  Emerging marine infectious diseases pose a substantial threat to marine
ecosystems and the conservation of their biodiversity. Compartmental models of
epidemic transmission in marine sessile organisms, available only recently, are
based on non-spatial descriptions in which space is homogenised and parasite
mobility is not explicitly accounted for. However, in realistic scenarios
epidemic transmission is conditioned by the spatial distribution of hosts and
the parasites mobility patterns, calling for a explicit description of space.
In this work we develop a spatially-explicit individual-based model to study
disease transmission by waterborne parasites in sessile marine populations. We
investigate the impact of spatial disease transmission through extensive
numerical simulations and theoretical analysis. Specifically, the effects of
parasite mobility into the epidemic threshold and the temporal progression of
the epidemic are assessed. We show that larger values of pathogen mobility
imply more severe epidemics, as the number of infections increases, and shorter
time-scales to extinction. An analytical expression for the basic reproduction
number of the spatial model is derived as function of the non-spatial
counterpart, which characterises a transition between a disease-free and a
propagation phase, in which the disease propagates over a large fraction of the
system.
