The red swamp crayfish, Procambarus clarkii, is regulated as a “conditionally specified invasive alien species” in Japan due to its invasive nature, and its management is guided nationwide by The Red Swamp Crayfish Control Guide issued by the Ministry of the Environment. In this study, we quantitatively compared the five main capture methods introduced in the guide, a D-framed net and four distinct trap types: conger eel cage trap (“cage trap”), plastic bottle trap (“plastic bottle”), PVC pipes, and seaweed net trap (“seaweed net”). Capture characteristics, including the number of individuals captured, sex ratio, body size, seasonal variation, bycatch, and time required for collection and retrieval, were evaluated. The test was conducted by simultaneously implementing all the capture methods at each of the five survey points within a wetland in Nagasaki City. This procedure was repeated 11 times between 2023 and 2024. Regarding the number of individuals captured, we analyzed the data using a generalized linear model. The results showed that the D-framed net and cage trap were effective methods for capturing P. clarkii and suggested a potential bias in their distribution within the wetland. No significant sex bias was detected for any capture method. Body size comparisons based on the carapace length as an indicator showed that the cage trap and plastic bottle tended to capture larger, mature individuals, whereas the D-framed net and seaweed net tended to capture smaller, immature individuals. Seasonal analyses showed that the D-framed net and cage traps were effective throughout the active season of this species. Furthermore, we found that cage traps were particularly more effective from late August to early September and that setting the seaweed net in early September and retrieving it in late September was also effective for targeting smaller, immature individuals. These setting and retrieval periods could be applied across a wide area of Western Japan. The cage trap had the highest number of bycatch organisms among the four trap types, the majority of which were shrimp and fish, highlighting the importance of early retrieval. Regarding the time required for collection and retrieval, the D-framed net, which involves scooping ten times, required approximately twice the time needed to retrieve the cage trap. Based on these characteristics, along with feasibility, we propose a control method that primarily uses cage traps in combination with seaweed nets.

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Freshwater unionid mussels (Unionoida) are threatened globally. However, stable and low-cost rearing techniques remain to be established. We developed a practical outdoor rearing system for Beringiana fukuharai by evaluating its survival, growth, and juvenile production under different soil and organic material conditions. In the first experiment, mussels were reared in plots containing mountain soil, pond mud, and chicken manure, each supplemented with one of the following three organic materials: reed leaves, leaf litter, or rice husks. All mussels survived and grew during the experimental period (approximately eight months), yielding the most juveniles in the reed leaf plot. In the second experiment, we compared plots containing four different soil components, each supplemented with reed leaves. The mussels survived and grew in all plots; especially, their growth was found to be greatest in the plots with mountain soil or pond mud than in those with sandy granite soil. Organic materials or their decomposition products contained in the mountain soil or pond mud likely contribute to a more stable rearing environment for the mussels by providing nutrient enrichment and favorable conditions.

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From August 2019 to August 2025, 56 specimens (19.3–61.0 mm in standard length) of the amphidromous gobiid fish Rhinogobius brunneus (Temminck and Schlegel, 1845) were collected from Hachijo-jima island, the Izu Islands, Tokyo Metropolis, Japan. No gobies of the genus Rhinogobius have previously been recorded in the Izu Islands except for a photographic record of R. brunneus from Hachijo-jima island. Therefore, the present specimens represent the first verifiable records of Rhinogobius in the region. The occurrence of this species, which adapted to small rivers, reflects the freshwater environment of the Izu Islands, characterized by numerous steep, small rivers.

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We examined the daily periodicity of otolith increment formation in reared Shishamo smelt Spirinchus lanceolatus larvae, and investigated the growth history of wild-caught larvae. The mean body length of newly hatched larvae was 8.4 mm, and the mean otolith radius was 12.7 µm (sagitta) and 12.4 µm (lapillus), respectively. Both otoliths displayed a check mark at hatching (1st check), and an additional check mark was observed later (2nd check). The relationship between the number of increments formed outside the 1st check on both otoliths (I_s, I_Ɩ) and age (A_c, days) was described by regression lines (I_s = 0.86A_c (n = 38, r² = 0.992, p < 0.01), I_Ɩ = 0.49A_c (n = 24, r² = 0.932, p < 0.01)), where the slopes significantly differed from 1 (t-test, both p < 0.01). We also examined the relationship between the number of increments outside the 2nd check (Iʹ_s, Iʹ_Ɩ) and age (A_c, days) for both otoliths. As a result, the regression lines (Iʹ_s = 0.99A_c–19.5 (n = 26, r² = 0.996, p < 0.01), Iʹ_Ɩ = 0.61A_c–16.3 (n = 15, r² = 0.928, p < 0.01)) showed that the slope for the sagitta was not significantly different from 1 (t-test, p = 0.38), while the lapillus was significantly different from 1 (t-test, p < 0.01). These results suggest that daily countable increments only form after the 2nd check mark in the sagitta. One possible reason why daily periodicity was not confirmed between the 1st and 2nd checks on both otoliths is that the increment width during this period was so narrow that it could not be distinguished under an optical microscope. This phenomenon has also been reported in cold-water fish species such as Atlantic herring and European sardine. Therefore, for cold-water fish species, which have increments after hatching that are generally difficult to distinguish, it is reasonable in age estimation to count increments only after they are clearly visible, and to compensate for earlier periods using check mark positions and growth indices. Based on the results of the rearing experiments, we examined the number and width of otolith increments in Shishamo smelt larvae and juveniles collected by sledge net along the Mukawa and Sarugawa coasts during June–July in 2023 and 2024. The ranges of hatching dates in both 2023 and 2024, calculated from age and sampling date, were respectively March 29–April 27 and March 23–April 27. A significant allometric relationship (r² = 0.959, p < 0.001) was found between otolith radius (OR, µm) and body length (BL, mm). The mean back-calculated body lengths of wild-caught individuals in 2023 and 2024 estimated using the Biological Intercept Method were respectively 11.4 mm and 11.3 mm at 20 days old, 17.0 mm and 17.6 mm at 40 days old, 21.8 mm and 23.3 mm at 60 days old, and 27.6 mm and 31.2 mm at 80 days old.

