Water temperature is a key environmental factor shaping sex determination in many gonochoristic fishes, with direct consequences for population responses to climate warming. This study is the first examination of temperature effects on sex determination in an atherionid fish, a family belonging to a taxonomic group (Atheriniformes) with abundant examples of temperature-dependent sex determination (TSD). It integrates field observations with controlled rearing experiments to analyze the possibility of TSD in the bearded silverside Atherion elymus population of Tokyo Bay, near the northern limit of distribution of this species. Histological analysis of the gonads and daily age estimation using otolith increment analyses indicated that sex determination in wild fish was completed in both males and females at about 14 days after hatching and 10 mm standard length. Reconstruction of the thermal experience of individual fish during this period did not support the assumption of thermal effects on sex determination in wild individuals. Laboratory rearing experiments conducted with five families in environmentally-relevant temperatures also consistently failed to produce sex ratio biases. These results demonstrate that sex determination in the bearded silverside population of Tokyo Bay is largely insensitive to temperature and highlight the importance of integrative field- and experiment-based approaches for assessing temperature effects on sex determination under climate warming. Although the current findings negate the presence of TSD in the bearded silverside, examination of more southern populations is warranted to rule out the possibility of a latitudinal cline in thermal sensitiveness in this species.
The Australian marine atherinid fish, Hypoatherina tropicalis (Whitley 1948), having long been the subject of taxonomic confusion because of its wide intraspecific variation, is redescribed as a valid species based on the holotype and 19 non-type specimens collected from the east coast of Australia. In this process, we describe three new atherinid species, Hypoatherina gracilis sp. nov., Hypoatherina platysoma sp. nov. and Hypoatherina polylepis sp. nov., which were previously confused with H. tropicalis and Hypoatherina temminckii (Bleeker 1854). Hypoatherina gracilis sp. nov. and H. polylepis sp. nov. are distinguishable from all congeners except H. tropicalis in having a short and blunt ascending process of the premaxilla, its height 2.4–2.8 and 2.3–2.8 times the maximum width, respectively, and shorter than half of the premaxillary horizontal length. Hypoatherina gracilis sp. nov. can be distinguished from H. tropicalis by having a narrower interorbital width, 31–33
Understanding how genetic differentiation is maintained between closely related species with overlapping habitats is a central question in evolutionary biology. The sticklebacks Gasterosteus aculeatus and Gasterosteus nipponicus occur in sympatry with reproductive isolation on the Pacific coast of Hokkaido Island. However, their distributions and hybridization patterns in more northern regions remain largely elusive. Here, we analyzed species distributions and hybrid occurrences in Sakhalin, the Northern Territories, the Kuril Islands, the Pacific and Okhotsk coasts of Kamchatka, Chukotka, and the open Bering Sea using genome-wide nucleotide variant data. We found that G. nipponicus is largely restricted to Sakhalin, while G. aculeatus predominates in other regions, with only rare hybrids detected. These findings indicate largely allopatric distributions around the Sea of Okhotsk and the Bering Sea. These allopatric populations provide valuable opportunities for comparing patterns of genetic differentiation and phenotypic divergence between allopatric and sympatric species pairs.
The South Basin of Lake Biwa, Japan, has undergone significant changes in fish fauna due to the introduction of invasive species and habitat alterations. This study assessed the current status of native fish species recovery through environmental DNA (eDNA) metabarcoding and capture surveys conducted in 2008 and 2021. The eDNA survey detected 40 fish taxa, including 31 native and nine non-native taxa, while capture surveys identified 18 taxa. Notably, ten native taxa absent from the 2008 survey were detected in both the 2021 capture and eDNA surveys, indicating their potential recolonization. Among these, eight taxa not subject to fisheries stocking suggest natural population recovery. The decline in invasive largemouth bass (Micropterus nigricans) and bluegill (Lepomis macrochirus) populations may have contributed to this resurgence. Additionally, eDNA surveys detected several benthic and elusive species, highlighting their effectiveness over traditional capture methods. A novel detection of Gobio gobio, a Eurasian cyprinid with no prior records in Japan, suggests possible human-mediated introduction. Conversely, nine native species previously recorded in the South Basin were not detected, warranting conservation attention. The study underscores the utility of eDNA metabarcoding for biodiversity monitoring and conservation, providing a non-invasive and comprehensive approach to assessing fish community dynamics in high-diversity lake ecosystems.
