Evidence that the climate is changing is indisputable (Berrang-Ford et al., 2011), and this phenomenon poses a potential threat to the planet’s biodiversity (Kappelle et al., 1999). Over the past 100 years, the Earth’s climate has visibly warmed, precipitation regimes have changed, and biologists have been concerned about the impact of these shifts on species distribution (Araújo and Rahbek, 2006). Under the background of global climate change, the study of species distribution range has become the premise of biodiversity conservation strategy, especially for those endemic species with narrow distribution range. For example, the endangered endemic annonaceae species in East Africa may lose some of their original suitable habitats due to future warming (Mkala et al., 2023). In addition, climate change will drive some species to higher altitudes or latitudes (Hickling et al., 2005; Chen et al., 2011; Liao et al., 2023). Over the past few decades, more than 1,700 species have been confirmed to migrate poleward at a rate of 6.1 km/decade (Parmesan and Yohe, 2003), and the rate of migration has doubled or tripled in recent years to a staggering 16.9 km/decade (Chen et al., 2011). Of the species studied, the current focus is mainly on plants and various insect groups, etc., while relatively little is known about vertebrates, especially these tropical endemic fish.

Hainan medaka (Oryzias curvinotus) is a small euryhaline fish that is confined to the coastal waters of the South China Sea, including mangroves, brackish water confluxes in estuaries, and even freshwater streams extending inland (Dong et al., 2021). It is small in size, usually growing up to 2 inches long. Similar to freshwater Japanese medaka (Oryzias latipes), Hainan medaka is an excellent euryhaline model fish resource to be developed. As a euryhaline fish, O. curvinotus exhibits unique adaptive traits that are crucial not only for its own survival but also serve as indicators of ecological balance and environmental health in the South China Sea region. Changes in the habitat of this species directly reflect subtle variations in environmental factors such as salinity and water temperature in the region, which may further impact the stability and biodiversity of ecosystems including mangrove forests, coral reefs, and seagrass beds. However, recent fieldwork indicates that the habitat of Hainan medaka appears to be under threat, and the population resources are not optimistic. Climate change, a long-term and irreversible factor, needs to be considered in addition to the immediate effects of human activities (Yao et al., 2022). In particular, the fate of Hainan medaka, which is located in a biological hotspot (Oriental region), is a key ecological issue in the context of global climate change. By conducting thorough investigations into the habitat shifts and adaptive mechanisms of O. curvinotus, a more precise understanding of the ecological health of the South China Sea can be gained, thereby providing scientific evidence for the formulation of effective environmental protection and management strategies.

Effectively predicting the species range and their impacts under climate change, as well as understanding their habitat attributes, is crucial for developing appropriate strategies for species conservation (Alabdulhafith et al., 2022). Species distribution models (SDMs), also known as climate envelope-models, habitat models, and (environmental or ecological) niche-models, largely address the urgent need for ecologists to understand the habitat attributes of species and their distribution range shifts. Maximum entropy model (MaxEnt) is the most widely recognized and used SDMs, due to its high prediction accuracy, automatic assessment of important environmental variables, fast operation, and small sample size requirements (Phillips et al., 2006; Ma and Sun, 2018; Li et al., 2019; Xiao et al., 2022; Liao et al., 2023). MaxEnt is therefore widely used to predict the potential distribution of plants, birds, insects, nematodes, corals, bryophytes, and fungi (Sérgio et al., 2007; Phillips and Dudík, 2008; Tognelli et al., 2009; Williams et al., 2009; Young et al., 2009; Alabdulhafith et al., 2022), providing valuable information in biogeography, invasion biology, conservation biology, and ecology.

In this study, we integrated our previous field data with reported occurrence sites and climate data to analyze the historical period (the Last Glacial Maximum, LGM, dating back approximately between 25,000 and 19,000 years ago), current scenarios, and future potential habitats under different warming levels for the euryhaline fish endemic to the coast of the northern South China Sea. The objectives of this study were to (1) comprehend the habitat properties of O. curvinotus, (2) investigate its distribution and refuge locations during historical periods, and (3) assess potential habitats in the present context while considering how future climate change may impact its distribution.

