ИСТИНА

Planktotrophic (p-trophic) larvae are hypothesized to be ancestral in annelids. Aiming to test this scenario, we study the development of the White Sea species Ophelia limacine from the Opheliidae/Capitellidae/Echiura (OCE) clade. First cleavages in Ophelia start as leotropic, equal, and synchronous with minor size differences in micro- and macromeres. Thoracophelia mucronate, Notomastus cf tenuis and two echiurids have strikingly similar pattern. Coeloblastulae such as in 7-20 hpf Ophelia are known in Notomastus and echiurids. Gastrulation by invagination (14-33 hpf in Ophelia) is typical for annelids without large yolky eggs. In early trochophore, 28-48 hpf, epithelization occurs, apical tuft appears. In trochophores, 1-4 dpf, wide episphere covers small hyposphere as in champignon a cap covers a stem, and forms a deep circular fold. Prototroch and metatroch run along the fold. Episphere forms a hood over a ciliated mouth. Esophagus makes a loop and leads to a stomach, which is separated by gastrointestinal valve from the intestine; anus dorsal. The first neurons along neurotroch. Ciliary episphere patch; long cilia of proto- and metatroch, 2-3 rows of short feeding cilia in the mentioned fold; some non-motile preoral cilia; telotroch. Late trochophore, 16-20 dpf, elongates hyposphere, somatic and visceral muscles form, two roots of circumesophageal connectives extend. Similar larval shape, ciliation, and gut structure in trochophores of Armandia polyophthalma, T. mucronate, N. cf tenuis, Urechis caupo, Lissomyema mellita. Most p-trophic larvae of OCE clade have similar pattern of cleavage, gastrulation and trochophore. Future analysis of p- and l-trophic development among the annelids is one of the basic requirements to understand the evolution of development of annelids. The studies supported by the grant MK-1164.2020.4.