A new ophidiid fish, Sirembo quinquefasciatus, is described based on the holotype and 13 paratypes, 125–172 mm in standard length, collected from the southern South China Sea (Gulf of Thailand, off the east coast of Peninsular Malaysia and off the west coast of Borneo). The new species can be distinguished from all other congeners by having two dark longitudinal bands on the anal fin, one along its base, the other submarginal. Furthermore, S. quinquefasciatus sp. nov. differs from Sirembo amaculata (Cohen and Nielsen 1982) and Sirembo wami Nielsen, Schwarzhans and Uiblein 2014 in having several blotches on the dorsal fin (vs. a single dark longitudinal band in S. amaculata and only two blotches in S. wami). The present new species can be distinguished from Sirembo metachroma Cohen and Robins 1986 by having dark bands along the dorsal-fin base and the lateral line (vs. no band along dorsal-fin base and a distinctive black streak along lateral line in the latter). The new species resembles Sirembo imberbis (Temminck and Schlegel 1846) and Sirembo jerdoni (Day 1888) in sharing many dark blotches on the dorsal fin and several dark longitudinal bands on the body and the anal fin. The former is distinguishable from S. imberbis by having 6–8 scale rows between the dorsal-fin origin and the lateral line (vs. 9–11), and from S. jerdoni by the position of the dorsal-fin origin (above fourth or fifth vertebra vs. first to third).
Size-dependent survival is a well-known phenomenon in fishes, typically positive but with exceptions. I investigated survival of chum salmon, Oncorhynchus keta, smolts in coastal waters of the Shiretoko Peninsula, northern Japan, using tethering experiments. Of 111 smolts, only 11 survived. Survival was explained solely by body size and was negatively correlated with it. These results suggest that predator foraging strategy and hatchery-wild differences may underlie the observed pattern, with implications for salmon management strategies.
Ocosia fasciata Matsubara 1943 (Synanceiidae: Tetraroginae) is redescribed on the basis of the lectotype, 37 available paralectotypes and 39 non-type specimens from Japan and Taiwan. A reassessment of diagnostic characters within the genus is made, with emphasis on comparisons with the morphologically similar congener Ocosia zaspilota Poss and Eschmeyer 1975, and two other Japanese congeners, Ocosia spinosa Chen 1981 and Ocosia vespa Jordan and Starks 1904. Ocosia fasciata is distinguished from O. zaspilota by a relatively narrower body and longer head, in addition to previously recognized differences in the lengths of the anterior dorsal-fin spines. Ocosia fasciata also tends to have slightly fewer lateral-line tubes and gill rakers that O. zaspilota. Ocosia spinosa and O. vespa share a combination of characters, including usually 16 dorsal-fin spines (almost always 15 in O. fasciata), the presence of lateral lacrimal and suborbital spines (almost always absent), moderately incised or weakly concave interspinous dorsal-fin membranes (strongly incised), and a relatively broad membrane between the last pelvic-fin soft ray and abdomen (narrow). Compared with O. spinosa, O. vespa has broader dorsal-fin membranes, apparently lacking incisions in life, although slightly concave in preserved specimens, and densely distributed papillae on both jaws. Molecular analyses based on mitochondrial COI sequences (443 bp) showed no clear interspecific divergence among the four species, despite clear morphological differentiation, suggesting that COI-based DNA barcoding does not consistently match morphological species definitions in Ocosia.