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A list of 363 species of benthic animals belonging to 12 phyla was compiled based on literature records from the lakes and streams of the Jūniko Lakes in the Shirakami Mountains in northern Honshū, Japan. The list was dominated by insects, representing 237 species or 65 % of the total fauna. The list includes seven introduced species, five of which are parasites of fish transported into the lakes. Distinct longitudinal changes were found in the faunal composition of streams along the Koikuchi-no-ike water system. The distributions of relatively large invertebrates and amphibians are biased towards the small lakes. This could be related to the absence of fish in the small lakes. An expansion in the distribution of the amphipod Jesogammarus jesoensis was observed in the last 30 years. This may be related to anthropogenic migration of fish. Significant declines in submerged plants have been observed in the Koikuchi-no-ike lake group over the past 20 years. It is suspected that the introduced red swamp crayfish is responsible for these declines.

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The freshwater pearl mussel Margaritifera laevis, with the epilithic red alga Virescentia sp. growing on its shell surface, was discovered in a small stream on the Noto Peninsula, central Japan. Although the cobbles provided sufficient attachment surfaces for red algae at the study site, red algae were also observed on freshwater pearl mussels. The study site, where both freshwater pearl mussels and red algae were distributed, is an infrequent habitat in the Hokuriku region of central Japan; therefore, the epizoic occurrence of freshwater pearl mussels with red algae in this stream is considered a unique case.

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A male-transitional stage pair of the cymothoid isopod Cinusa nippon was collected from the buccal cavity of the pufferfish Takifugu alboplumbeus along the coast of Hiroshima Prefecture, Japan. This finding represents the first confirmed record of this species from the Seto Inland Sea, extending its known geographic distribution. Morphological examination generally agreed with the original description of C. nippon. It was shown that characteristics of the adult female appear even at the transitional stage, enabling species identification. On the other hand, the male specimen possessed four recurved robust setae on the maxilla, a character that has not been previously reported for this species. This condition is interpreted as intraspecific variation, which is consistent with recent observations in related studies. To further assess host utilization, additional surveys of tetraodontid fishes were conducted at coastal sites in Hiroshima Prefecture, and 136 individuals were examined. However, no further infestation was detected. The extremely low prevalence observed suggests that C. nippon occurs at very low infection rates in this study area. This study provides new distributional and morphological information on C. nippon and contributes baseline data for understanding its host–parasite relationships in Japanese coastal waters.

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A single specimen of Ctenophoraster donghaiensis was collected south of Kagoshima Prefecture, Kyushu, Japan, in October 2025. This represents the first record of this species in over 35 years since its original description. Most diagnostic characters agree well with the original description of C. donghaiensis, with only minor differences. These differences are interpreted as intraspecific variation; notably, however, the presence of unique pedicellariae, previously unreported in this species, further supports the placement of C. donghaiensis within Ctenophoraster. This study provides new morphological information on this poorly known species and extends its known distribution in the East China Sea.

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In Kochi Prefecture, the field surveys for subterranean worm gobies, the Luciogobius pallidus Regan, 1940 complex and L. cf. albus Regan, 1940, continued from Okamura et al. (2024) were newly recorded based on specimens from 17 and 6 river systems, respectively. As a result, the former and latter have been known from 39 (excluding one well record) and 10 river systems in the area, respectively. Further, the specimens of the former complex collected from Kochi Prefecture are subdivided into two types, L. pallidus FW (L. pallidus Regan, 1940 Freshwater type) and L. pallidus BW (L. pallidus Regan, 1940 Brackish-water type), which are separable by some morphological characters such as skulls, external ones in live and fresh condition, and molecular data (COI region of mtDNA). Although these groups were first named by Okumura et al. (2021), their morphological diagnoses were not enough to separate them. After, Okamura et al. (2024) indicated some differences in external features on the head (i.e., dorsal contour, coloration, and pigmentation) between some live individuals and fresh specimens (they used L. sp. FW and L. sp. BW as the same taxa in this study), but did not find apparent differences between their preserved specimens, which had almost lost the diagnosis. However, it is most likely that the two types are separated by cranial morphology (e.g., elongation of the frontals and parasphenoids in adults of L. pallidus FW), suggesting distinct ontogenies and life histories associated with the different habit environments of each type. On the other hand, 14 specimens of L. cf. albus have been collected from Kochi Prefecture, possibly including two nominal species, L. albus and L. fonticola Kanagawa, Itai and Senou, 2011, as identified by a few morphological differences. However, their molecular analysis of the COI region of mtDNA revealed that they formed a single clade with no genetic differences.

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The lizardfish Synodus pacificus Ho, Chen and Shao, 2016 (Aulopiformes: Synodontidae) is known to be distributed in the western Pacific Ocean, recorded off the coast of Japan, Taiwan, the Philippines, and New Caledonia. Here, we report additional distribution records based on 36 specimens (59.5–208.5 mm standard length) of the species from Japan, Vietnam, and Indonesia. The specimens from Vietnam and Indonesia represent the first records from these countries. The specimens from Sagami Bay, Japan, represent the northernmost and easternmost records, and those from Vietnam represent the westernmost record.

