Corals are an important habitat for a diverse fauna which 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 during three months. Colony size and bleaching were also considered as two additional factors. We demonstrated that even at the early stage of the colonies’ formation, the diversity of obligate symbiontson on these corals was higher than in any other studied local faunas of the World Ocean.Species richness and abundance of symbionts were positively related to colony size and density, while no impact of bleaching was found. Species richness and abundance of obligate symbionts increased along with the host population density, while in facultative symbionts, these indexes decreased. We hypothesized that the negative effect of the host density on species richness and abundance of facultative symbionts is caused by the competitive pressure of obligates, which increases with their abundance. Reasons for the different responses of obligate and facultative symbionts to host density were considered.

The material flow dynamic and velocity distribution on the melted domain surface play a crucial role on the joint quality and formation of welding defects. In this study, authors investigated the effects of the low and high currents of plasma arc welding on the material flow and thermodynamics of molten pool and its relationship to the welding defects. The high-speed video camera (HSVC) was used to observe the convection of the melted domain and welded-joint appearance. Furthermore, to consider the Marangoni force activation, the temperature on the melted domain was measured by a thermal HSVC. The results revealed that the velocity distribution on the weld surface was higher than that inside the molten weld pool due to the difference of the massive density between the air and the steel. Moreover, in the case of low welding current (80A) the convection speed of molten was faster than that of the high welding current case (160A) owing to the difference of main driving forces direction and strength, which leading to undercut and humping defects on the weld surface and excessive convex (burn-through) defect at the bottom weld side, respectively. The medium welding current (120A) had two convection patterns with the main flow in backward direction, which resulted in better welding quality without defect. The interaction between the shear force and Marangoni force played a solid state on the convection and heat transportation processes in the plasma arc welding process.