According to classical competition theory, interspecific competition increases with relatedness between species, rendering coexistence of closely related species unlikely. Cryptic species -species that are morphologically indistinguishable, but show genetic differences- do co-occur at small geographical scales, which seems at odds with classical competition theory. The underlying mechanisms of this co-occurrence remain unknown. We study interspecific interactions between four cryptic species of the Litoditis marina species complex. Interactions are high between the species in lab experiments, but permanent coexistence of closely related species may exist if the different species are locally adapted to different resources or/and abiotic variables. Differential effects of salinity, temperature, pollution, etc. are studied. Local dispersal can be another mechanism to facilitate in which competing species become spatially or temporarily segregated to some degree. Nematode dispersal is considered to be mostly passive, but active dispersal over short distances also occurs and may facilitate active emergence from the sediment. The effects of active dispersal on the competition and different drivers for dispersal are also an important aspect of this research. The cryptic diversity is important in terms of biodiversity estimates and ecosystem functioning. We also study if closely related nematodes play a differential role in decomposition processes.