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Argulus coregoni was collected from Opsariichthys platypus in Nakanoho River Gifu Prefecture, Japan. This is the second case of Opsariichthys platypus parasitized by Argulus coregoni.

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DNA barcoding using mitochondrial DNA relies on accurate DNA databases. However, it is not uncommon for these DNA databases to contain nucleotide sequences with erroneous species labels. Furthermore, sequencing errors and nuclear mitochondrial DNA (NUMT) sequences may impact DNA barcoding. In the present study, we applied phylogenetic, nucleotide sequence, and deduced amino-acid sequence analyses to the 1,157 unique COI haplotypes of the 22 spiny lobster species of the genus Panulirus collected from the GenBank database and from a published article. Based on the large number of amino-acid substitutions, large nucleotide-sequence divergence, and odd phylogenetic placement, one haplotype was found to be misidentified to species. Of 55 haplotypes determined to be problematic, 27 haplotypes obtained from cloned amplicons were presumed to be NUMTs and 28 haplotypes obtained by direct nucleotide sequencing were presumed to be NUMT-contaminated sequences. A small number of non-synonymous nucleotide substitutions were observed in 96 other haplotypes, possibly caused by sequencing errors or slight NUMT-contamination. The unnoticed inclusion of NUMTs or NUMT-contaminated sequences in nucleotide sequence datasets may inadvertently inflate intraspecific genetic diversity and distort phylogenies.

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A clingfish of the family Gobiesocidae was collected using a scoop net from shallow coastal waters of the Oki Island, Shimane Prefecture, Japan. Based on detailed morphological examination and DNA barcoding of the mitochondrial 16S ribosomal RNA gene, the specimen was identified as Aspasma ubauo. This species has previously been recorded from several localities along the Japanese coast, however no confirmed records existed from the Sea of Japan coast west of the Noto Peninsula, Ishikawa Prefecture, Japan. The present specimen therefore represents the first record of A. ubauo from Shimane Prefecture, Japan and fills an important distributional gap along the Sea of Japan. The finding suggests either a previously overlooked local population or a recent range extension influenced by regional oceanographic conditions.

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We tested whether the non-marine ostracod Heterocypris spadix Munakata, Tanaka, and Kakui, 2021 is capable of producing desiccation-tolerant eggs. Under ambient light and room-temperature conditions, we transferred live ostracods together with brackish water and bottom substrate containing eggs from a long-maintained rearing aquarium to a small experimental aquarium. The contents were dried gradually over five days until completely dry, then maintained for five more. Freshwater was then added, and the aquarium was placed in an incubator (25 °C; 14 h light : 10 h dark cycle). Living individuals were observed six days after rehydration. In a separate trial, eggs laid on an old valve were subjected to a similar drying–rehydration treatment, and living individuals were again observed after rehydration. Our observations demonstrate that H. spadix can produce desiccation-tolerant eggs.

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A sponge-associated crab of the family Pilumnidae, Ericiolumnus symbioticus Ng and Rahayu, 2020, previously known only by the type specimens from Lombok, Indonesia, is reported from Japan for the first time. Notes on the morphological features and color in life of the female collected from an unidentified sponge at subtidal zone in Okinawa Island, Ryukyu Islands are provided.

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A specimen of Armina papillata was collected from the tidal flats Hachi-no-higata, Hiroshima Prefecture, Japan. The individual was found in proximity to the sea pen Cavernularia obesa, a known prey species of this nudibranch. This represents a record indicating the recent occurrence of A. papillata from Hiroshima Prefecture. Although the species has been documented from several localities within the Seto Inland Sea, regional distribution records remain sparse, particularly in inner-bay environments. The collection site is currently facing potential habitat alteration associated with coastal development, highlighting the importance of documenting baseline biodiversity data. This record provides valuable information for understanding the distributional range and conservation status of A. papillata in the Seto Inland Sea region.

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A fish survey was conducted from May 2023 to April 2024 in the Onosato River estuary, located in southern Osaka Bay, Japan. A total of 1,883 individuals representing 63 species, spanning 27 families and eight orders, were identified. Gobiid fishes were the most species-rich group, comprising 20 species. The fish species confirmed in this survey included endangered and rare species, as well as those newly documented in this bay. The occurrence patterns and growth trajectories of the yellowfin goby (Acanthogobius flavimanus) and the grey-spotted puffer (Takifugu albophumbeus) were examined in detail. The yellowfin goby was recorded from May to January, with individuals exhibiting continuous growth during this period. Its absence from February to April suggests that this species likely migrates out of the estuary to spawn over this timeframe. In contrast, the grey-spotted puffer was observed year-round within the estuary. It breeds in the estuary in June and July, followed by the appearance of juvenile recruits in July and August. These findings suggest that the Onosato River estuary is a critical habitat that supports diverse ecological roles, including growth, reproduction, and recruitment of multiple fish species.

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The bagrid catfish Tachysurus nudiceps (Sauvage, 1883) is a freshwater fish naturally distributed in western Honshu, Shikoku, and northeastern Kyushu in Japan. This species has been introduced to other areas outside of its natural distribution range in Japan and has become established there, likely through the stocking of Plecoglossus altivelis (Temminck et Schlegel, 1846) from Lake Biwa. This has raised concerns about its potential impact on native fishes of the family Bagridae. The first specimen of this species in Kanagawa Prefecture was discovered in the Tsurumi River in 2019, and its presence was subsequently confirmed in 2022 and 2023. Here, we report on additional specimens of this species collected in the Tsurumi River, representing the first specimen-based record from this river. A total of nine individuals were collected by angling in one night of April 10, 2025. Their standard length ranged from 139.5 to 246.0 mm. There is concern that this species may significantly impact the endangered native species, Tachysurus tokienesis.

