ИСТИНА

Summer is the season of active growth of diatoms on the stems of colonial hydroids inhabiting the White Sea, especially littoral species in the family Campanulariidae, which are characterized by large elaborately branched stems covered with hydranths enclosed by hydrothecae. Hydroids feed on zooplankton: food is captured when prey comes into contact with the tentacles of hydranths. This feeding strategy is more efficient in streaming water. As the flow of water around the stems slows down, the number of prey captured per unit of time declines as well. Diatomic fouling (microepiphyton) undermines the efficiency of feeding in hydroids. As the density of periphytic communities increase, the velocity of water in the area of direct contact with hydroids declines, obstructing the contact of prey with hydranth tentacles. The process of microepiphytic biofouling overlaps with the hydranths' own development cycles: in the case of Obelia longissima the life span of hydranths is short (usually 7–10 days), after which they are resorbed and their hydrothecae fall off, and new hydranths are formed in their place, with hydrothecae not yet colonized by periphyton. The purpose of the study was to describe the diatomic colonization of the perisarc of large stems of O. longissima, taking into account the dynamics of their spatial distribution and possible impact on the life of the hydroid. At the end of August — beginning of September, the period of hydroids' active growth, two actively branching colonies, 20 cm long each, were selected for observation. Throughout the experiment the colonies were kept in the natural environment, i.e. growing on the lateral surface of a floating platform, submerged to a depth of 0.015 m. Every 24 hours for 10 days in a row the colonies were removed from water and one internode on each of the selected branches was observed, including the hydrotheca, pedicel and segment of the stalk; the distribution of periphytic diatoms was mapped, and division, attachment (immigration) and loss (elimination) of cells were quantified. To describe the general pattern of periphyton distribution on peripheral branches of a colony of O. longissima, it was decided to perform simultaneous count of fouling cells on hydrothecae and internodes of 61 branches with varying numbers of hydranths (between 4 and 10). Comparison of the intensity of fouling on the top ten hydrothecae of a stem demonstrated that fouling processes are most active at 3–5 hydrothecae proximal from the tip of the branch, and less pronounced those closer to the base of the branch. Accumulation of diatoms is more rapid on the perisarc of hydrotheca than on the pedicels and stalk. It is caused by increased water circulation activated by moving tentacles. It was also established that the species composition of periphytic communities is dominated by Synedra and Licmophora. Settling begins on the third day after the stem internode and the hydrotheca that crowns it are fully formed. Maximum rate of microepiphytic biomass increase is observed on the 6–7th day on internodes and on the 5–6th day on hydrothecae, and is mainly due to cell division and to a lesser extent to immigration. Peak biomass is observed on the 7th day after organ formation on stems, and on the 6th day on hydrothecae. This is followed by bulk elimination of diatom cells on the 7th day on hydrothecae and on the 8th day on internodes. Resorption of hydranths is observed around the same time. The regeneration cycle of hydrothecae does not affect their pedicels and the perisarc of internodes. Population decline on the internodes is due to sloughing off of larger colonies (>4 cells) of Synedra and Licmophora, which are characterized by higher drag — only one cell is left of each colony. Epiphytes growing on hydrothecae are eliminated by virtue of hydrothecae themselves being lost. Less abundant secondary fouling is observed on the 9th day on internodes and on the 10th day on hydrothecae (on newly formed 'cups'); it is associated with the division of residual cells on the internodes (to a lesser extent — by immigration), the availability of space on newly formed hydrothecae, and the proliferation of newly settled cells. It can be concluded that biofouling of O. longissima occurs along the margins of the colony, affecting the distal parts of peripheral branches.