Study of the biological connectivity of land-sea continuum: a case study of fishes in Shanghai waters of the Yangtze River Estuary

The land-sea continuum is a transitional zone connecting terrestrial and sea ecosystems and the connectivity has a significant impact on the migration of organisms, while description and quantification of the land-sea continuum is still being explored. Based on fish diversity data obtained by amplified a portion of the V5 high-variable region of the 12S rRNA gene in fish mitochondria using universal primers targeting vertebrates, we developed a method to quickly assess connectivity of the land-sea continuum. The method is used to estimate the comprehensive index of biological connectivity (CIBC) from the Beta diversity index between adjacent cloud computing sites. The value ranges from 0 to 1 and the higher the value means a better connectivity. In October 2023, eDNA technology was successfully used to detect 117 species fish of 86 genera in 35 families of 21 orders from the main streams and tributaries of Shanghai waters in the Yangtze River Estuary where the numbers of fish in the tributaries were more than that in the main stream. Alpha diversity analysis shows that fish communities in tributaries are richer than those in main streams, but less diverse than those in main streams. The CIBC values of Cyprinidae show that the connectivity of trunk branches is generally fine while the values of Xenocypridae are better at the upstream and downstream. The values of Acheilognathidae are relatively good at the upstream while the values of Gobiidae are also high at the middle and upper reaches. The overall CIBC values of the fish community indicated a good connectivity in the main stream, but it is poor in the lower reaches near the Yangtze River Estuary and it various significantly in the tributaries. Overall connectivity in main stream is better than that of the tributaries. The method developed in this study can provide new insights for the study of land-sea continuum connectivity and contribute to the ecological restoration and integrated management for ecological corridors in Shanghai waters of the Yangtze River Estuary.