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Twelve and three specimens of two species of the ghost shrimp genus Neocallichirus calmani and N. jousseaumei were collected from sandy beaches in Honshu and/or Shikoku of the Japanese mainland. These locations in central and western Japan represent the temperate region. Notably, N. calmani was the dominant ghost shrimp in the lower tidal zone of the latter site throughout the sampling period. The identifications were confirmed through both morphological analysis and DNA barcoding of the mitochondrial 16S rRNA gene. These findings represent the northernmost records for both species, which were previously known only from subtropical and tropical regions. Furthermore, the collection timing and specimen sizes suggest that these species are capable of overwintering in these temperate areas. Given that species of this genus can function as an influential ecosystem engineer—acting as both a bioturbator and a foundation species—continued monitoring of its population and distribution dynamics in these regions is essential.

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The alpheid shrimp, Potamalpheops yamakawai Yamashita, Komai & Koreeda, 2024, has previously been recorded from only estuarine along the Pacific coast of Shizuoka Prefecture and the East China Sea coast of Kagoshima Prefecture, Japan. In this study, 10 specimens of the species were newly collected from five rivers in Kanagawa, Wakayama, Tokushima, and Miyazaki Prefectures along the Pacific coast of Japan. The specimens were found from not only vegetation along the bank, but also interstices of pebbles on the riverbed in limited area of tidal reaches. Thus, this species can inhabit various environments. However, these habitats have been severely affected by human activities, therefore, urgent conservation is needed.

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Two Hydrophilidae species, Hydrobius fuscipes (Linnaeus, 1758) and Laccobius (Microlaccobius) oscillans Sharp, 1884, were newly recorded on Rebun Island, northern Hokkaido, Japan, based on specimens collected in July 2025. Hydrobius fuscipes represents the first record from the island and was collected from a shallow spring-fed pool in Lake Kushu, whereas L. oscillans was found in a small stream flowing directly into a rocky coastal area. These records add two freshwater hydrophilid species to the known fauna on Rebun Island.

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The striped bitterling (Acheilognathus cyanostigma), a species of the family Cyprinidae, is naturally distributed in the Kinki region of central Japan. This species is spreading to various other areas due to the release of juvenile ayu (Plecoglossus altivelis) from Lake Biwa. This species has been reported in the Monobe, Kagami, and Niyodo river systems in central Kochi Prefecture, where it has been considered as an artificially introduced species. However, there is no documented record based on voucher specimens. We conducted meristic and morphometric analyses on bitterling specimens deposited at the Lake Biwa Museum and confirmed that three specimens collected from the Monobe and Watari rivers in Kochi Prefecture were A. cyanostigma.

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At Eishin High School, Fukuyama City, Hiroshima Prefecture, an ex-situ conservation program has been conducted for a population of the endangered bittering fish Rhodeus atremius suigensis from the Ashida River system. This population has been maintained in aquaria through successive generations by artificial insemination for approximately 30 years. In July 2021, 31 juveniles obtained from these generations were introduced into a conservation pond, where they were reared without supplemental feeding and natural reproduction was encouraged using artificial spawning beds with mussels. In May and June 2022, seven and 26 juveniles of R. a. suigensis were confirmed, respectively, after emerging from the mussels collected from the pond and subsequently reared in an aquarium. A survey in the pond in October 2022 confirmed 17 individuals born in 2022. These results indicate that the individuals released in 2021 had spawned in the mussels. This paper presents a case in which an endangered bittering fish species, maintained in aquaria for about 30 years through successive breeding by artificial insemination, successfully reproduced by spawning in mussels under semi-natural conditions.

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A single specimen of the coastal tank goby, Glossogobius laticeps (De Vis 1884) (79.6 mm standard length), was collected from the lower reach of the Mimi River, Miyazaki Prefecture, Kyushu, Japan. This constitutes the first record of the species in Japan and extends its range northwards from the previously known northernmost record in Taiwan. Two additional specimens from Hong Kong, identified as Gl. laticeps during an examination of specimens deposited in the Kanagawa Prefectural Museum of Natural History (105.8‒140.1 mm standard length), represent the first specimen-based records of the species from mainland China’s coast. The diadromous habit of this tropical/subtropical species suggests that its presence on the east coast of Kyushu, which is located in a warm temperate zone, is probably accidental and is most likely due to planktonic larvae being transported from tropical/subtropical rivers to the sea by the Kuroshio Current. Detailed morphological descriptions, illustrations of the cephalic sensory systems, and color photographs of the specimens are provided here.

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The genetic population structure of Pacific saury (Cololabis saira) was investigated using nucleotide sequences of a 926 to 932 bp stretch of mitochondrial DNA containing the entire control region. A total of 309 individuals collected in 2021 from 12 locations across the western to eastern North Pacific were analyzed. No significant spatial heterogeneity in haplotype distributions was detected (p > 0.3). Comparison with published data from 2002–2003 revealed no significant temporal heterogeneity despite the two-decade gap, although the difference was marginal (p = 0.056). Overall, the low level of genetic variation and the absence of detectable spatiotemporal differentiation indicate a high degree of gene flow across the North Pacific. However, these findings should be interpreted with caution, as the analysis is based solely on a maternally inherited marker with limited resolution.

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This study examined parturition timing, reproductive patterns, offspring morphology, and post-parturition maternal condition in the viviparous surfperch Ditrema temminckii temminckii, using pregnant females collected from Uwajima Bay, Ehime Prefecture, Japan. Under captive conditions, parturition occurred from late March to late April, with a peak between April 10 and 26, and was more frequent at night than during the day. The offspring obtained from the 24 females included normal, premature, and morphologically malformed individuals. The normal offspring had greater mean body length and weight than the premature and malformed offspring. The premature offspring lacked swimming ability. Variation in body size was also observed among normal offspring, with multiple size groups identified. All females died within 18–20 days after completing parturition, and almost no offspring remained in the ovary. These findings clarify the characteristics of parturition timing, variation in offspring size, and the relationship between parturition and post-parturition mortality in this species, providing insight into reproductive strategies in viviparous marine fishes.

