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Abstract

The seasonal and spatial dynamics of the main trophic characteristics (the food spectrum, the intensity of feeding and ingestion of mesozooplankton by the jellyfish Aurelia aurita (Linnaeus 1758) and ctenophore Mnemiopsis leidyi A. Agassiz 1865 populations were studied in four cruises of RV “Professor Vodyanitski” in the shelf regions of Crimean peninsula in January – Octobers 2016. The whole region was divided in inner (<50 m depth) and outer (51–150 m) shelves. Collection and treatment of macrozooplankton were fulfilled according to earlier described method [18]. To study the food spectrum and feeding rate of gelatinous predators the food items composition in the gastric cavity was gbserved under binocular microscope. Daily ration values (R, mg ind.-1 day-1) were calculated by the formula: R= Bz × DT-1 × 24, where Bz is zooplankton biomass in the predator gastric cavity (mg), DT is digestion time of zooplankton (h). The consumption rate of zooplankton by gelatinous macroplankton populations was estimated by the daily ration and mesozooplankton biomass values. Mesozooplankton was collected with the Juday plankton net with mouth diameter 38 cm and mesh size 140 mkm. Vertical net hauls from the bottom to the sea surface were performed on the inner shelf stations and to the boundary of the hydrogen sulfide zone (δt = 16.2) on the outer shelf stations. In the samples fixed with 4 % formalin solution the zooplankton abundance, its taxonomic composition and size-age structure were assessed with standard method. Seasonal differences in the food spectrum of jellyfish revealed prevalence of bivalve mollusks veligers in the spring period and a wide species composition of crustaceans and other groups of prey in the summer. The nutritional intensity of the studied species was close: the specific daily rations in winter, spring, and autumn did not exceed tenths of a percent of the carbon content in the body. Both species fed at a maximum rate in the summer on the outer shelf, where the specific rations reached 12.9 and 5.1 % C of the body in jellyfish and ctenophores, respectively. A. aurita and M. leidyi populations consumed 0.2–5 % of the biomass of fodder zooplankton per day, which did not result in a drastic reduction in the abundance of zooplankton community and provided the favorable nutritional conditions for small planktivorous pelagic fish.

Authors

G. Finenko

leading researcher, PhD

https://orcid.org/0000-0002-3303-0906

https://elibrary.ru/author_items.asp?id=499604

N. Datsyk

junior researcher

https://orcid.org/0000-0002-8113-8786

https://elibrary.ru/author_profile.asp?id=762916

B. Anninsky

leading researcher, PhD

https://orcid.org/0000-0001-9614-9392

https://elibrary.ru/author_profile.asp?id=260698

Yu. Zagorodnyaya

leading researcher, PhD

https://orcid.org/0000-0002-9502-4923

https://elibrary.ru/author_profile.asp?id=815221