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Eel larvae apparently feed on marine snow, but many aspects of their feeding ecology remain unknown. The eukaryotic 18S rRNA gene sequence compositions in the gut contents of four taxa of anguilliform eel larvae were compared with the sequence compositions of vertically sampled seawater particulate organic matter (POM) in the oligotrophic western North Pacific Ocean. Both gut contents and POM were mainly composed of dinoflagellates as well as other phytoplankton (cryptophytes and diatoms) and zooplankton (ciliophoran and copepod) sequences. Gut contents also contained cryptophyte and ciliophoran genera and a few other taxa. Dinoflagellates (family Gymnodiniaceae) may be an important food source and these phytoplankton were predominant in gut contents and POM as evidenced by DNA analysis and phytoplankton cell counting. The compositions of the gut contents were not specific to the species of eel larvae or the different sampling areas, and they were most similar to POM at the chlorophyll maximum in the upper part of the thermocline (mean depth: 112 m). Our results are consistent with eel larvae feeding on marine snow at a low trophic level, and feeding may frequently occur in the chlorophyll maximum in the western North Pacific.

Giant clams have evolved to maximize sunlight utilization by their photosymbiotic partners, while affording them protection from harmful ultraviolet (UV) light. The presence of UV absorbing substances in the mantle is thought to be critical for light protection; however, the exact localization of such compounds remains unknown. Here, we applied a combination of UV liquid chromatography (LC), LC-mass spectrometry (MS), MS imaging, and UV micrography to localize UV absorbing substances in the giant clam Tridacna crocea. LC-MS analysis revealed that the animal contained three classes of mycosporines: progenitor, primary, and secondary mycosporines. MS imaging revealed that primary and secondary mycosporines were localized in the outermost layer of the mantle; whereas progenitor mycosporines were distributed throughout the mantle tissue. These findings were consistent with the results of UV micrography, which revealed that the surface layer of the mantle absorbed UV light at 320 ± 10 nm. This is the first report indicating that progenitor and primary mycosporines are metabolized to secondary mycosporines by the giant clam and that they are differentially localized in the surface layer of the mantle to protect the animal from UV light.

The spawning behavior of a Japanese flying squid (Todarodes pacificus) is described based on up-close observation of a captive female. The squid was first transferred from a 10-ton tank to a polystyrene plastic box containing 45 liters of seawater. About one hour later, the mantle-contraction rate increased rapidly, followed by a brief convulsion of the mantle and arms and a whitening of the body. The mantle contractions become shallow and rapid, and several seconds later, semitransparent jelly presumably from the nidamental glands emerged from the funnel and passed between the ventral pair of arms. Approximately 90 seconds after the egg mass first emerged, the female began ejecting oocytes through the funnel into the egg mass using rapid, powerful mantle contractions. Soon after the oocytes were ejected, translucent strands (presumably sperm) emanated from the buccal membrane. The female continued to eject oocytes for approximately two minutes, after which the mantle convulsed, and the mantle-contraction rate decreased slowly for about one minute until the contractions stopped. The squid died soon afterwards.

Diatoms play important roles in primary production and carbon transportation in various environments. Large-scale diatom bloom occurs worldwide; however, metabolic responses of diatoms to environmental conditions have been little studied. Here, we targeted the Oyashio region of the western subarctic Pacific where diatoms bloom every spring and investigated metabolic response of major diatoms to bloom formation by comparing metatranscriptomes between two depths corresponding to different bloom phases. Thalassiosira nordenskioeldii and Chaetoceros debilis are two commonly occurring species at the study site. The gene expression profile was drastically different between the surface (late decline phase of the bloom; 10 m depth) and the subsurface chlorophyll maximum (SCM, initial decline phase of the bloom; 30 m depth); in particular, both species had high expression of genes for nitrate uptake at the surface, but for ammonia uptake at the SCM. Our culture experiments using T. nordenskioeldii imitating the environmental conditions showed that gene expression for nitrate and ammonia transporters was induced by nitrate addition and active cell division, respectively. These results indicate that the requirement for different nitrogen compounds is a major determinant of diatom species responses during bloom maturing.

