The concept of the ecological niche, defined as the basic habitat requirements for a species, is central to understanding species geographic distributions and predicting their responses to environmental change. However, identifying the essential niche for large regional communities remains a challenge because niche axes can be “hidden” by the complexity of the underlying ecological processes. Here, applying advanced species distribution modelling to nationwide environmental DNA survey data, we identified hidden niche axes of the Japanese coastal fish community and investigated the response diversity to these axes. Our survey detected 1,220 coastal fish species. The hidden niche axes collectively explained most of the variation in fish biodiversity and revealed five biogeographic boundaries for the regional community. These niches of the Japanese fish community may primarily relate to several processes due to ocean currents, such as environmental filters, transport from source areas and dispersal barriers. We also found that the response diversity to niche axes was positively correlated with species richness, although local communities with particularly high response diversity were geographically biased. A better understanding of the niche axes of the regional ecological community should help to mitigate the loss of biodiversity and ecosystem services caused by ongoing environmental change.

Taxonomy of the genus Rhinogobius was investigated for the first time in Central Vietnam. Five species occurring in this region can be distinguished by shape of their head, number of pectoral-fin rays, vertebral counts, arrangements of scales and cutaneous sensory papillae, and coloration. Two species are described as new, Rhinogobius rong sp. nov. and R. nami sp. nov., and the remaining three were identified as R. taenigena, R. leavelli, and R. similis. Though the morphology of R. leavelli and R. similis differed slightly between localities, counts and measurements overlapped, and coloration was variable. This study tentatively identifies all of those populations as R. leavelli or R. similis, but further research using additional specimens from a wider geographic range is required to clear the identity of the Rhinogobius occurring in this region. Rhinogobius rong, R. similis, and R. leavelli are distributed in both Da Nang City and Thua Thien Hue Province, with the first two species occurring widely in coastal areas, and R. leavelli restricted to inland areas. On the other hand, Rhinogobius nami and R. taenigena occur only in inland areas of Da Nang City and Thua Thien Hue Province, respectively.

- Short-steep-coastal-streams of the tropical Indo-Pacific region are largely dominated by faunal assemblages of diadromous fish, crustacean and mollusc species. These streams are inherently difficult to survey owing to their characteristic dense vegetation, steep terrain and shallow riffle-run-pool sequences. In this study, we document the first records of four sicydiine species (Sicyopterus lagocephalus, Sicyopus zosterophorus, Stiphodon atropurpureus, and Stiphodon percnopterygionus) from snorkel-based surveys in five streams near Da Nang in central Vietnam. Specimens were preserved and identified to validate field-based identifications. Here we discuss the implications of these new records for conservation planning in Vietnam and for understanding the biogeography of sicydiine gobies in the Indo-Pacific region.

Colonisation of freshwater habitats by marine animals is a remarkable evolutionary event that has enriched biodiversity in freshwater ecosystems. The acquisition of tolerance to hypotonic stress during early life stages is presumed to be essential for their successful freshwater colonisation, but very little empirical evidence has been obtained to support this idea. This study aimed to comprehend the evolutionary changes in osmoregulatory mechanisms that enhance larval freshwater tolerance in amphidromous fishes, which typically spend their larval period in marine (ancestral) habitats and the rest of their life history stages in freshwater (derived) habitats. We compared the life history patterns and changes in larval survivorship and gene expression depending on salinity among three congeneric marine-originated amphidromous goby species (Gymnogobius), which had been suggested to differ in their larval dependence on freshwater habitats. An otolith microchemical analysis and laboratory-rearing experiment confirmed the presence of freshwater residents only in G. urotaenia and higher larval survivorship of this species in the freshwater condition than in the obligate amphidromous G. petschiliensis and G. opperiens. Larval whole-body transcriptome analysis revealed that G. urotaenia from both amphidromous and freshwater-resident populations exhibited the greatest differences in expression levels of several osmoregulatory genes, including aqp3, which is critical for water discharge from their body during early fish development. The present results consistently support the importance of enhanced freshwater tolerance and osmoregulatory plasticity in larval fish to establish freshwater forms, and further identified key candidate genes for larval freshwater adaptation and colonisation in the goby group.

