Seasonal Variations in the Dynamics of the Number and Biomass of the Black Sea Macrozooplankton Populations

The abundance and biomass dynamics of the jellyfish Aurelia aurita and ctenophores-invadors Mnemiopsis leidyi and Beroe ovata in 2019 on the northeastern shelf of the Black Sea were studied. Data on the maxima of reproduction of each species of gelatinous macrozooplankton were obtained, as well as conclusions on the dynamics of individuals growth in populations and average life expectancy were drawn.

The impact of polychlorinated biphenyls on the bioluminescence rates of the Black Sea ctenophore Mnemiopsis leidyi A. Agassiz (Ctenophora: Lobata)

The toxic impact of a mixture of polychlorinated biphenyls Aroclor 1254 (PCB) on the luminescence characteristics of the ctenophores Mnemiopsisleidyi A. Agassiz was studied. The control group was kept in an aquarium without a toxicant; experimental individuals were placed in three aquariums with water with PCB concentrations of 1, 10, and 100 µg -1 with an exposure of 15, 30, 40 and 60 minutes. Bioluminescence of M. leidyi was studied using the “Svet” complex. The toxic impact of PCBs caused a decreased amplitude and duration of luminescence at all given concentrations, whereas one-hour exposure to mechanical agitation resulted in a maximum decrease by 35% in the glow duration. At a PCB concentration of 1 μg ∙ L -1 , a maximum decrease in the luminescence intensity by a factor of 20 was observed compared with the control group.

Distribution, diet and relationships of the invasive ctenophore Mnemiopsis leidyi with anchovies and zooplankton, in the northeastern Adriatic Sea

Blooms of invasive ctenophore Mnemiopsis leidyi can have massive consequences on fish stocks and marine food webs. The distribution, abundance and diet of this ctenophore were investigated in the northeastern (NE) Adriatic between 2016 and 2019. The abundance of M. leidyi was determined daily along the coast of Rovinj (Croatia), and its spatial distribution monitored by visual census from research vessels and by pelagic trawl during acoustic surveys in September of each year. Mesozooplankton samples were collected along the western coast of Istria by vertical tows from the bottom to the surface. Spatial distribution and abundance of anchovy (Engraulis encrasicolus) assemblages were determined by geo-referenced hydro-acoustic sampling using a scientific echosounder. Large swarms of M. leidyi covering several km2 were regularly reported between July and November extending up to 25 NM from the western coast of the Istrian peninsula and reaching maximum offshore densities of 270 individuals per m2. The abundance of anchovy in the areas where M. leidyi was present evidently decreased. The content of M. leidyi’s digestive tract and plankton samples consisted both mainly of cladocerans, copepods, pteropods, echinodermata and bivalvia larvae. Results indicate that M. leidyi may accumulate and increase its abundance in the stagnant and trophically rich areas of the northern Adriatic gyres, spreading to the surrounding larger areas with currents. We assume that M. leidyi presence correlates with a decrease in anchovy population due to competition for food (zooplankton). Our findings support the importance of implementing an international monitoring program throughout the Adriatic Sea and demonstrate the ability of current MEDIAS surveys to detect changes in the pelagic ecosystem throughout surveyed areas.

Krüppel-like factor gene function in the ctenophore Mnemiopsis leidyi assessed by CRISPR/Cas9-mediated genome editing

The Krüppel-like factor (Klf) gene family encodes for transcription factors that play an important role in the regulation of stem cell proliferation, cell differentiation, and development in bilaterians. While Klf genes have been shown to functionally specify various cell types in non-bilaterian animals, their role in early diverging animal lineages has not been assessed. Thus, the ancestral activity of these transcription factors in animal development is not well understood. The ctenophore Mnemiopsis leidyi has emerged as an important non-bilaterian model system for understanding early animal evolution. Here we characterize the expression and functional role of Klf genes during M. leidyi embryogenesis. Zygotic Klf gene function was assessed with both CRISPR/Cas9-mediated genome editing and splice-blocking morpholino oligonucleotide knockdown approaches. Abrogation of zygotic Klf expression during M. leidyi embryogenesis results in abnormal development of several organs including the pharynx, tentacle bulbs, and apical organ. Our data suggest an ancient role for Klf genes in regulating endodermal patterning, possibly through regulation of cell proliferation.

Whole-Body Regeneration in the Lobate Ctenophore Mnemiopsis leidyi

Ctenophores (a.k.a. comb jellies) are one of the earliest branching extant metazoan phyla. Adult regenerative ability varies greatly within the group, with platyctenes undergoing both sexual and asexual reproduction by fission while others in the genus Beroe having completely lost the ability to replace missing body parts. We focus on the unique regenerative aspects of the lobate ctenophore, Mnemiopsis leidyi, which has become a popular model for its rapid wound healing and tissue replacement, optical clarity, and sequenced genome. M. leidyi’s highly mosaic, stereotyped development has been leveraged to reveal the polar coordinate system that directs whole-body regeneration as well as lineage restriction of replacement cells in various regenerating organs. Several cell signaling pathways known to function in regeneration in other animals are absent from the ctenophore’s genome. Further research will either reveal ancient principles of the regenerative process common to all animals or reveal novel solutions to the stability of cell fates and whole-body regeneration.

Spatial Distribution, Population Structure, and Predatory Impact of Gelatinous Predators on Zooplankton Community in Inshore Waters off Crimean Coast of the Black Sea

The study reports the data on the distribution and predatory impact of the gelatinous macroplankton on mesozooplankton in the inshore waters of Crimea in April 2016. In the study areas, gelatinous macroplankton comprised Scyphomedusae Aurelia aurita (Linnaeus, 1758) and three ctenophore species (Mnemiopsis leidyi A. Agassiz, 1865, Beroe ovata Bruguire, 1789, and Pleurobrachia pileus (O. F. Müller, 1776). The biomass of A. aurita dominated everywhere and varied from ~ 62 to 330 g·m‑2 in different areas. The largest A. aurita biomass was observed in the South Coast of Crimea and the smallest in Karkinitsky Bay, where the number of jellyfish was high, suggesting the predominance of small individuals in the population. The size structure of A. aurita population differed by region: the proportion of large animals increased from the West to the East. The abundance and biomass of M. leidyi were rather low: 0.33–1.45 ind·m‑2 and 1–51.7 g·m‑2, respectively (the exception was the position in the Kerch Strait, where M. leidyi abundance reached 2 ind. m‑2 and biomass 126.3 g·m‑2); that species occurred only at 33–45 % of the stations (the exception was Karkinitsky Bay – 17 %), with the maximum values in the South Coast of Crimea. Large, 55–70-mm, adult individuals predominated. P. pileus biomass was lower than the biomass of A. aurita and M. leidyi in all areas, but its abundance was much higher. The daily rations of A. aurita varied widely both in the entire area and from one region to another. The daily ration values correlated with the carbon content in A. aurita body. The predation pressure of A. aurita on zooplankton in the inshore waters of Crimea was very low and did not result in a crucial decrease in mesozooplankton abundance. The effect of M. leidyi, even with its low abundance and biomass, was much stronger