Indian Journal of Geo Marine Sciences Vol. 49 (08), August 2020, pp. 1389-1394 Understanding the dietary relationship between extensive Noctiluca bloom outbreaks and Jellyfish swarms along the eastern Arabian Sea (West coast of India) L C Thomas, S B Nandan & K B Padmakumar* Department of Marine Biology, Microbiology and Biochemistry, School of Marine Sciences, Cochin University of Science and Technology (CUSAT), Kochi-16, Kerala, India *[E-mail: kbpadmakumar@gmail.com] Received 12 November 2018; revised 21 December 2018 The paper attempts to understand the interrelationship between recurring blooms of dinoflagellate, Noctiluca scintillans as well as increasing jellyfish swarms along the coastal waters of eastern Arabian Sea. The grazing of N. scintillans on diatoms in the productive waters with reduced competition pressure due to the opportunistic feeding of jellyfishes on zooplankton are described here. With the development of N. scintillans in the favourable environmental conditions, jellyfishes utilize this dinoflagellate as their food source and thrive in the coastal waters. Hence, trophic interaction between Noctiluca and jellyfishes leading to their proliferation in the coastal waters are delineated. Introduction Marine ecosystems are experiencing serious threats that affect the diversity, sustainability and services they render to human beings. Eutrophication, expansion of hypoxia, increased ocean acidification, dwindling fisheries, algal blooms and increasing gelatinous zooplankton or jellyfish swarms are some among them. Harmful algal blooms (HABs) along the productive eastern Arabian Sea are increasing in the last few decades 1,2 . The bloom of Noctiluca scintillans that results in red tide during summer monsoon mainly along the southeastern Arabian Sea and green tide along the northeastern Arabian Sea during winter monsoon is a recurrent phenomenon 3-9 . These are opportunistic dinoflagellates exhibiting high growth and reproduction rate within a particular season 10 . Noctiluca is a heterotrophic dinoflagellate that feeds on small diatoms, dinoflagellates, eggs of copepods and microzooplankton 10,11 . They are a significant competitor for mesozooplankton that prey upon mixed diatom blooms during summer monsoon associated upwelling 7,12,13 . The bloom of Noctiluca generally appears in red causing a red tide. Green Noctiluca harbours green photosynthetic prasinophyte endosymbiont Pedinomonas noctilucae which is absent in red Noctiluca and provides nutrition to the dinoflagellate and imparts a green colour to its bloom 14,15 . However, green Noctiluca is also observed to depend on phagotrophy which is evident from the food vacuoles with diatoms inside these dinoflagellates from the northern Arabian Sea winter blooms 16 . Owing to its large size and high ammonia concentration N. scintillans are often avoided by smaller zooplankton and fishes, while the major predators of N. scintillans include gelatinous zooplankton and certain large fishes 16,17 . Gelatinous zooplankton mainly jellyfishes are characterised by broad diet spectrum, high reproduction as well as growth rates and experience less predation pressure 18 . Underpinned with these benefits they can form huge swarms in the pelagic water column under favourable environmental conditions. Climate change, alteration in aquatic habitats, eutrophication and unsustainable fisheries are the primary external factors contributing to their population outburst 19,20 and are referred to as synanthropic, benefitting from the environmental alterations caused by anthropogenic influences 21,22 . Materials and Methods Frequent blooms of N. scintillans were observed and reported from the eastern Arabian Sea as a part of regular monitoring programs for harmful algal blooms and associated microalgal studies. Samples were collected on the incidence of bloom events along the west coast of India. Taking into account on these studies conducted as well as from various published reports on the blooms caused by N. scintillans 1,2,3-9 the