm in standard length. The size range of prey increased with growth, but all fish sizes examined fed mainly on 1–4 mm long prey. Luminescent copepods and ostracods were the most abundant prey, suggesting that G. gracile detects its prey visually.

Observations on the spawning behavior, egg masses and paralarval development of the ommastrephid squid Todarodes pacificus in a laboratory mesocosm.

ABSTRACT The spawning behavior of ommastrephid squids has never been observed under natural conditions. Previous laboratory observations of Japanese flying squid (Todarodes pacificus) suggest that pre-spawning females might rest on the continental shelf or slope before they ascend above the pycnocline to spawn, and that the egg masses might settle in the pycnocline. Here, two mesocosm experiments were conducted in a 300 m3 tank that was 6 m deep to investigate this hypothesis. In the first experiment, a thermocline (2.5–3.5 m) was established in the tank by creating a thermally stratified (17–22°C) water column. In the second experiment, the temperature was uniform (22°C) at all depths. Prior to spawning, females did not rest on the tank floor. In the stratified water column, egg masses remained suspended in the thermocline, but in an unstratified water column, they settled on the tank bottom, collapsed and were infested by microbes, resulting in abnormal or nonviable embryos. Eleven females spawned a total of 18 egg masses (17–80 cm in diameter), indicating that females can spawn more than once when under stress. Paralarvae hatched at stage 30/31 and survived for up to 10 days, allowing us to observe the most advanced stage of paralarvae in captivity. Paralarvae survived after consumption of the inner yolk, suggesting they might have fed in the tank. Highlighted Article: Mesocosm observations on the reproduction of the squid Todarodes pacificus reveal the animals spawning behavior, egg mass properties and paralarval development in a simulated natural environment.

Lipase from neon flying squid hepatopancreas: purification and properties

Lipase from hepatopancreas of the neon flying squid (Ommastrephes bartramii) has been partially purified. The molecular weight determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 33 000. Optimum pH was around 7.0 and the enzyme was relatively stable between pH 6.0 and 9.0 for 6 h at 25 °C. Optimum temperature of the enzyme reaction was around 25 °C. The enzyme was tolerably stable up to 37 °C. Several triglyceride (TG) molecular species were used as substrates for investigating the positional and fatty acid specificities of the enzyme. With respect to the hydrolysate of TG, the enzyme examined was not an sn-1,3 or an sn-2 positionally specific lipase. The enzyme appeared to have specific activity on monounsaturated and saturated chain TGs, most likely oleic and/ or palmitic acid. In this study, colipase was also separated from the enzyme.

Morphometry of olfactory lamellae and olfactory receptor neurons during the life history of chum salmon (Oncorhynchus keta).

It is generally accepted that anadromous Pacific salmon (genus Oncorhynchus) imprint to odorants in their natal streams during their seaward migration and use olfaction to identify these during their homeward migration. Despite the importance of the olfactory organ during olfactory imprinting, the development of this structure is not well understood in Pacific salmon. Olfactory cues from the environment are relayed to the brain by the olfactory receptor neurons (ORNs) in the olfactory organ. Thus, we analyzed morphometric changes in olfactory lamellae of the peripheral olfactory organ and in the quantity of ORNs during life history from alevin to mature in chum salmon (Oncorhynchus keta). The number of lamellae increased markedly during early development, reached 18 lamellae per unilateral peripheral olfactory organ in young salmon with a 200 mm in body size, and maintained this lamellar complement after young period. The number of ORNs per olfactory organ was about 180,000 and 14.2 million cells in fry and mature salmon, respectively. The relationship between the body size (fork length) and number of ORNs therefore revealed an allometric association. Our results represent the first quantitative analysis of the number of ORNs in Pacific salmon and suggest that the number of ORNs is synchronized with the fork length throughout its life history.