O. curvinotus is a typical small tropical fish mainly found in the northern coast of the South China Sea. Generally, species with limited distributions tend to have narrower ecological niches, such as smaller temperature variations within their range, making them more susceptible to climate change (Liao et al., 2022). The habitats of species in tropical regions are typically characterized by high humidity and warmth (Skendžić et al., 2021). For instance, Neurobasis chinensis inhabits the hot and humid tropical regions of East Asia and is projected to expand its range northward under future warming climates (Skendžić et al., 2021; Liao et al., 2022). However, humidity is not a significant factor for underwater species throughout their lives. In this study, we determined that a mean diurnal temperature range of 8 °C is the critical point for the distribution range of O. curvinotus. In areas where the mean diurnal temperature exceeds 8 °C, O. curvinotus has virtually no suitable habitat. This is consistent with its status as a tropical species. Tropical species, especially tropical plants, are affected more by climate change than by topographic gradients (Toledo et al., 2012). Climate is a strong driver of species distribution, with 72% of tropical species affected by temperature (Toledo et al., 2012). In this study, the habitat of O. curvinotus has typical characteristics of tropical species, and only inhabits within the range of mean diurnal temperature of 8°C, especially within 6°C, which is a highly suitable habitat for it. This reveals that one of the habitat characteristics of O. curvinotus is a small range of the mean diurnal temperature. Another relatively important climatic factor shaping the distribution of O. curvinotus is the mean temperature of wettest quarter (bio8, contribution 11.62%, permutation importance 46.34%). The mean temperature of wettest quarter in the habitat area of O. curvinotus is more than 25 °C, with the highly suitable habitat exceeding 28 °C. These findings again confirm the habitat properties of O. curvinotus as a tropical species.

Our results revealed that the current distribution area is mainly concentrated in northern Vietnam, northeastern Hainan Province, the coast of Guangdong Province and Guangxi Zhuang Autonomous Region, and a small part of Taiwan Province, in total, covered ca. 17.82×10⁴ km². Within these ranges, a small number of simulated habitats exist in Taiwan Province of China, which is unexpected and requires further field work to confirm. In addition, it is worth noting that our field work experience confirmed the existence of a large population of O. curvinotus in Sanya, Hainan, and we brought the population of O. curvinotus in this area back to the laboratory for morphological and DNA barcoding identification, confirming the identity of O. curvinotus (Yao et al., 2022). However, the simulation results of this study suggest that Sanya in Hainan Province is not a suitable habitat for O. curvinotus. We speculate that the Sanya population may have undergone adaptive evolution in order to meet the changing climate environment, but the evolution has not yet reached the species level. Our earlier study corroborated these results across various dimensions (Dong et al., 2021; Yao et al., 2022). In our previous research, we discovered that the Sanya population lacked the sex determination gene “dmy” when compared to other populations (Dong et al., 2021). Additionally, Yao et al. (2022) revealed a significant level of genetic differentiation between the Sanya population and others, without indicating species diversification. Currently, the Sanya population is experiencing a bottleneck phase (characterized by a small population size), confirming that it has undergone intense environmental selection (Yao et al., 2022). All of these evidences indicate that the Sanya population is a unique group, exhibiting genetic variations that have emerged as adaptations to the challenging environment of Sanya, located in the China’s Hainan province. Regarding the relationship between the unique physiological response mechanism of O. curvinotus populations in Sanya and climate change, we have conducted a thorough analysis of the species’ resilience or vulnerability in the face of climate change. Firstly, as a euryhaline fish, O. curvinotus possesses physiological characteristics that endow it with remarkable tolerance to salinity fluctuations. Against the backdrop of climate change, the rise in sea level and fluctuations in freshwater input may lead to drastic changes in salinity levels in mangrove tidal creeks and streams. Nevertheless, through a series of unique physiological response mechanisms, such as the fine-tuned regulation of ion transport proteins, O. curvinotus effectively maintains the balance of salts within its body, ensuring the continuation of normal physiological functions even in varying salinity environments. This genetic adaptability provides significant resilience to O. curvinotus in the face of salinity fluctuations, facilitating its survival and reproduction in diverse environments.

