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Ctenophores are quick responders to coastal environmental changes and play a crucial role in marine food web dynamics. We report the environmental drivers of a ctenophore swarm (Pleurobrachia spp.) and associated ecological changes in estuarine and coastal waters of the Sundarbans mangrove–estuarine complex in the northern Bay of Bengal (BoB). Plankton and fish samples were collected from estuarine and coastal waters at the peak of estuarine outflow in the northeast monsoon (NEM). Sampling locations at the estuarine mouth witnessed ctenophores (Pleurobrachia pileus and Pleurobrachia globosa) in swarming proportions. Multivariate statistical analysis revealed significant differences in hydrographical and biological properties between the swarm and non-swarm areas. The sea surface salinity, ammonium, and mesozooplankton (MSP) density were positively correlated while microzooplankton (MCZ) density was negatively correlated with the ctenophore swarm. Fish caught from swarm locations, dominated by microbial food web consisted of species of less market value, while those from the non-swarm locations, dominated by conventional plankton food web consisted mainly of commercially important species. Thus, in the first-ever record from a commercially important fishing ground in the BoB, this study provides evidence of how a ‘dead end’ plankton predator affects the plankton food web organization and fisheries in tropical coastal waters. HIGHLIGHTS Occurrence of a ctenophore swarm and its hydrobiological links investigated in the BoB.; The ctenophore swarm and non-swarm locations favoured different plankton trophic pathways.; Active ctenophore predation on copepods favoured the microbial loop in swarm locations.; Predation on swarm-forming ctenophore favoured the conventional food chain.; Ctenophore swarm negatively affected fish community composition.;

A four-decade data of jellyfish aggregation from 1980 to 2020 were taken to discern whether there has been an actual rise in jellyfish swarm in Indian coastal and estuarine waters. Despite frequent jellyfish aggregations and beach strandings in Indian waters, jellyfish aggregations have been poorly investigated and there is a dearth of information on the swarm-forming jellyfish, their preferred season, and the location of swarming. Therefore, our review aims to account for the frequency of swarming phenomenon annually and the appearance of new swarm-forming jellyfish species. The term ‘jellyfish’ refers to the medusae stage of phylum Cnidaria (Cubozoa, Hydrozoa, and Scyphozoa) only in this review. The present work postulates a geospatial spread and swarm-forming jellyfish species to increase in recent times. More than 23 coastal locations of India have witnessed jellyfish aggregations and beach stranding in the last four decades. Seasonal oceanographic conditions prevailing during the summer monsoon, fall, and early winter promoted jellyfish aggregations and swarming. Only two jellyfish species were known to form aggregates during 1981–1990, but the diversity of jellyfish species increased to nine by 2011–2020. The development of predictive models from remote sensing data can be useful to warn humans and coastal industries of the approaching swarm. HIGHLIGHTS Geospatial spread, frequency of jellyfish swarms, and the number of swarm-forming species increased in the coastal and estuarine waters of India.; More than 23 locations along the coast of India have witnessed jellyfish aggregations, swarms, and beach stranding in the last four decades.; Seasonal oceanographic conditions during the summer monsoon, fall, and early winter promote jellyfish aggregations.;

Paramacrobiotus bengalensis sp. nov. was discovered in a moss sample collected from a tree in West Bengal, India. We describe this new species using detailed morphological and morphometric data obtained from phase contrast microscopy and scanning electron microscopy, along with molecular and phylogenetic data analyses. Due to the presence of a cap-like structure at the distal portion of egg processes, the new species showed the highest similarity with Paramacrobiotus garynahi (Kaczmarek, Michalczyk & Diduszko, 2005), Paramacrobiotus alekseevi (Tumanov, 2005), Paramacrobiotus filipi Dudziak, Stec & Michalczyk, 2020, Paramacrobiotus sagani Daza, Caicedo, Lisi & Quiroga, 2017, Paramacrobiotus vanescens (Pilato, Binda & Catanzaro, 1991) and Paramacrobiotus gadabouti Kayastha, Stec, Mioduchowska & Kaczmarek, 2023. However, it can be differentiated from them by some morphological and morphometric characteristics. The genetic data corroborated our phenotypic outcome further supporting the new species hypothesis.

Epibiosis among plankton communities and the role of environmental forcing therein are discussed in this study. We hypothesized that a sub-surface thermal inversion phenomenon and associated hydrographical changes during winter monsoon promoted group/species-specific epibiosis among the plankton in the western Bay of Bengal. Plankton samples were collected from discrete depths along five transects placed perpendicular to the coast. Water column profiling revealed thermal inversion below the mixed layer depth along three southern transects (APB, APV and APK) and its absence along the two northern transects (ODP and ODG). Multivariate analysis revealed a significant difference in hydrographical properties between these areas. The warmer, highly saline waters of the thermal inversion layer, sandwiched at 10–20 m, between the colder, less saline waters of surface and bottom, supported higher density and diversity of epibionts on copepods and diatoms. The density of epibionts and intensity of epibiosis were significantly lower outside the thermal inversion layer and in the northern transects. Epibionts preferred copepods over diatoms and carnivorous/omnivorous copepods over herbivorous copepods of comparable size. These results suggest that hydrographical variability associated with thermal inversion during winter monsoon support epibiosis among the plankton in the coastal waters of the western Bay of Bengal (BoB). The present investigation is the first-ever study from a coastal marine ecosystem connecting hydrographical features with epibiosis among the plankton. Further studies are warranted to explore the higher frequency of epibiosis in the thermal inversion layer, the nutrient preference and indicator properties of epibionts, and their ecological role in the coastal and open ocean ecosystems.