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Rhinogobius brunneus is an amphidromous goby distributed in Japan and Korea, whose larvae drift to the sea after hatching and later return to freshwater habitats. Hachijo-jima Island, in the southern Izu Islands, lies immediately downstream of the Kuroshio and has recently yielded the first specimen-based records of R. brunneus. To evaluate whether larvae could realistically reach the island from mainland Japan, I conducted backward particle-tracking simulations using daily surface fields from FRA-ROMS II for the years 1993–2025. Virtual particles released from Hachijo-jima during the assumed recruitment season were traced backward for up to 45 days, corresponding to the pelagic larval duration. Estimated coastal origins were classified into four source regions: Ryukyu, Kyushu, Shikoku, and Kii–Tokai. To address uncertainty in larval depth, I used the 0-m current field as the baseline and repeated the analysis with the 10-m current field. I also conducted a forward validation following the logic of two-way particle tracking, in which particles were re-released from backward-inferred coastal origins and tracked forward to test whether they could return to the Hachijo-jima target area. The 10-m analysis reduced absolute arrival ratios but retained the main interannual pattern observed at 0 m. Kyushu provided the most persistent background contribution, whereas Kii-Tokai provided episodic but efficient transport opportunities, including high two-way-supported connectivity in several years. These results support the hypothesis that the recent establishment of R. brunneus on Hachijo-jima reflects episodic long-distance transport from mainland Japan via the Kuroshio. More broadly, opportunities for immigration to oceanic islands may be temporally intermittent rather than continuous, even for amphidromous fishes with pelagic larval stages.

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Fish occurrence patterns and habitat characteristics in eight rivers discharging into Lake Hinuma, a designated Ramsar wetland and the only natural brackish lake (9.35 km2) on the Kanto Plain, eastern Japan, were investigated in April and May (spring) and July and August (summer) in both 2019 and 2020. A total of 69 fish species, including 16 red-listed taxa (e.g., threatened and near threatened), 7 exotic species and 9 introduced Japanese species, were collected during the study. Species that spent at least part of their life cycle in brackish and/or marine habitats (i.e. estuarine, marine and diadromous fishes) accounted for approximately 52 % of all species. A cluster analysis conducted on species composition data for each river revealed that the 8 rivers fell into one of two groups, a large river (Hinuma River) group with high species richness, and small river group with low species richness. The latter group also exhibited a distinctive fish fauna comprising a variety of species, reflecting the salinity gradient within the lake. Generalized linear mixed model analyses based on the occurrence patterns of 15 major fish species and environmental variables showed marked interspecific differences in preferred habitat conditions. For example, some endangered species, such as Japanese eel (Anguilla japonica), Japanese ricefish (Oryzias latipes) and the sculpin Cottus reinii, were frequently found in areas with well-developed aquatic vegetation. In addition, upstream migration of several species, including A. japonica, Japanese dace (Pseudaspius hakonensis), mullet (Mugil cephalus cephalus) and two gobies (Gymnogobius urotaenia and Tridentiger brevispinis), were likely to be negatively influenced by weirs. To appropriately conserve essential habitats for native fish species, including both red-list and commercially important species, in rivers flowing into Lake Hinuma, it is desirable that river management plans incorporate regular monitoring of the population status of each species, in addition to the protection offered by natural riverbanks, including aquatic vegetation zones, restoration of artificial banks to semi-natural conditions, connectivity maintained between rivers and the brackish lake via operation or improvement (e.g., installation of fishways) of weirs, and control of invasive exotic species [e.g., piscivorous smallmouth bass (Micropterus dolomieu dolomieu)].

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Specimen of twelve species of Astrabe, Clariger, Inu and Luciogobius (Gobiidae), including Astrabe lactisella Jordan and Snyder, 1901, Clariger papillosus Ebina, 1935, Clariger sp. sensu Akihito et al. (2013), Inu koma Snyder, 1909, Inu sp. 1 sensu Shibukawa et al. (2020), L. martellii Di Caporiacco, 1948, L. grandis Arai, 1970, Luciogobius sp. 5 sensu Shibukawa et al. (2019), Luciogobius sp. 6 sensu Shibukawa et al. (2019), L. platycephalus Shiogaki and Dotsu, 1976, L. parvulus (Snyder, 1909) and L. elongatus Regan, 1905 were collected from Fukushima and Ibaraki prefectures, Joban Region, eastern Japan. The present specimens of Luciogobius sp. 5 and sp. 6 represent the first record from the region. Comparing differences in each species' habitats suggests that bottom material, including grain size and presence of bedrock, elevation and dryness influence species segregation.

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Territorial and courting behaviors of the barbel steed Hemibarbus labeo (Pallas, 1776) were observed in an irrigation canal of the Tamekawa River system in Kurashiki City, Okayama Prefecture, Japan, on April 28, 2026. The courting behavior was distinguished by four phases: tracking, parallel swimming, reversal, and rubbing. The occurrence of these behaviors in the irrigation canal suggests that H. labeo uses agricultural canals as spawning sites.

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The Polychelida, often referred to as a ‘living fossil’, has been poorly documented in basic biological data among the decapod infraorders to date. The current knowledge on the polychelid families, is summarized along with the relevant palaeontological information.