Sex changes in fish are commonly driven by social or reproductive advantages associated with body size, and protogyny is particularly prevalent in species exhibiting male territoriality or haremic mating systems. Within the family Serranidae, several species exhibit sexual dimorphism and protogyny; however, in the genus Selenanthias, only Selenanthias analis Tanaka 1918 has been reported to exhibit sexual dimorphism. The present study investigated sexual dimorphism and the occurrence of protogynous hermaphroditism in two serranid species, S. analis and Selenanthias sp. sensu Ikeda and Nakabo 2015, based on external morphology, gonadal histology, and mitochondrial DNA sequences (partial COI region). Morphological and histological analyses revealed consistent sex-specific coloration and differences in the fin filament lengths in both species. Six individuals of S. analis possessed gonads containing both ovarian and testicular tissues. Among these, two individuals morphologically identified as males and four individuals represented both female and male characteristics. In addition to these six individuals, two individuals exhibited both female and male morphological characteristics but possessed only ovarian tissue. Genetic analyses supported the recognition of two distinct species (S. analis and Selenanthias sp.) and confirmed that the color morphs within each species represent intraspecific sexual dimorphism. These findings provide the first evidence of sex change in the genus Selenanthias, establishing S. analis as a protogynous species and suggesting a similar potential for sex change in Selenanthias sp.
Although a ninespine stickleback (Pungitius d'Annone 1760) inhabiting the Kanto region of Honshu Island, Japan, was reported as an undescribed species in the early 1960’s, it has at no time been formally described. Morphologically similar to Pungitius sinensis (Guichenot 1869), the former is now described as Pungitius nakamurai sp. nov., being usually differentiated from all other congeners by the following combination of characters: dorsal-fin rays IX + 11; anal-fin rays I + 9; pectoral-fin rays 10; pelvic-fin rays I + 1; procurrent caudal-fin rays 5 + 5 = 10; lateral plates in an incomplete row, forming a distinct lateral keel on caudal peduncle; 6 plates on ventral surface of caudal peduncle; vertebrae 33 (14 abdominal and 19 caudal vertebrae); dorsal-fin spines inclining alternately to sides of mid-line; first dorsal-fin spine antero- or postero-dorsal to upper end of pectoral-fin base; ascending process of pelvis well-developed, upper end of pelvis reaching to level of fifth pectoral-fin ray base; antero-ventral process of ectocoracoid present, right and left ectocoracoids articulating anteriorly with each other; no distinct ridge on cleithrum and ectocoracoid; body dark yellow or dark green; membranes of dorsal-fin spines hyaline or with a few minute black spots; membrane of pelvic-fin spine of adult males white, becoming blue during courtship behavior; membrane of anal-fin spine of adult males whitish with a distinct white blotch (turning blue during courtship).
Priacanthus starnesi sp. nov., a bigeye (Perciformes: Priacanthidae) from the western Pacific Ocean, previously misidentified as Priacanthus blochii Bleeker 1853, is described based on its morphological and genetic distinctiveness. The new species is characterized by the following combination of characters: dorsal-fin soft rays 13 or 14; anal-fin soft rays 14 (rarely 15); total gill rakers on first arch 20–22 (modally 21); scales in lateral series 82–87 (rarely 80 or 81); scale rows below lateral line B (counted posteriorly from anal-fin origin to lateral line) 30–32 (rarely 29); 10th dorsal-fin spine short, 12.6–15.1
In China, the eightbarbel loach Lefua costata (Kessler 1876) is widely distributed across northern provinces, with the Huai River as its southern limit. However, anecdotal reports suggested its presence further south in Zhejiang Province. In this study, we collected 17 putative L. costata specimens from four Zhejiang sites, alongside 27 specimens from five northern populations for comparison. Analyses included 23 morphometric measurements, five meristic traits, and mitochondrial COI and cytb genes. Morphological characteristics (separated nostrils, tube-like anterior nostrils, absent epural, lower lip with four distinct median papillae) confirmed the Zhejiang specimens as L. costata. Phylogenetics showed they represent a distinct southern lineage, sister to the Huang River population within L. costata. This disjunct distribution may suggest southeastward glacial dispersal followed by postglacial isolation of this species. The finding also aids in reevaluating the taxonomy of Lefua sayu (Herre and Lin 1936), known from a single specimen collected from Zhejiang.
