ИСТИНА

As demonstrated by laboratory experiments, the patterns of feeding intensity in colonial hydroids that prey on zooplankton depend on hydrodynamics (Marfenin, Malyutin, 1995). Given expressed periodicity of tidal currents, food intake may depend on the phase of the tidal cycle. This assumption was tested on specimens of Obelia longissima collected near the White Sea Biological Station of the Moscow State University during eight semi-diurnal tidal cycles between the 21st and 25th of July, 2013. The tidal cycle was divided into 14 phases, about one per hour. Specimens of O. longissima were gathered from a platform floating 50 meters from shore and preserved in 4% formaldehyde solution. Feeding intensity was determined based on the share of food containing hydranths. Five samples were obtained from each specimen, 100 hydranths (gastrozooids) per sample. When a hydranth captures food, it changes color. «Starved» (whitish) and «satiated» (rufous colored) hydranths were counted in each 100-hydranth sample, and the percentage of «full» and «empty» hydranths was thus determined. Environmental conditions at the time of specimen collection: calm, water temperature 12—15°C, northeast wind at 3–7 m/s, 60% cloud cover, atmospheric pressure 760.7 mm Hg. Velocity of tidal currents around and inside the hydroid thickets is non-homogeneous, which made it an unsuitable parameter for the purposes of this study. The percentage of food containing gastrozooids (FG) varies depending on the stage of the tidal cycle. Minimum values are observed at slack water stages: 22% at low tide and 24% at high tide. During tidal flow the share of FG increases, reaching 74% three hours before high tide and 84% three hours after high tide. The share of FG declines as the tide approaches high or low water. Food lumps in the stomach of O. longissima have been found to contain: Oithona similis (Claus 1866), Centropages hamatus (Lilljeborg 1853), Acartia longiremis (Lilljeborg 1853), Temora longicornis (Müller 1785), Pseudocalanus minutus (Kröyer 1848) and Microsetella norvegica (Boeck 1865); Podon leuckarti (Sars 1862) and Evadne nordmanni (Loven 1835); nauplii and cypris larvae of the cirripedian species Verruca stroemia (Müller 1776). We also discovered nauplii of other non-cirripedian crustaceans, as well as unidentified remains including crustacean fragments and «slurry» of unclear origin. The content of O.similis in the gastrozooids of O.longissima measures between 9% (an hour after high tide) and 27% (an hour before low tide) (Table 1). The content of C. hamatus can reach 4% at the stages «one hour before low tide» and «two hours before high tide», and the content of A.longiremis — from 5% two hours after low tide up to 21% three hours before high tide. The content of V. stroemia nauplii in the diet of O.longissima amounts to 30% at the stages «two hours before low tide» and «two hours before high tide», and the share of unidentified remains may add up to 48% an hour after high tide. M.norvegica is barely present in the stomach contents (1—3% of the total amount of food), however at the stages «one hour after low tide» and «two hours after high tide» its share reaches 9% and 13%, accordingly. P.calanus, T.longicornis, and cypris larvae of V.stroemia do not prevail in the diet of O.longissima, but individual members of these species were occasionally found in the food lumps. The share of food containing gastrozooids and the composition of food lumps in the hydranths of the colonial hydroid O. longissima varies considerably depending on the stage of the tidal cycle.