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The diving beetle Allodessus megacephalus (Gschwendtner, 1931) is known to inhabit coastal rocky pools and coastal ponds. We found this species in the inland waters of Saitama Prefecture, Japan, located approximately 32 km from Tokyo Bay. This is the first record from Saitama Prefecture and the most inland record currently known for the species.

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Fourteen subtidal crabs of thirteen genera in seven families (Crustacea: Decapoda: Brachyura) are recorded from Suruga Bay, the Pacific coast of Japanese mainland. All of them listed in the following lines are southern in their distribution, and seven of them with an asterisk are extending the range northwards to Suruga Bay. Hyastenus borradailei (Rathbun, 1907) (Figs. 1A⎼B, 2) [Epialtidae]; Micippa philyra (Herbst, 1803) (Fig. 1E⎼F) [Majidae]; Charybdis amboinensis Leene, 1938 (Fig. 1C⎼D), Laleonectes nipponensis (Sakai, 1938), and Thalamita mitsiensis Crosnier, 1972 (Fig. 3E⎼F) [Portunidae]; Carpilius convexus (Forskål, 1775) (Fig. 3G) [Carpiliidae]; *Actaeodes mutatus Guinot, 1976 (Fig. 3B), *Etisus odhneri Takeda, 1972 (Fig. 3A), Macromedaeus orientalis (Takeda and Miyake, 1969) (Figs. 3H, 4), *Polydectus cupulifer (Latreille, 1812) (Fig. 3C⎼D), *Pseudoliomera lata (Borradaile, 1902) (Fig. 5E⎼F), and *P. variolosa (Borradaile, 1902) (Fg. 5A⎼D) [Xanthidae]; *Actumnus marissinicus Takeda and Kim, 1977 (Fig. 6A⎼D) [Pilumnidae]; *Crinitocinus alcocki (Borradaile, 1902) (Fig. 6E⎼F) [Acidopsidae].

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Two species of the genus Hydrovatus Motschulsky, 1853— H. onigiri Watanabe & Biström, 2022 and H. stridulus Biström, 1997—were collected from a pond in Nikaho City, Akita Prefecture, northern Honshu, Japan. Two species were found sympatrically in a pond characterized by rich emergent vegetation and humic bottom sediments. Their coexistence in this habitat, where exotic species such as Procambarus clarkii and Micropterus nigricans were also observed, suggests that Hydrovatus populations in this pond are potentially vulnerable. Continuous monitoring of the habitat is recommended for conservation purposes.

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Athelges pileus, a bopyrid isopod parasitizing the hermit crab Diogenes edwardsii, was collected off Ehime Prefecture, Japan, by beam trawl sampling. Prior to this study, this species was known only from its type locality in the Sea of Japan. Therefore, the present specimen represents the second record of A. pileus, as well as its first occurrence in the Seto Inland Sea.

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A molecular genetic analysis of freshwater unionid mussels collected in the Sagamigawa River (near Lake Sagami in northern Kanagawa Prefecture in central Japan) revealed that four of the mussels were Beringiana fukuharai. This is the first record of this species in this river basin. Morphological analyses were performed on the four adult specimens and the larvae they produced, and the results were compared with those of other congeneric species reported previously.

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The striped bitterling Acheilognathus cyanostigma (Cyprinidae: Acheilognathinae) is naturally distributed from the Chubu region centered around Lake Biwa to the Chugoku region of Japan. Although this species has been reported in the Nishiki and Saba River systems in Yamaguchi Prefecture, there are no documented records based on voucher specimens. The present study conducted morphological and molecular genetic analyses of specimens of this species collected from the Saba River system. The results of the molecular phylogenetic analysis indicated that the specimens originated from an introduced population, raising concerns about the potential negative impact on the native bitterling species in the river system.

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Early life stages of benthic decapod crustaceans are critical for dispersal and recruitment. We investigated the low-salinity tolerance of Petrolisthes japonicus, an intertidal anomuran crab, to infer its larval dispersal strategy. This species undergoes two zoeal stages and one megalopal stage. Given its broad distribution in East Asia, we hypothesized that it employs a larval export strategy, in which larvae develop in stable coastal or offshore waters before returning to fluctuating intertidal habitats. We exposed larvae and juveniles from three broods to various salinities at 24°C for 24 h to determine the median lethal salinity (MLS). MLS values for first zoeae (18.0–20.0 ppt) and second zoeae (16.7–17.8 ppt) were significantly higher than those for megalopae (7.0–8.0 ppt) and first crabs (6.1–10.3 ppt). These findings indicate an ontogenetic shift in salinity tolerance, with megalopae and juveniles being significantly more euryhaline than zoeae. This supports the export strategy hypothesis, suggesting that stenohaline zoeae rapidly emigrate to higher-salinity coastal or offshore waters, whereas the more tolerant megalopae subsequently return to the parental intertidal habitat for settlement.

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The freshwater fish fauna of the Koishibara River in the Chikugo River system, Fukuoka Prefecture, Japan were investigated in the alluvial fan section. Habitat-scale surveys were conducted in the autumn of 2022 and 2023, encompassing the heavy rainfall period of July 2023, and the two years were compared. The number of sites with presence of Coreoperca kawamebari, Rhynchocypris oxycephala jouyi, Pseudogobio esocinus, and Tachysurus aurantiacus increased, whereas those of Odontobutis obscurus, Carassius sp., Cobitis kaibarai, and Rhinogobius sp. decreased. Investigations of each species in the ”wando” (embayment connected to the mainstream) and ”tamari” (embayment closed off from the mainstream) revealed that, following periods of heavy rainfall, the population sizes of riverine fish such as P. esocinus, Cobitis matsubarae, C. kawamebari, and R. oxycephala jouyi increased, whereas those of Carassius sp. and Rhinogobius sp. decreased. These results indicate that river-dwelling species utilize embayments as refuges during major floods. The number of species found in natural and artificially dug ”wandos” was almost the same, and several threatened species not found in the natural ”wandos” were collected from the artificial ”wando”. Furthermore, the presence of species such as P. esocinus, C. kawamebari, and R. oxycephala jouyi increased in the artificial ”wando” after flooding. These results demonstrate the significance of digging ”wandos” even in the alluvial fan section.