Colonisation of freshwater habitats by marine animals is a remarkable evolutionary event that has enriched biodiversity in freshwater ecosystems. For successful freshwater colonisation, high physiological plasticity is presumed to be necessary, but its evolutionary basis has not been detailed. Marine-originated amphidromous species, which regularly migrate between freshwater and marine environments, have repeatedly lost migratory behaviour in many lineages, which sometimes triggered species radiation in freshwater habitats. Since amphidromous species typically visit the sea during the larval period, the difficulty in the evolution of larval freshwater tolerance is a bottleneck for freshwater colonisation. To elucidate the key evolutionary changes that enhance the physiological plasticity for freshwater colonisation, we compared larval gene expression changes depending on salinity conditions among three congeneric amphidromous goby species (Gymnogobius) with varying dependences on freshwater habitats. First, an otolith microchemical analysis and rearing experiment under laboratory conditions confirmed the presence of freshwater residents only in G. urotaenia and higher larval survivorship of this species both in seawater and freshwater conditions than the obligate amphidromous G. petschiliensis and G. opperiens. Larval whole-body transcriptome analysis revealed that G. urotaenia exhibited the greatest differences in the expression levels of several osmoregulatory genes, including aqp3, which is critical for water discharge from their body during early fish development. Thus, we obtained the results that consistently support the importance of enhanced osmoregulatory plasticity for establishing freshwater forms, and further identified some important evolutionary changes for larval freshwater adaptation and colonisation in the goby group.

The larval period is a critical stage for many aquatic organisms, because larvae are small, fragile, and have high mortality rates. Amphidromous fish spend their larval stages in the sea, and larval durations vary, possibly reflecting their dispersal tendencies. The genus Rhinogobius (suborder Gobioidei) is widely distributed throughout Asia and exhibits a variety of migratory patterns. Despite some studies of their migratory histories, larval durations of Rhinogobius species are uncertain. This study used analyses of otolith microstructure and trace-elemental chemistry to investigate habitat use throughout the life history of R. nagoyae. Twenty-six adult gobies were collected from seven rivers on Sado Island, in northern Japan, and otolith Sr : Ca ratios were quantified using electron microprobe analysis to infer amphidromous movement. All individuals showed shifts in Sr : Ca ratio across the otolith transect, which indicated movement between freshwater and saltwater. Marine larval duration was estimated by otolith increment counting, combined with Sr : Ca ratio. Larval duration ranged from 29 to 45 days (mean, 35.3), and there were no significant differences among rivers on the island. The relatively short larval duration of R. nagoyae, compared with other amphidromous fish such as ayu and galaxids, may reflect environmental factors such as island size, cool weather or species characteristics.

The larval period is a critical stage for many aquatic organisms, because larvae are small, fragile, and have high mortality rates. Amphidromous fish spend their larval stages in the sea, and larval durations vary, possibly reflecting their dispersal tendencies. The genus Rhinogobius (suborder Gobioidei) is widely distributed throughout Asia and exhibits a variety of migratory patterns. Despite some studies of their migratory histories, larval durations of Rhinogobius species are uncertain. This study used analyses of otolith microstructure and trace-elemental chemistry to investigate habitat use throughout the life history of R. nagoyae. Twenty-six adult gobies were collected from seven rivers on Sado Island, in northern Japan, and otolith Sr:Ca ratios were quantified using electron microprobe analysis to infer amphidromous movement. All individuals showed shifts in Sr:Ca ratio across the otolith transect, which indicated movement between freshwater and saltwater. Marine larval duration was estimated by otolith increment counting, combined with Sr:Ca ratio. Larval duration ranged from 29 to 45 days (mean, 35.3), and there were no significant differences among rivers on the island. The relatively short larval duration of R. nagoyae, compared with other amphidromous fish such as ayu and galaxids, may reflect environmental factors such as island size, cool weather or species characteristics.

AbstractThe silver pomfret (Pampus argenteus) is one of the most commercially important marine fish species in the Bay of Bengal, Indian Ocean. However, detailed information on the reproductive biology of silver pomfret is limited for the Bay of Bengal, Bangladesh. To understand their reproductive biology, especially the gonadal developmental cycle and spawning season, a total of 373 individuals were collected monthly from February 2016 to January 2018 and examined. The total length (TL) and weight (TW) of P. argenteus ranged from 18.82–35.73 cm and 89.26–617.60 g, respectively. The highest gonadosomatic index (GSI) values were observed from April–June with a second (smaller) peak in October. The highest oocyte diameter was observed in May (630.50 ± 96.70 μm), when the fish reached full maturity, and the lowest was observed in July (35.10 ± 7.12 μm), when new eggs began their development for the next spawning season. The maximum number of eggs in the ovary were found in the yolk granule stage in May with a number of post-ovulatory follicles, which indicated that the ovaries developed to a peak leading up to the spawning season. A number of spermatids were also seen in males in April–June along with females. From these detailed observations, it may be concluded that P. argenteus shows two spawning peaks, in May–June and October. This observation will be helpful for the artificial breeding, sustainable management and conservation of this species in the study area.

