Progress of Evo‐Devo requires broad phylogenetic sampling providing the data for comparative analysis as well as new objects suitable for experimental investigation. Representatives of the early‐branching animal phylum Cnidaria and particularly hydrozoans draw great attention due to the high diversity of embryonic and post‐embryonic development and life‐cycles in general. Most detailed studies on embryonic development in hydrozoans were performed on the species shedding their gametes with subsequent embryo development in the water column. Widely distributed thecate hydrozoan Gonothyraea loveni broods its embryos within reduced medusae attached to the colony until development of a free‐swimming metamorphosis competent planula‐larva. In the current essay we present a detailed description of G. loveni embryonic development based on in vivo observations, histology, immuno‐cytochemistry, and electron microscopy. Starting from early cleavage, the embryo becomes a morula without any sign of blastocoele. Gastrulation proceeds as mixed delamination and ends with parenchymula formation. The first morphological sign of primary body axis appears only in the beginning of parenchymula‐preplanula transition. In mature metamorphosis competent planula only the cells of the oral two‐thirds of endoderm retain proliferative activity resulting in accumulation of great number of i‐cells and nematoblasts, which can be used during metamorphosis accompanied with essential reorganization of larval tissues. G. loveni demonstrates the diversity as well as evolutionary plasticity of hydrozoans development: in brooding hydrozoans embryonic and larval development is highly embryonized in comparison with the spawning species with free‐swimming embryos.

Corals provide an important habitat for diverse fauna. The habitat is especially rich in branching species. Each colony harbors several species and dozens of individuals, forming an integrated symbiotic community. Factors affecting the diversity and abundance of coral symbionts are poorly understood. The present study tested experimentally the relationship between the population density of coral Pocillopora verrucosa and the diversity and abundance of symbionts in planted coral fragments exposed for three months. Colony size and bleaching were also considered as two additional factors. We demonstrated that, even at the early stage of colony formation, the diversity of obligate symbionts on planted corals in Nha Trang Bay was higher than the diversity of any other studied local faunas of the world’s ocean. Species richness and the abundance of symbionts were positively related to colony size and density, while no impact of bleaching was found. Species richness and the abundance of obligate symbionts increased with host population density, while in facultative symbionts, these indices decreased. We hypothesized that the negative effect of host population density on species richness and the abundance of facultative symbionts is caused by the competitive pressure of obligates, which increases with their abundance. The reasons for the different responses of obligate and facultative symbionts to host density were considered.