First record of Acromitus flagellatus (Maas, 1903) (Cnidaria: Scyphozoa) swarm from the world's largest deltaic ecosystem, the Sundarbans, India

The present study describes the first record of Acromitus flagellatus (Maas, 1903) swarm from the east coast of India, Bay of Bengal. Specimens of A. flagellatus were collected from the estuarine waters of Saptamukhi river channel, of the Sundarbans deltaic region during February 2021. The exumbrella was 160 ± 40 mm in diameter, with oral arms measuring 115 ± 45 mm and possessing a single long flagellum at the distal end of each oral arm. A. flagellatus swarm had an abundance of 5200 ± 440 ind. m^‑3. Sea surface temperature and salinity in the study area were 25.3 ±0.7°C and 24.5 ± 0.5 ‰ respectively. Common phytoplankton species encountered were Biddulphia sinensis, Planktoniella blanda and P. sol; while microzooplankton comprised mostly of larval forms of invertebrate metazoans. Mesozooplankton species during A. flagellatus swarm were dominated by Centropages orsinii, Bestiolina similis, Subeucalanus subcrassus, Clytia simplex, Pleurobrachia globosa, Zonosagitta pulchra and Hyperia macrocephala. Our study points to the non-specific, opportunistic feeding pattern of A. flagellatus across different trophic levels, from phytoplankton to fish and will extend the distribution of A. flagellatus up to the Sundarbans delta of the Bay of Bengal.