The genus Acanthogobius of gobiioid fish has been reported for six species from East Asia, and inhabits estuarine and coastal waters. Within this genus, Acanthogobius insularis is an endangered and endemic goby in the Amami-oshima and Okinawa-jima Islands, southern Japan, and its range is restricted to the lowermost course of a few river basins. Basic knowledge on this species is scarce in spite of its vulnerable conservation status. The purpose of this study was to elucidate the life history of A. insularis. Monthly sampling was conducted at five stations in the Taiho River, Okinawa-jima Island, from November 2014 to November 2015. Monthly standard length (SL) distributions were unimodal except in April during the recruiting period, suggesting that A. insularis is an annual species. Analysis of the gonadosomatic index and histological observations of the ovaries revealed that this species spawns from January to May. The beginning of the spawning season seems to be related to a decline in water temperature in December. Growth rates appeared to be lower from April to December and higher in winter months. From monthly collections, A. insularis was found to move upstream with growth, and gather at spawning grounds during the reproductive season. Acanthogobius insularis might be threatened by increasing water temperature due to climate change, since low water temperatures appear to be important for their reproduction and growth. Moreover, habitat diversity, from tidal flats for recruiting grounds to upstream sites with cobbles for spawning, is needed to complete their life cycle, and should be conserved.

Mochizuki and Fukui (Jpn J Ichthyol 30 () 27-36) studied the development and replacement of the upper jaw teeth in a Japanese fish species, Sicyopterus japonicus (Gobioidei: Sicydiinae), and they reported that worn-out functional teeth in the upper jaw were not shed outside the skin but were taken into the soft tissue of the upper jaw and completely resorbed there. To date, however, this phenomenon appears poorly documented. Furthermore, the mechanism for the resorption of these teeth remains to be determined. In this study, we examined this phenomenon by using 3D microcomputed tomography (m-CT), scanning electron microscopy (SEM), and various techniques of light (LM) and electron (EM) microcopy. This study demonstrated that the upper jaw dentition of this fish was more or less simultaneously replaced with the replacement occurring during short time periods and that the lingual movement of the replacement teeth to the functional tooth position advanced simultaneously in a given row. Furthermore, our study also revealed that many worn-out functional teeth were engulfed by the oral epithelium, invaginated into the lingual shallow ditch of the premaxilla, and were resorbed/degraded completely by numerous foreign body giant cells rather than by odontoclasts during periods of at least three intervals of tooth replacement. The complete resorption/degradation of worn-out functional teeth in the soft tissue of the upper jaw suggests the possibility of the reuse of their components (minerals such as Ca and P, including Fe) for rapid and successional production of new replacement teeth in the upper jaw of adult S. japonicus. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 301:111-124, 2018. © 2017 Wiley Periodicals, Inc.

The ayu (Plecoglossus altivelis) is an annual, amphidromous, plecoglossid fish, distributed in Vietnam, China, Taiwan, Korea, and Japan. To date, ayu have been found only in two rivers in northern Vietnam, where little is known about their life history. The Tien Yen River is believed to be the most southwestern habitat for this species. To determine whether newly hatched ayu larvae drift and to understand their downstream migration, intensive surveys were conducted in the Tien Yen River from October to March of 2013–2016. In total, 529 drifting ayu larvae were collected from four of six sampling stations along the river. Thus, ayu reproduction has been confirmed in this river for the first time, where only adult fish had been found previously. However, we did not successfully collect larvae in the eastern branch of the river, which has a hydroelectric dam, suggesting that ayu do not inhabit this branch or else do not reproduce there. The presence of drifting larvae in the western branch from mid-December to late January implies that they spawn from late November to mid-January. Drifting larvae were captured primarily at night, but peak occurrences varied depending upon the day and the sampling station. With the range of body sizes and variable diel abundance patterns, ayu in the Tien Yen River probably employ multiple spawning grounds. This study provides fundamental life history data for the vulnerable ayu populations in northern Vietnam.

