International Journal of Marine Science, 2025, Vol.15, No.3, 118-129 http://www.aquapublisher.com/index.php/ijms 119 by using destructive means such as fried fish and poisoned fish has long been banned, and collection methods such as trawling are not applicable in reef areas. Therefore, in the context of the growing demand for ecological environment monitoring, it is urgent to introduce new technical means to obtain more efficient and comprehensive information on the diversity of coral reef fish. In recent years, the rise of environmental DNA (eDNA) technology has brought revolutionary changes to ecological monitoring. eDNA refers to trace DNA fragments left by organisms in the environment. By collecting water samples and analyzing the eDNA in it, information about the existence of species in the environment can be obtained indirectly. Compared with traditional methods, eDNA technology has significant advantages such as non-invasive, high sensitivity and high throughput (Gold et al., 2021). Based on these characteristics, eDNA technology has developed rapidly since the mid-2010s and has been widely used in species monitoring in various ecosystems such as rivers, lakes and oceans. A large number of cases have proven that eDNA can compensate for the shortcomings of traditional methods for inadequate detection of rare, occult and small species. Therefore, introducing eDNA technology into the survey on fish diversity in coral reefs in Hainan Island is expected to break through the limitations of existing monitoring and provide more comprehensive and scientific data support for coral reef protection. 2 Basic Principles and Technical Processes of eDNA Technology 2.1 Source, characteristics and stability of environmental DNA Environmental DNA is usually derived from metabolites or tissue fragments of organisms in the environment. These gene fragments exist in water as free DNA or cellular forms and can be collected as "genetic traces" of the presence of the target species. However, the stability of eDNA in the environment is affected by a variety of physicochemical factors, and its concentration and fragment length rapidly decline over time (Mauvisseau et al., 2022). The eDNA in water can be in various forms (such as dissolving state, adhering to particles, ingesting by microorganisms, etc.), and the degradation rates of DNA in different forms vary. Environmental conditions such as temperature, pH, UV radiation, microbial activity, etc. will significantly affect the degradation process of eDNA (Sahu et al., 2025). Generally speaking, in tropical nearshore waters, high temperatures and strong light will accelerate DNA degradation, making the eDNA signal relatively short, and the detection of species information that has existed in recent days. Some studies have reviewed the retention time of eDNA in water bodies and found that in most cases, environmental DNA is completely degraded within a few days to weeks. On the other hand, the spatial distribution of eDNA is also localized and localized. In open seas, water flow will cause DNA to spread, but the complex structure of coral reefs can form a small locally closed environment, so that eDNA still mainly represents the composition of local species within the scale of tens of meters. Jaquier et al. (2024) study in the Western Indian Ocean scattered archipelago shows that the composition of eDNA in water bodies near the reef disk is significantly different from the offshore 250 meters away: samples close to the reef are mainly settled benthic fish DNA, while more oceanic fish are detected (Jaquier et al., 2024). This discovery supports the spatial limitations of eDNA signals, that is, environmental DNA mainly reflects biological communities within a certain range around the sampling point, rather than information at infinite distances. Therefore, in practical applications, it is necessary to reasonably design sampling and distribution points based on the ecological characteristics and current conditions of the target species to ensure that representative eDNA signals are obtained. 2.2 Methods for collecting and preserving water samples The first step in environmental DNA analysis is water sample collection. A reasonable sampling plan should consider factors such as time, space and frequency to cover the heterogeneity of the target area as much as possible. In the coral reef environment of Hainan Island, multiple sampling points can be set according to the distribution of reefs and current conditions, including typical habitats such as lagoons, reef slopes and open waters in the reef. Each point is repeatedly sampled in different seasons or day-night cycles to capture information on dynamic changes in communities. Contamination should be avoided during the sampling process: the water collector and bottle utensils should be sterilized in advance, the sampling personnel should avoid contacting the bottle mouth, and collect blank control samples on site to monitor background DNA. Usually a certain volume of
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