Abstract
To assist in detection of offshore spawning activities of the Japanese eel Anguilla japonica and facilitate interpretation of results of environmental DNA (eDNA) analysis in their spawning area, we examined the eDNA concentration released by each life history stage of artificially reared Japanese eels in the laboratory using quantitative real-time PCR (qPCR). We also compared eDNA concentrations between before and after artificially induced spawning activities. eDNA was not detected from three 30 L seawater tanks containing each single fertilized egg, but eDNA was found from other tanks each containing single individuals of larval stages (preleptocephalus and leptocephalus), juvenile stages (glass eel, elver and yellow eel) or adult stage (silver eel). The eDNA concentrations increased in the life history stages, showed a significant difference among all stages, and were positively correlated with the total length and wet weight. Moreover, the eDNA concentration after spawning was 10–200 times higher than that before spawning, which indicated that the spawning events in the ocean would produce relatively high eDNA concentration. These results in the laboratory suggested that eDNA analysis appears to be an effective method for assisting oceanic surveys to estimate the presence and spawning events of the Japanese eel in the spawning area.
Highlights
The catadromous eels of the genus Anguilla are famous for their remarkable migrations between ocean and freshwater environments[1]
Another technique has been used during recent oceanic surveys we conducted in the Japanese eel spawning area, which is to analyze environmental DNA from water samples to search for the presence of the adult eels
The water sampling, filtration, extraction and PCR system for the environmental DNA (eDNA) analysis worked successfully on board during an oceanic survey, and we could detect eDNA of the Japanese eel from 2 L seawater samples from water depths at 250 and 400 m using real-time PCR14. This indicated that the onboard eDNA analysis was a viable technique for identifying locations or depths where Japanese eels may be within their spawning area during an oceanic survey
Summary
The catadromous eels of the genus Anguilla are famous for their remarkable migrations between ocean and freshwater environments[1]. Recent efforts have been made to further understand the spawning ecology of the Japanese eel through attempts to directly observe their natural spawning behavior using submersibles and underwater camera systems[11,12] This requires that the precise location and depth of likely spawning are narrowed down to a specific area in advance for the underwater surveys. Www.nature.com/scientificreports where the eels may be aggregating before new moon periods, and underwater surveys can be conducted where the eels are present during their spawning time. Another technique has been used during recent oceanic surveys we conducted in the Japanese eel spawning area, which is to analyze environmental DNA (eDNA) from water samples to search for the presence of the adult eels. It appears likely that offshore aggregations and spawning events of Japanese eels should be detectable by high eDNA concentrations
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