Recruitment from the sea to freshwater is a turning point in the life history of amphidromous fish. The amphidromous gobioid subfamily Sicydiinae is distributed widely in Indo-Pacific regions. To understand their ecology at recruitment, which is poorly known for this subfamily, underwater observations of recruiting post-larvae of Sicyopterus japonicus were conducted and their swimming depth was measured in the estuary of the Ota River of Wakayama, Japan. During 16 observation periods from April to July in 2006,2007 and 2008, newly recruiting post-larvae were observed swimming in shoals in the middle layer of the lower half of the water column near the river banks in the upper area of study site and in the upper half of the water column near the river mouth. Although the estuary appears to be a temporary transit area for newly recruiting post-larvae, it may be an important place for them to adapt to their later benthic herbivorous life in freshwater.

Objective: The present study was aimed at elucidating the time and order of eruption of first functional teeth in the upper jaw of post-larval life of Sicyopterus japonicus (S. japonicus) during cranial metamorphosis at the time of river recruitment.Design: Fishes were caught at the post-larval stage at a river mouth and maintained for 7 days in a water tank. Each of 10 specimens was evaluated every day for 7 days by using microcomputed tomography, scanning electron microscopy, and light microscopy with peculiar attention to the development of the upper jaw teeth.Results: Fishes caught at the river mouth were mostly transparent, with a rostral terminal mouth, and no teeth could be found in either the upper or lower jaw. At 2 days after collection, the mouth position changed from terminal to subterminal, resulting from a change in head shape. The initial eruption of first functional teeth was detected at the anterior two-thirds region of each upper jaw. These teeth erupted in adjacent positions, most had a tricuspid crown, and they represented miniature versions of adult teeth. At 5 days, the position of the mouth became further relocated from terminal rostral to ventral. The number of erupted teeth increased, followed by spreading of them anteriorly and posteriorly. At 7 days, they formed a single row of close-set tricuspid teeth along the entire length of each upper jaw.Conclusions: The present study demonstrated that even under laboratory conditions a rapid and drastic cranial metamorphosis took place within a week after the time of collection of post-larval S. japonicus from a river. The eruption of first functional teeth in the upper jaw of S. japonicus, which teeth are adapted to scraping algae off the substrate, was initially detected at 2 days after collection, and first functional dentition of the upper jaw was set up within 7 days after it. (C) 2016 Elsevier Ltd. All rights reserved.

Amphidromy is a diadromous life history pattern where fish spawn in freshwater, and their larvae drift downstream to the sea; the larvae develop in marine environments then migrate back in rivers to grow and reproduce. Two amphidromous types with different life history characteristics, such as egg and larval sizes, exist. To understand the ecology and early life history of amphidromous gobioid fish, six species from Okinawa Island were selected—two large egg-type species (Rhinogobius similis and Tridentiger kuroiwae) and four small egg-type species (Stiphodon percnopterygionus, Stenogobius sp., Sicyopterus lagocephalus, and Eleotris acanthopoma). The migratory pattern of four of these species was confirmed using otolith Sr:Ca and Ba:Ca ratios combined with water chemistry analysis. Although these species showed amphidromous migratory patterns, the timing of migration from estuarine to freshwater habitats was species-specific. The large egg-type, R. similis, showed three different migratory patterns: a long marine larval phase with a relatively fast migration from estuarine to freshwater habitats, a short marine larval phase with a relatively fast migration, and a gradual migration. Similar patterns of a long and fast migration or a gradual migration were seen in T. kuroiwae; however, the two small egg-type species, Sti. percnopterygionus and Stenogobius sp., showed rapid migration to freshwater after entering the river. To estimate larval ecology in the sea, ontogenetic changes in specific gravity (SG) were examined in all species. The SG was measured day and night for 1–5days until settlement in R. similis and T. kuroiwae, and until 10days after hatching in the other species. The SG of all species ranged from 1.0138 to 1.0488, and varied among ontogenetic stages and between day and night and species. Larval SG was relatively similar between R. similis and T. kuroiwae, with low SG in the early stages and high SG after yolk absorption. During the late larval stages and until settlement, T. kuroiwae showed diel changes in SG, with higher SG during the day, whereas R. similis had a relatively constant pattern. The diel changes of T. kuroiwae larvae suggest different activity during the day and at night (e.g. diel vertical migration). In the four small egg-type species, SG was high at hatching and decreased thereafter, not showing large diel changes. The results suggest that sympatric amphidromous gobioid species have various early life histories that may be influenced by several larval traits, including SG.

