ИСТИНА

Planulae of scyphozoans of the order Semaeostomae exhibit negative phototaxis (Brewer, 1976), positive gravitaxis alternating with negative gravitaxis, a preference for hard, rough substrates (Holst and Jarms, 2006), and predominantly choose the undersides of substrates for settlement. The present study was aimed at determining the effect of lighting on settlement site selection by Cyanea capillata planulae, considering substrate type, substrate depth, and settlement on the upper or lower surface. The substrates used were plates of transparent plastic, dark opaque plastic, and Mytilus edulis shells of approximately equal area. One element of each of these substrates was placed in the cylinder at three depth levels. Light was provided from above, below, or from the side throughout the experiment. In some experiments, light was provided from the side only on the first or second day of the experiment. Planulae were counted two days after being placed in the cylinder. A total of 14721 planulae settled on substrates, from 269 to 5178 per experiment. Our experiments demonstrated that the least frequently chosen substrate was shells, ranging from 3.9 % under constant side lighting to 12.5 % under top lighting. Transparent and opaque plastic were chosen in 53.9 % and 41.0 % of cases, respectively, with constant side light, and 31.6 % and 56.1 % of cases with upper light. This choice can be attributed to the substrate shape—plastic plates, unlike shells, lacked a pronounced relief. The lower level was always the most preferred. Gravitaxis was most evident in the darkness — 93.3 % of the recorded planulae chose the lowest substrates. Level preference did not change dramatically between experiments. With constant side light, substrate side preference changed significantly comparing with most experiments with constant light — 51.2 % chose the upper sides (versus 27.6 % with top light, P = 0.268, 8.2 % with bottom light, P = 0.037, and 5.9 % in the dark, P = 0.027). A similar pattern was observed when the lights were turned off — 58.6 % of the planulae settled on the upper sides (P = 0.177 for upper light, 0.025 for lower light, and 0.019 for darkness). However, when the lights were turned on, only 24.1 % preferred the upper sides. This may indicate that it is light causes changes in settlement patterns. Planulae most frequently chose the undersides of substrates in the dark (93.2 % of cases) and under illumination from below (91.8 %). Thus, light primarily affects the settling side and does not fundamentally affect the patterns of choosing the height and type of substrate. The study was conducted under the state assignment of Lomonosov Moscow State University № 1025031900094-4-1.6.12-1.6.12.