ИСТИНА

We studied the body plan formation in the sedentary annelid Ophelia limacine. The animals burrow in the sand of the upper subtidal zone of the Great Salma Strait, the White Sea. Notably, for the first glance the sediment and hydrological conditions are the same along the beach of the Salma Strait, but Ophelia inhabits the very restricted area. Here, we aimed to find out peculiarities of larval morphology of O. limacine in order to explain its restricted habitat. In June-July 2015-2018, we collected adult worms in close vicinity of the WSBS MSU, performed artificial fertilizations, kept cultures under 12°C degree, and salinity 25%о, fed with algae Nanochloropsis, Rhynomonas and Nitzschia. During the 30-35 days the embryos develop to larvae, settle, and metamorphise into juveniles. We have studied external morphology with light and scanning electron microscopies, cilia beating and swimming behavior with the high speed camera, ciliogenesis and neurogenesis with the application of the immunocytochemistry combined with the confocal laser scanning microscopy, the cell proliferation in the forming segments by EdU labeling. The 30 hpf gastrulae with the prototroch start swimming. The 10-12 dpf larvae which are ready to settle have the active cell proliferation in the zone between prototroch and telotroch, they have triple ventral nerve cord, circumesophageal connectives , and one transverse commissure, set of ciliary bands (proto-, telo-, and neurotroch) and apical tuft, capillary chaetae, gut develops (though does not function). Larvae at this stages are swimming in the water column. As larva settles, the first four segments appear simultaneously. The next segments appear in the posterior zone in front of the telotroch. Larvae start feeding and crawling on the bottom of the dishes. As a result, we hypothesize that larvae remain in the water column until they have small compact body shape; whereas the settled larvae have the segmented and extended body. As a side result, we hypothesize the interconnection of two processes: the segment formation process triggers the mechanism of the larval settlement. We acknowledge Tatiana Belevich (Dept. of Hydrobiology, MSU) and Andreas Hejnol (Sars Center, Univ. of Bergen) for the algae package; WSBS labs for immunocytochemical and EdU labelling.