However, the impact of climate change on O. curvinotus is not entirely positive. With the persistent increase in global temperatures and the frequent occurrence of extreme climate events, these environmental factors may exceed the adaptive range of O. curvinotus. For instance, high-temperature environments may significantly increase the metabolic rate of O. curvinotus, subsequently escalating its demand for food and oxygen. Under resource-limited conditions, such an elevation in metabolic rate could pose a threat to the survival of O. curvinotus. Furthermore, extreme climate events like heavy rainfall or drought can directly destroy the habitats of O. curvinotus, leading to severe degradation of its living environment. In such scenarios, the genetic adaptability of O. curvinotus may be insufficient to fully cope with these drastic changes, thus exhibiting a degree of vulnerability. However, the adaptation of O. curvinotus to this sudden climate change requires at least several generations of natural selection. Therefore, we believe that the genetic adaptability of O. curvinotus exhibits both resilience and vulnerability in response to climate change. To gain a more comprehensive understanding of this relationship, further research is needed to explore the interaction between genetic variations in O. curvinotus and climate change, as well as to delve deeper into its adaptive mechanisms. This will aid us in providing a more scientific basis for the conservation and utilization of O. curvinotus, while also serving as an important reference for addressing the biodiversity challenges posed by climate change.

The historical species distribution pattern in the Quaternary largely determines their current species range. The conservatism of climatic niches, combined with the cycles of glacial and interglacial periods, has forced many species to reduce their ranges in order to survive under the advance of Pleistocene ice sheets. Refugia provide geographical opportunities for species to retreat, survive, and later recolonize under favorable environmental conditions (Morales-Barbero et al., 2018). Our research has revealed three refugia, all located in the northern part of Sundaland (the northern edge of the South China Sea). Among them, the largest one, Refugium 1, is speculated to be the starting point of the dispersal of highly suitable habitats in the northern part of Vietnam and Leizhou Peninsula. While Refugium 2 and Refugium 3 are speculated to be the sources of species migration for the highly suitable habitats in the Pearl River Delta region. Typically, species range may expand or contract based on the suitability of climate change for its ecological niche (Morales-Barbero et al., 2018). Migrant species may be geographically restricted to a particular location, or continue their colonization route through a habitat matrix, depending on the suitability of climatic conditions (Hewitt, 2000; Petit et al., 2003; Barnosky, 2005). Due to the disappearance of Sundaland after the glacial period, many habitats of O. curvinotus sank into the sea, and highly suitable habitats migrated to neighboring northern Vietnam, Leizhou Peninsula, and a small part of the Pearl River Delta region. Overall, the habitats of O. curvinotus have been in a stage of retraction from the Quaternary to the present day.