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Individual rearing of cone snail larvae enables direct monitoring of survival, development, and growth on a daily basis and provides a greater number of replicates for evaluating the effects of environmental factors on these traits. We established an individual larval culture system using cell culture plates for two planktotrophic species, Conus fulmen and Conus pulicarius, and evaluated the effects of microalgal feeding concentration on veliger survival and growth. Veligers were cultured individually in artificial seawater (salinity 34 ppt; 25 °C) for 50 days after hatching at three concentrations of Chaetoceros calcitrans (1 × 10⁴, 5 × 10⁴, and 1 × 10⁵ cells/mL). Larval survival remained high (83–100 %) and did not differ significantly among feeding concentrations in either species. In contrast, feeding concentration significantly affected larval growth. In C. fulmen, growth was low at 1 × 10⁴ cells/mL but similarly high at 5 × 10⁴ and 1 × 10⁵ cells/mL. In C. pulicarius, growth was greatest at 5 × 10⁴ cells/mL. In both species, veliger larvae reached a shell-growth plateau accompanied by fully developed velar lobes, suggesting the acquisition of metamorphic competence. This study establishes a reliable individual culture method for Conus larvae and identifies suitable microalgal concentrations for sustained growth to the fully developed veliger stage.

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A single specimen of the little-known alpheid shrimp, Salmoneus aduncus Komai, 2022 was collected by Smith-McIntyre grab from Ehime Prefecture, western Japan. Although well sculptured major chela of the present material was closer to S. rostratus Barnard, 1962, morphological characters of other parts were well matched to the original description of S. aduncus. DNA barcoding of 16S rDNA region also supported the conspecificity between the present material and S. aduncus holotype. We regard that relatively shorter rostrum with more vestigial lower teeth and blunt posteroventral tooth of pleura 4 and 5 as potential diagnostic characters comparable to S. rostratus. The living colouration of the present material was reddish and more similar to that of S. rostratus than the original description of S. aduncus, described as pale orange. The symbiotic relationship between S. aduncus and Alpheus digitalis De Haan, 1844 was also suggested.

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The hybrid of the native freshwater mussel Sinohyriopsis schlegelii (Martens, 1861) and Sinohyriopsis cumingii (I. Lea, 1852), introduced from China, has been reported to be established in natural freshwater bodies in multiple localities in Japan. In Okayama Prefecture, possible hybrid specimens were recorded in 2025 in a channel of the Kurashiki River system at Kawahari, Minami-ku, Okayama-shi for the first time. In this study, five individuals assumed to be hybrid specimens were collected from four newly identified sites in Okayama Prefecture. Three of the sites are part of the Sasagase River system: Higashiune and Onoue, both irrigation ditches, and Yonekura, located in the main stream. The remaining site, Futsukaichi, is part of the Kurashiki River system. Although the record of hybrid specimens in Okayama Prefecture was limited to a single locality, this study confirmed that hybrid specimens occur across multiple water systems within the prefecture.

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We investigated the effects of temperature on the survival and growth of veliger larvae of the cone snails Conus fulmen and Conus pulicarius using an individual larval rearing system. Newly hatched veligers of C. fulmen were cultured at 21, 24, 27, and 30 °C, whereas those of C. pulicarius were cultured at 21, 24, 27, 30, and 33 °C. Larval survival and shell growth were monitored daily, and shell growth was analyzed using logistic growth equations. No larvae of either species metamorphosed during the experiments. Survival of C. fulmen remained high (91.7–100 %) at all temperatures and was not significantly affected by temperature. In contrast, survival time of C. pulicarius was significantly affected by temperature, and all larvae died by day 38 at 21 °C. Shell growth of both species was well described by logistic growth equations. The lower temperature limits for larval growth were estimated to be 18.6 °C for C. fulmen and 20.5 °C for C. pulicarius. These findings indicate greater low-temperature tolerance in veliger larvae of C. fulmen than in those of C. pulicarius and reveal species-specific thermal responses in larval performance that correspond to the geographic distributions of these species.

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Skeletal deformities are rarely documented in adult aquarium-reared grass carp (Ctenopharyngodon idella). This study reports an opportunistic observation of external spinal deformities in three adult individuals maintained in a public display aquarium in Kerala, India. Body curvature was consistent with kyphosis, lordosis, and kypho-lordosis. Despite these abnormalities, all affected fish were alive and exhibited routine swimming and feeding behavior. No radiographic or anatomical examinations were performed; therefore, the observations were limited to external examination and the underlying vertebral abnormalities could not be confirmed. This report expands the limited documentation of spinal deformities in captive adult grass carp and highlights the value of future radiological and anatomical investigations.