Amphidromy is a type of diadromous life history that includes migrations between the ocean and freshwater. The amphidromous fish Sicyopterus japonicus spends its early life in the ocean and then migrates back to estuaries as post-larvae, but little is known about their transition between marine and freshwater habitats. To understand their oceanic larval duration and recruitment mechanism, an intensive three-year survey was conducted in the Ota River, Wakayama, Japan. Their general amphidromous migratory patterns were confirmed using otolith Sr:Ca ratios that all showed an oceanic larval phase. Newly recruited S. japonicus post-larvae were sampled daily in the estuary from March to September 2006, 2007 and 2008. Post-larvae were collected mainly from April to June (spring to early summer), but the number of post-larvae collected varied among years (N = 12,766 in 2006, 372 in 2007 and 942 in 2008). No clear relationships were observed between environmental factors, such as moon phase and water temperature, and the number of post-larvae collected each day, but post-larvae were observed to recruit with flood tides (flood tide transport). Standard lengths of collected post-larvae ranged 22.5-34.0 mm, and the oceanic larval durations estimated by their otoliths ranged from 176 to 283 days (mean: 219 days). Although there were no clear trends in length, body weight, condition factor and larval duration during each recruitment season, most post-larvae recruited at a constant size of approximately 27 mm. Our results indicated that amphidromous S. japonicus have a limited recruitment season and a generally constant body size when they recruit. Their recruitment season was shorter than that of similar species inhabiting tropical and subtropical regions, which might be an adaptation to seasonality of the temperate region. It is possible that body size is a more important factor affecting the timing of recruitment of S. japonicus post-larvae than environmental factors. (C) 2015 Elsevier B.V. All rights reserved.

Amphidromy is a distinctive form of diadromy, but differences in the life histories of tropical and temperate amphidromous fishes suggest that there are two types of freshwater amphidromy. The life histories of Sicydiinae gobies, ayu (Plecoglossus altivelis), Japanese sculpins (Cottus) and galaxiids (Galaxiidae), suggest that the Sicydiinae are representatives of tropical freshwater amphidromy, whereas ayu, sculpins and galaxiids are representatives of temperate freshwater amphidromy. The Sicydiine larval stage may be required to occur in the ocean for all species, but ayu, sculpins and galaxiids have landlocked or fluvial forms with larvae that do not need to enter the ocean for larval feeding and growth. This suggests that Sicydiine larvae have a high oceanic dependency whereas ayu, sculpins and galaxiid larvae have a low oceanic dependency. Freshwater amphidromous fish in tropical and temperate zones appear to have developed two different strategies in the evolution of their life histories. It is likely that the evolutionary direction of the larval stage of tropical amphidromy is to remain in the sea and that of temperate amphidromy is towards having the ability to remain in freshwater if needed. Tropical and temperate amphidromy appear to be biologically informative categories and evaluations of this hypothesis will facilitate better understanding of the various forms of amphidromy in the future.

Even if amphidromous fish species contribute most to the diversity of fish communities in the tropical insular rivers, their biological cycle remain poorly known. For the first time, the otolith elemental composition and microstructure of two ancestral gobioids, Rhyacichthys guilberti and Protogobius attiti, were investigated to describe their biological cycle and pelagic larval duration (PLD). The otolith analysis using a femtosecond laser ablation coupled to an inductively coupled plasma–mass spectrometer (fs-LA-ICP-MS) revealed an amphidromous life history for R. guilberti and it suggested a progressive habitat shift from a marine habitat to a freshwater environment for P. attiti. For the first time, an endemic species, P. attiti, showed longer and more variable PLD (55.2 ± 13.5 days) than did a widespread one (R. guilberti: ~30 days). These results need to be confirmed by analysing more samples but suggest that factors other than the PLD control endemism and dispersal processes. In association with this first description of the biological cycle for both species, such an approach is a prerequisite for the management and conservation of both patrimonial species.