Distribution of nematocytes differs in two types of gonophores in hydrozoanSarsia lovenii

Hydrozoan cnidarians are widely known for a diversity of life cycles. While some hydrozoan polyps produce medusae, in most species the gonophore remains attached to the polyp. Little is known about the mechanisms behind the loss of the medusal stage in hydrozoans. Hydrozoan Sarsia lovenii is a promising model for studying this issue. It is a polymorphic species with several haplogroups. One haplogroup produces attached eumedusoids and the other one buds free-swimming medusae. Here, we compared patterns of cell proliferation and distribution of nematocytes in medusoids, medusa buds and medusae of S. lovenii . Cell proliferation is absent from exumbrella of late medusa buds and medusae, but presumably i-cells proliferate in exumbrella of medusoids. In exumbrella of medusoids, we also observed evenly distributed nematocytes with capsules and expression of late nematogenesis-associated gene, Nowa . Nematocyte capsules and Nowa expression were also observed in exumbrella of medusa bud, but we did not detect prominent Nowa signal in the bell of developed medusa. It is also known that abundance of exumbrellar nematocysts signs immaturity in medusae of Sarsia genus. Our data demonstrate that nematocyte distribution and associated gene expression in medusoids resemble medusa buds rather than developed medusae. Thus, sexually mature medusoids exhibit juvenile somatic characters, demonstrating signs of neoteny. Research highlights Hydrozoan Sarsia lovenii has attached eumedusoids and free-swimming medusae. The distribution of nematocytes in eumedusoids resembles that in medusa buds. This may indicate neoteny of eumedusoids.