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Experimental assessment of copepod survival in response to the harmful dinoflagellate Karenia selliformis from the southeastern coast of Hokkaido, Japan
Get PDFIn the autumn of 2021, a large-scale harmful algal bloom (HAB) emerged along the southeast coast of Hokkaido, Japan, and this was predominantly composed of the dinoflagellate Karenia selliformis. The emergence of K. selliformis-dominated HABs was the first observation in Japan; therefore, no previous reports exist on the impact of HABs on zooplankton in the region. This study investigated the effects of K. selliformis on the survival of copepods—a critical component of the zooplankton community. The results indicated that the survival rates of all six copepod species examined were markedly reduced, particularly at elevated K. selliformis concentrations or during extended exposure. The copepod survival rate decline occurred even in the absence of direct contact or ingestion of K. selliformis, thus implying that K. selliformis may excrete harmful substances extracellularly. Feeding experiments revealed that Neocalanus plumchrus consumes K. selliformis cells; however, the ingestion rate diminished with increased concentrations of K. selliformis cells. The results suggest that larger copepods with smaller surface-to-volume ratios may possess greater resilience to the harmful substances compared to smaller species. Further, the HAB in southeast Hokkaido likely exerted deleterious effects on lower trophic levels within the marine ecosystem by disrupting copepod survival and feeding activity.journal articl
Interspecific and intraspecific difference in egg size of two mackerel (Scomber spp.) species in relation to sea surface temperature in the western North Pacific: A new approach to species identification
Get PDFChub mackerel (Scomber japonicus, Scombridae) and blue mackerel (Scomber australasicus) are two important fishery resources in the western North Pacific that spawn eggs during the same season. Although estimating the total egg production (TEP) is important for evaluating the spawning stock biomass of mackerel species, it is difficult to accurately identify formalin-preserved eggs collected during ichthyoplankton surveys. Hence, a new identification criterion that incorporates the effects of water temperature on egg size was developed in this study. The diameter of 37,304 mackerel eggs collected over 16 years (2006–2021) was measured, and frequency distributions of egg size across sea surface temperature (SST) ranges at 1°C interval were constructed. The frequency distributions were classified into two groups using a Gaussian mixture model; based on the results of DNA analysis, the small group was chub mackerel, while the large group was blue mackerel. The SST at the sampling stations and the mean egg size of both groups were negatively correlated. The new identification criterion, incorporating the relationship between SST and egg size, provided reasonable estimates of the TEP of the two mackerel species compared with the conventional criterion. The new species identification approach is applicable to other fish taxa in the western North Pacific.journal articl
Targeted Metabolite Fingerprints of Thirteen Gambierdiscus, Five Coolia and Two Fukuyoa Species
Get PDFThe genus Gambierdiscus produces an array of bioactive hydrophilic and lipophilic secondary metabolites that range in mode of action and toxicity. In this study, the metabolite fingerprint was mapped for thirteen Gambierdiscus, five Coolia and two Fukuyoa species (34 isolates) by assessing the production of 56 characterised secondary metabolites. Gambierdiscus polynesiensis was the only species to produce Pacific-ciguatoxin-3B (P-CTX3B), P-CTX3C, iso-P-CTX3B/C, P-CTX4A, P-CTX4B and iso-P-CTX4A/B. G. australes produced maitotoxin-1 (MTX-1) and MTX-5, G. cheloniae produced MTX-6 and G. honu produced MTX-7. Ubiquitous production of 44-methylgambierone was observed amongst all the Gambierdiscus isolates, with nine species also producing gambierone. Additional gambierone analogues, including anhydrogambierone (tentatively described herein), were also detected in all Gambierdiscus species, two Coolia and two Fukuyoa species. Gambieroxide was detected in G. lewisii and G. pacificus and gambieric acid A was detected in ten Gambierdiscus species, with G. australes (CAWD381) being the only isolate to produce gambieric acids A–D. This study has demonstrated that the isolates tested to date produce the known CTXs or MTXs, but not both, and highlighted several species that produced ‘unknown’ compounds displaying characteristics of cyclic polyethers, which will be the focus of future compound discovery efforts.journal articl
Evaluation of the Precision of Age Determination and Variability in Growth for Japanese Sardine (Sardinops melanostictus), Tsushima Warm Current Subpopulation
Get PDFAccurate estimates of fish age and growth are essential for stock assessment and management. We evaluated the differences in aging precision of Japanese sardines (Sardinops melanostictus) from the Sea of Japan and East China Sea determined from the counts of annual rings on scales and otolith observation by two readers. Changes in growth parameters between 2011 and 2018 year-classes were also evaluated. Although the aging precision between readers was relatively precise for both scales and otoliths, otoliths may be more useful for determining the age of this population because of the low discrepancy of the age counts between readers. The average standard length of Japanese sardines aged 2-3 years was relatively stable for 2011-2017 year-classes but declined sharply after 2018 year-class, along with increased stock biomass. Recent (2018 year-class) estimated theoretical asymptotic length also declined compared with those of early 2010s year-classes. Changes in the mean standard length at an age of 2-3 years and theoretical asymptotic length of Japanese sardines in the Sea of Japan and East China Sea may be a density-dependent response to increased stock abundance.journal articl