Future warming is an undeniable reality. Over the past century, the global temperature has risen by 0.6°C and is continuing to change at an accelerated pace. Projections indicate that by the end of the 21st century, the minimum temperature increase is expected to be between 0.3°C and 1.7°C, with a maximum increase of 2.6°C to 4.8°C (Root et al., 2003; Kumar and Rawat, 2022; Liao et al., 2022, 2023). Climate warming is inevitable, and it will undoubtedly alter the current distribution patterns of various species. The impacts of warming have already been observed in numerous organisms, including plants, dragonflies, butterflies, grasshoppers, lacewings, spiders, reptiles, woodlice, ground beetles, longhorn beetles, soldier beetles, harvestmen, millipedes, aquatic bugs, freshwater fish, birds, and mammals (Parmesan and Yohe, 2003; Root et al., 2003; Hickling et al., 2005, 2006; Lenoir and Svenning, 2015). Tropical species with narrow ecological niche are theoretically vulnerable to the stress of climate warming and migrate northward like most species. However, the subjects studied in this research did not follow this typical pattern. In our study, climate warming under four different carbon emission levels caused the current habitat of O. curvinotus to expand within a short timeframe (by 2040). After a period of adaptation (by 2100), the expansion trend was mitigated, and the expansion area retreated to slightly larger than the current climate scenario. The increase of habitats primarily occurred in northern Vietnam, southern Guangxi, Hainan Island and Taiwan Island, with a notable increase in highly suitable habitats, such as the Red River Delta in Vietnam and the central part of Leizhou Peninsula. Contrary to the typical northward migration pattern observed in other species under the influence of warming (Hickling et al., 2006), the highly suitable habitats for O. curvinotus did not shift latitudinally but instead increased their area around 21°N. East Asia, at 21 degrees north latitude, is a global biodiversity hotspot, and the region’s geological history has shaped a complex climate and geological environment in this area (Hekinian and Walker, 1987; Marchese, 2015), resulting in a complex network of biological and abiotic factors. Numerous species have come to rely on this hotspot habitat, making it a crucial ecosystem for various organisms (Gathorne-Hardy et al., 2002; Gorog et al., 2004). Our subject, O. curvinotus, despite its adaptability, is currently found only in a small area along the northern coast of the South China Sea, including coastal freshwater extending inland. Hayakawa et al. (2015) discovered this species in mangrove plantations and inland paddy fields in Bang La, Do Son, Vietnam, and conducted population genetics studies, recognizing it as a promising experimental model species in tropical Asia (Hayakawa et al., 2015). These provide valuable insight into the dispersal of O. curvinotus into inland freshwater habitats at the same latitude. Indeed, our simulations of O. curvinotus habitats in the context of future climate warming support this notion and underscore the need for further investigation into the species’ ecological niche and potential adaptability to changing environmental conditions.

Despite our optimistic findings regarding the recent survival status of this species, proactive conservation measures must still be implemented to secure its long-term survival. To this end, we propose the following conservation strategies: Firstly, monitoring and surveillance: We plan to establish a regular monitoring program to closely track the species’ population dynamics, habitat utilization, and any emerging threats. This will involve conducting surveys, monitoring breeding habitats, and collecting crucial data using remote sensing techniques. These data will not only aid in evaluating the effectiveness of our conservation efforts but also guide future actions. Secondly, habitat restoration and enhancement: We prioritize the restoration and enhancement of the species’ existing habitats, including contaminant removal, controlling invasive species, and promoting native vegetation growth. Additionally, we aim to establish new habitats in suitable areas to mitigate the potential negative impacts of predicted habitat shifts.

This study summarizes the habitat attributes, potential distribution areas and its long-term temporal changes of a small euryhaline fish species. Results show that O. curvinotus is currently mainly distributed along the coastal areas of northern Vietnam, northeastern Hainan, Guangdong and Guangxi Zhuang Autonomous Region, as well as in a few areas of Taiwan Province, China, covering a total area of 17.82 ×10⁴ km². Highly suitable habitats are mainly found in northern Vietnam, central Leizhou Peninsula, and the western coast of the Pearl River Delta. Three refugia existed in historical times, with the largest one, Refugium 1, speculated to be responsible for the dispersal and establishment of highly suitable habitats in northern Vietnam and central Leizhou Peninsula. Refugium 2 and Refugium 3 are hypothesized to be the sources of immigrant individuals with highly suitable habitats in the Pearl River Delta, with only Refugium 2 remaining to current. Climate warming in the future is not expected to pose a threat to the species’ distribution. Instead, it may lead to a rapid expansion of its habitat in a short period of time, followed by a stabilization and gradual decrease to slightly above the current habitat area. The expansion of its habitat will not shift northward as most species do, but will expand around the 21°N latitude. This study provides valuable insights for the conservation of such small euryhaline fish species and the development of model species suitable for ecological environmental research in the region.