In bluefin tuna aquaculture, high mortalities of hatchery-reared juveniles occur in sea cages owing to wall collisions that are caused by high-speed swimming in panic due to changes in illuminance. Here, we report that targeted gene mutagenesis of the ryanodine receptor (RyR1b), which allows the sarcoplasmic reticulum to release Ca 2+ in fast skeletal muscle, using highly active Platinum TALENs caused slow swimming behaviour in response to external stimuli in Pacific bluefin tuna (PBT) larvae. This characteristic would be a useful trait to prevent wall collisions in aquaculture production. A pair of Platinum TALENs targeting exons 2 and 43 of the PBT ryr1b gene induced deletions in each TALEN target site of the injected embryos with extremely high efficiency. In addition, ryr1b expression was significantly decreased in the mutated G0 larvae at 7 days after hatching (DAH). A touch-evoked escape behaviour assay revealed that the ryr1b-mutated PBT larvae swam away much less efficiently in response to mechanosensory stimulation at 7 DAH than did the wild-type larvae. Our results demonstrate that genome editing technologies are effective tools for determining the functional characterization of genes in a comparatively short period, and create avenues for facilitating genetic studies and breeding of bluefin tuna species.

The upstream Kuroshio flows through Okinawa Trough and the Tokara island chain, the region near the continental shelf of the East China Sea and shallow seamounts, where the Kuroshio can induce strong mixing over the shallow topography. Also, tidal currents over the rough topography may produce internal tides, and associated turbulence. The previous observations show energetic high vertical wavenumber near-inertial wave shear in the Kuroshio thermocline, which implies strong turbulent mixing. However, direct turbulence measurements in this region are very scarce. Using high lateral resolution (1-2 km) direct turbulence measurements, we show here, for the first time, that strong turbulent layers form spatially coherent banded structures with lateral scales of >O(10 km), associated with bands of near-inertial wave/diurnal internal tide shear of high vertical wavenumber in the upstream Kuroshio. The turbulent kinetic energy dissipation rates within these turbulent layers are >O(10 -7  W kg -1 ), and estimated vertical eddy diffusivity shows >O(10 -4  m 2  s -1 ) on average. These results suggest that the high vertical wavenumber near-inertial waves propagating in the upstream Kuroshio could have large impacts on the watermass modifications, momentum mixing, nutrient supply, and associated biogeochemical responses in its downstream.

The relationship between biodiversity and ecosystem functioning is an important theme in environmental sciences. We propose a new index for configuration of the biomass pyramid in an ecosystem, named integrated trophic position (iTP). The iTP is defined as a sum of trophic positions (i.e. the average number of steps involved in biomass transfer) of all the animals in a food web integrated by their individual biomass. The observed iTP for stream macroinvertebrates ranged from 2.39 to 2.79 and was negatively correlated with the species density and the Shannon-Wiener diversity index of the local community. The results indicate a lower efficiency of biomass transfer in more diverse communities, which may be explained by the variance in edibility hypothesis and/or the trophic omnivory hypothesis. We found a negative effect of biodiversity on ecosystem functioning.

Male coercive mating exerts a strong evolutionary pressure on mating-related traits of both sexes. However, it is extremely rare in externally fertilizing species probably because the male mating behaviour is incomplete until females release their eggs. Here we showed that males of the externally fertilizing fish Rhabdoblennius nitidus coercively confine females to the nests until spawning, and investigated why females accept male coercive mating. The females entered the males' nests following male courtship displays, but they usually tried to escape when there were no eggs because males tended to cannibalize all the eggs when there were few. Most males that used small, tight nests acquired new eggs but with experimentally enlarged nests, 90% of the males without eggs failed to confine the females. Spawning tended to occur during the early/late spawning period in nests with no eggs (i.e. male coercive mating). In the nests where the first eggs were deposited in the early period, subsequent matings with other females were more likely to occur, whereas in the late period, most parental care of the eggs failed without additional matings. The females that spawned in the late period may have been compelled to accept male coercive mating due to time constraints.

Hermaphroditism is ubiquitous among plants and widespread in the animal kingdom. It is an unsolved problem why reversed sex change has evolved in polygynous and protogynous reef fish. We have previously suggested that facultative monogamy occurs in low-density populations of polygynous species and that males that become single as a result of accidental mate loss may change sex when they meet larger males. In this study, to test this 'low-density hypothesis', we conducted field experiments with the coral reef fish Labroides dimidiatus in which a portion of females were removed to create a low-density situation. The 'widowed' males moved to search for a new mate when no male, female or juvenile fish migrated into their territories and paired with nearby single fish, whether male or female. Alternatively, males expanded their territories to take over the nearest pair whose male was much smaller. These results support our low-density hypothesis.