Animal-borne soundscape logger as a system for edge classification of sound sources and data transmission for monitoring near-real-time underwater soundscape
Get PDFThe underwater environment is filled with various sounds, with its soundscape composed of biological, geographical, and anthropological sounds. Our work focused on developing a novel method to observe and classify these sounds, enriching our understanding of the underwater ecosystem. We constructed a biologging system allowing near-real-time observation of underwater soundscapes. Utilizing deep-learning-based edge processing, this system classifies the sources of sounds, and upon the tagged animal surfacing, it transmits positional data, results of sound source classification, and sensor readings such as depth and temperature. To test the system, we attached the logger to sea turtles (Chelonia mydas) and collected data through a cellular network. The data provided information on the location-specific sounds detected by the sea turtles, suggesting the possibility to infer the distribution of specific species of organisms over time. The data showed that not only biological sounds but also geographical and anthropological sounds can be classified, highlighting the potential for conducting multi-point and long-term observations to monitor the distribution patterns of various sound sources. This system, which can be considered an autonomous mobile platform for oceanographic observations, including soundscapes, has significant potential to enhance our understanding of acoustic diversity.journal articl
N-Desmethylmajusculamide B, a lipopeptide isolated from the Okinawan cyanobacterium Okeania hirsuta
Get PDFA new lipopeptide, N-desmethylmajusculamide B (1), was isolated from the Okinawan cyanobacterium Okeania hirsuta along with 2 known compounds majusculamide A (2) and majusculamide B (3). The planar structure of (1) was elucidated by a detailed analysis of mass spectrometry and nuclear magnetic resonance spectra. The absolute configurations of the amino acid residues were determined using Marfey's analysis. The configuration of C-16 in the α-methyl-β-keto-decanoyl moiety was determined unambiguously to be S by conducting a semisynthesis of N-desmethylmajusculamide B from 3. The cytotoxicity against mouse L1210 leukemia cells was evaluated for majusculamides (1-3).journal articl
Inter-annual variation in the relationship between early growth rate and potential survival of the western sand lance Ammodytes japonicus in the Seto Inland Sea in western Japan
Get PDFThis study examined whether the potential early survival of sand lance in the Bisan Strait, central Seto Inland Sea, can be explained by variations in the growth rate and duration in the early life stages. Otolith microstructure analysis was conducted to examine the growth trajectories of specimens collected in six sampling years: 2011–2014, 2019, and 2020. There were significant differences in the growth rates of larvae and juveniles among sampling years, but such differences were likely attributable to variations in temperature in given seasons. Relative growth rates standardized by temperature were negatively correlated with the duration of the larval stage, suggesting that faster-growing individuals at given temperatures had a shorter larval stage. In each sampling year, there was a significant relationship between standardized hatch date and relative growth rate during the larval stage but not the juvenile stage. Within a given season, later-born larvae seemed to grow faster relative to the temperature experienced, and the higher percentages of the faster-growing larvae were likely associated with higher recruitment and recruitment per egg production. These findings suggest that the proportion of faster-growing sand lance larvae in a cohort at given temperatures explains the variation in potential early survival, but the higher temperatures in recent years may not necessarily contribute to improved early survival.journal articl
A new species of the genus Paradistomum (Platyhelminthes: Trematoda) from Iwasaki’s snail-eating snake Pareas iwasakii, with a note on morphological variations of Paradistomum megareceptaculum (Tamura, 1941)
Get PDFDicrocoeliid trematodes were detected from Iwasaki’s snail-eating snake Pareas iwasakii in Iriomote Island, Okinawa Prefecture, Japan, and described as a new species Paradistomum dextra n. sp. in the present study. This new species can be distinguished from the type series of the other members of the genus based on size of eggs and morphological characteristics of body, oral and ventral suckers, and reproductive organs. However, the new species was hard to distinguish from Paradistomum megareceptaculum infecting snakes in Japan, including Iriomote Island where is the type locality of the new species, because it is closely similar to some part of the broad range of morphological variations in P. megareceptaculum. On the other hand, a partial sequence of 28S ribosomal DNA clearly distinguished these two species. Moreover, the new species’ host snake Pareas iwasakii is reported to exclusively feed on land snails while host snakes of P. megareceptaculum feed on small vertebrates, indicating that the new species is also ecologically different from P. megareceptaculum. We also redescribed P. megareceptaculum based on adults sampled in this study and past studies to record the morphological variations of this species.journal articl