High-throughput DNA metabarcoding of mitochondrial 12S rRNA and Cyt b gene sequences was coupled with a morphology-based identification tool to assess ichthyoplankton community structure in Oujiang River Estuary, China. The performances of 12S and Cyt b barcoding markers were compared in terms of taxonomic resolution, detection and coverage, and their suitability was established for use as a quick and powerful ichthyoplankton assessment tool. A total of 30,138 ichthyoplankton (2462 eggs and 27,676 larvae) samples were collected from April to August 2015 and identified to 145 taxa belonging to 57 families and 105 genera. June and July were the main spawning months. Ichthyoplankton were more abundant around Lingkun and Qidu Islands and the upper parts of Oujiang River Estuary. The 12S gene marker presented higher species coverage and detection rate than Cyt b. DNA metabarcoding exhibited more representative species identification power than morphology. The findings reported in this study provided a key attempt towards the development of time-efficient and cost-effective ichthyoplankton identification and assessment tool.

Since it was first observed in Israel in the 1970s, and due to its subsequent negative impact on human activities, the nomad jellyfish Rhopilema nomadica has earned itself a spot on the list of the 100 Worst Invasive Alien Species in the Mediterranean. It was assumed to originate in the Red Sea, or in the Indo-Pacific region, but in the absence of additional reports of live specimens outside the Mediterranean, its origins have remained a mystery. Here, via molecular analysis, we present the first verified results of the existence of R. nomadica in the Western Indian Ocean. Moreover, using additional evidence from Cassiopea andromeda and R. nomadica, we propose that the construction of the Aswan High Dam may have led to the proliferation of R. nomadica in the Levantine Basin.

Yueqing Bay in China supports important marine resources that sustain local fisheries and food security, but increasing anthropogenic pressures and natural environmental changes threaten its biodiversity. This study used morphological identification, COI DNA barcoding, and 12S DNA metabarcoding to assess ichthyoplankton composition and fish spawning patterns. A total of 13,415 eggs and 17,291 larvae were collected using horizontal and vertical plankton nets. Morphological analysis identified 58 taxa, while molecular methods detected 21 species (COI) and 48 species (12S), with an overall total of 105 species from 78 genera and 42 families. Spawning activity showed clear seasonal and spatial patterns, with the highest abundance and diversity in spring and summer, particularly around Ximen Island and the Bay mouth. These areas function as key spawning and nursery grounds. 12S DNA metabarcoding contributed strongly to species detection, especially for cryptic and morphologically indistinguishable larvae, complementing traditional surveys. The findings provide essential baseline information for monitoring fish stocks, protecting critical habitats, and improving fishery management strategies under increasing anthropogenic and climate-related pressures in Yueqing Bay.