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Hermit crabs and their parasitic crustaceans constitute an ecologically important component of coastal benthic communities, yet their diversity and distribution in the northern Sea of Japan remain insufficiently documented. To update regional faunal records, we conducted a comprehensive coastal survey across Sado Island, Niigata Prefecture, from 21 to 25 October 2024. Eight hermit crab species belonging to two families were collected from several intertidal and shallow subtidal sites. Among them, Pagurus quinquelineatus and P. nigrivittatus were newly recorded from Niigata Prefecture, extending their known distributional ranges northward. Parasitological examination of all collected individuals revealed four species of parasitic crustaceans: the rhizocephalans Peltogaster postica and P. aff. ovalis, and the bopyrid isopods Athelges takanoshimensis and Parathelges enoshimensis. Both P. postica and P. aff. ovalis represent the first records from the Sea of Japan, with the former infecting P. filholi, and the latter infecting P. japonicus. Species identifications were supported by detailed morphological comparisons, and rhizocephalans were further confirmed by COI barcoding. These findings considerably expand the known biogeography of hermit crabs and their parasites in Japan. The discovery of multiple northernmost records highlights Sado Island as a major biogeographical boundary for warm-temperate hermit crab–parasite assemblages. The host specificity and spatial variation in parasitism documented here provide an essential baseline for future ecological, parasitological, and conservation-oriented studies in the Sea of Japan region.

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The cymothoid isopod Nerocila japonica is an ectoparasite known to infect a variety of marine fishes in Japanese waters. This study reports new host records for N. japonica from commercial fish species based on photographic and specimen evidence collected in Hiroshima Prefecture between 2022 and 2026. The Japanese white seaperch Sebastes cheni and the black scraper Thamnaconus modestus are newly identified as host species. Additionally, this study confirms that the red seabream Pagrus major, previously known only as a host for the aegathoid stage, is also utilized by the parasite after this stage. Based on the recent fish classification system, the host range of N. japonica is updated to 11 orders, 15 families, and 24 species, including eight commercially important fishes. Given that N. japonica causes significant tissue erosion at attachment sites, which degrades commercial value and poses health risks to hosts, continuous monitoring from a fisheries science perspective is warranted. Despite the expanded host spectrum, further investigations are needed to elucidate its geographical distribution, particularly in the Sea of Japan where records remain scarce.

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Temperature is a key determinant of species distributions in ectotherms, yet low-temperature tolerance remains poorly understood in cone snails of the genus Conus. This study quantified the lower critical temperatures for locomotor activity in five Conus species (C. coronatus, C. ebraeus, C. fulmen, C. lividus, and C. pulicarius) using adult specimens under controlled laboratory conditions. We predicted that southern-ranging species would exhibit greater sensitivity to low temperatures than the more northerly distributed C. fulmen. Individuals were exposed to stepwise decreases in temperature, and the temperature at which locomotor activity ceased was recorded. The lower critical temperature was lowest in C. fulmen (mean 6.0 °C), whereas the other species exhibited similar values ranging from 9.7 to 11.5 °C, supporting our prediction. These findings indicate lower cold tolerance in the southern-ranging species than in C. fulmen, broadly supporting the predicted relationship between low-temperature sensitivity and geographic distribution. Because the present study evaluated locomotor inactivity but not survival or recovery after prolonged low-temperature exposure, further studies are needed to clarify the ecological significance of these activity thresholds. The thermal thresholds estimated here provide useful baseline information for evaluating cold tolerance in Conus species.

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Metanephrops japonicus and M. sagamiensis (Decapoda: Astacidea: Nephropidae) inhabit deep-sea sandy-muddy bottoms at 200-400 m. This study examined egg coloration, stomach contents, and gastrolith-like structures in specimens from Suruga Bay. Ovigerous M. japonicus carried blue-green (Munsell 5PB 6/12) or yellow-amber eggs (Munsell 10R 6/10). Blue-green eggs were smaller (2.34 ± 0.18 vs. 2.96 ± 0.25 mm; t = 6.24, p < 0.001). Hard-shelled individuals fed mainly on detritus (62.1%, SD = 24.08); biological fragments comprised 14.3% (SD = 17.96), indicating a detritivorous diet. Soft-shelled M. japonicus and M. sagamiensis contained fragments derived from whitish spherical structures in their stomachs. These were absent from all hard-shelled individuals, occurred exclusively in recently molted individuals, and effervesced in hydrochloric acid, suggesting calcium-rich gastrolith-like structures for postmolt mineral storage. These findings clarify reproduction, feeding, and calcium storage in lobsters and suggest the occurrence of gastrolith-like structures in Nephropoidea.

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In Lake Kasumigaura, three species of sphaeromatid isopods have previously been recorded; however, their current distribution and habitat conditions remain poorly understood. We surveyed sphaeromatid isopods Gnorimosphaeroma in reed beds at 24 littoral sites in the Nishiura Basin of Lake Kasumigaura between 18 and 30 January 2026. Relationships between species occurrence and water quality variables, including dissolved oxygen (DO), chemical oxygen demand (COD), suspended solids (SS), and chloride ion concentration (Cl⁻), were analyzed. As a result, sphaeromatid isopods were collected from 15 sites, and all specimens were identified as Gnorimosphaeroma chinense. The species was widely distributed, although several areas lacked continuous occurrence. No significant relationships were detected between species occurrence and any water quality variables. Further research on local habitat conditions is needed to elucidate the factors affecting the distribution pattern of this species.

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Tokyo Port Wild Bird Park, a metropolitan park located in Ota Ward, Tokyo, is situated on reclaimed land created in the 1960s and contains multiple water bodies within its grounds. In 2024, the authors conducted three fauna surveys targeting aquatic insects in the park. As a result, a diverse range of aquatic insects was recorded, including multiple extremely rare species such as Hyphydrus japonicus, Coelambus chinensis, and Helochares pallens. This revealed that the park serves an important function as a habitat for aquatic insects. The coastal area around Tokyo Bay is one of the most highly developed regions in Japan, and water bodies capable of supporting the habitation and breeding of aquatic insects are limited. However, it is believed that the implementation of appropriate maintenance and management activities within the park has enabled aquatic insects dispersing by flight from other water bodies to establish populations there.

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