In metacommunities, the dispersal of individuals between populations is very important for the dynamics of each population and the maintenance of the system and the diversity therein. Among different aspects of structures of dispersal connectivities, whether dispersal is symmetric or asymmetric is considered to have a large impact on the biodiversity of the system, especially in cases such as oceanic metacommunities, where dispersal is often directed. In order to obtain a clear picture about the degree to which asymmetry affects biodiversity, we simulate dispersal and compute coalescent time in networks with different degrees of asymmetry using two different approaches. First, we develop a theoretical framework in which we use small ideal networks with two nodes with parameters to measure asymmetry. Second, we analyse data from realistic coral larvae dispersal networks based on a directed network representing dispersal among coral populations in a part of Indo-Pacific Ocean modeled by ocean current simulation, and an undirected modification of this network. We use these approaches together to explore the mechanisms by which dispersal network asymmetry affects overall biodiversity.