Maturation and spawning in the Japanese codling Physiculus japonicus were described in terms of breeding behavior, spawning period, daily spawning time, and gonad analysis for captive individuals (n = 46). The spawning of Japanese codlings could be observed daily from 1 January to 23 May 2024. The average egg batch weight (n = 5) per spawning by a pair was 12.7 g (9.5–16.2 g), while the average number of eggs per batch was 18,161 (13,585–23,166). Observing daily spawning activity, there was no spawning during the day from 0400 to 1600 hours. Spawning activity was observed at night, with peak spawning occurring from 2000–2400 hours. Sampling twelve fishes (6 males and 6 females, total length 327–418 mm) for gonad analysis, the gonadosomatic index of females was significantly higher than that of males. The oocyte diameter distribution in almost all females was bimodal, with two peaks corresponding to yolked oocytes at various stages and hydrated oocytes. These showed typical maturation patterns - spawning many batches in a spawning season. Histological observations confirmed this conclusion.
As global warming progresses, many tropical and subtropical species are now observed in temperate regions, including the Japanese Archipelago. Similar range shifts are also expected within species, with increasing influxes of individuals from southern populations, potentially altering their population genetic structures. The flagtail Kuhlia marginata and Kuhlia rupestris (Perciformes: Kuhliidae) are catadromous fishes distributed across the tropical and subtropical western Pacific and adjacent areas. They are believed to reproduce around the Ryukyu Islands in Japan as well. Carried by warm currents, such as the Kuroshio Current, they appear in the Japanese Archipelago, but are generally thought to die during winter. However, recent reports suggest their successful overwintering in certain areas in the archipelago. Global warming may enable such individuals to reproduce around the Japanese Archipelago or increase the influx of individuals from more southern regions to both the Japanese Archipelago and the Ryukyu Islands, raising the possibility of changes in population structures in Japan. To explore such potential changes, we investigated the current population genetic structure of these two species in Japan. Population analyses based on partial mitochondrial DNA sequences (including publicly available data) and genome-wide polymorphism data obtained by the MIG-seq method supported the hypothesis that each species constitutes a single population within Japan. In addition, there was no significant difference in the genetic diversity indices between regional samples from the Japanese Archipelago and the Ryukyu Islands. These results propose two possible scenarios: K. marginata and K. rupestris reproduce in the Ryukyu Islands and a significant number of juveniles born from genetically unbiased parentals are transported to the Japanese Archipelago by the Kuroshio Current, sufficient to maintain the genetic diversity of the original population; or these species reproduce mainly in more southern regions, not even around the Ryukyu Islands, and the individuals born in the former are transported to Japan. In addition, the subtle population structure observed exclusively in the mtDNA of K. rupestris across the western Pacific suggests that ecological differences between K. marginata and K. rupestris have contributed to their different population structures, and may further lead to divergent changes in their future population structures.
The arrangement of the sensory epithelium over the surface of the olfactory lamellae in teleosts is variable, with a diverse pattern ranging from large expanses of continuous epithelium to numerous discrete islets. As the olfactory epithelium is the location where water-borne odorants interact with olfactory receptors, its spatial distribution pattern serves as an important anatomical proxy for olfactory ability. However, nothing is known about the olfactory epithelial pattern in southern bluefin tuna, Thunnus maccoyii. Scanning electron microscopy was used to examine the spatial distribution pattern, area, and number of olfactory epithelial patches over the olfactory lamellar surface within the olfactory rosettes of both juvenile and mature T. maccoyii. Juvenile T. maccoyii exhibit a novel olfactory epithelial topography, where the lamellae in the caudal parts of the olfactory rosette display a combination of Type IV (olfactory epithelium arranged into numerous, small discrete islets) and Type III (olfactory epithelium irregularly intermingled with non-sensory epithelium) topographies. The lamellae in the rostral parts of the olfactory rosettes primarily have Type IV topography. In both arrangement patterns, the olfactory epithelia are smaller and more numerous rostrally. In mature T. maccoyii, the Type IV topography disappears, and lamellae in all parts of the olfactory rosette display exclusively Type III topography. The combined results of this study reveal a new and unique distribution pattern of olfactory sensory epithelia in T. maccoyii, with a striking change in distribution pattern associated with both lamellar location and growth, indicating adaptation to the different hydrodynamic conditions.
