This paper focuses on the biology and ecological impacts of Ichthyodinium chabelardi (phylum Dinophyta, class Syndiniophyceae, order Syndiniales), a virulent endobiotic parasite of yolk sacs and young larvae of many species of marine fin fish. Its infections have been observed in warm and temperate open oceanic environments and crowded marine fish tanks. The prevalence of I. chabelardi and the range of its host fishes is not well studied, and our understanding of its life cycle is incomplete. Here, we describe what is known about I. chabelardi infections in fish and we compare this with several other protistan parasites of fish, including Amyloodium ocellatum, Saprolegnia parasitica, Sphaerothecum destruens and the ‘X-cell’ clades Gadixcellia and Xcellia, all of which are considered emerging generalist parasites infecting a wide variety of fin fish species. Recent findings suggest that rising seawater temperatures might lead to higher infection rates in fishes, and we expect that these changing conditions could also expand the ranges of some of these parasitic species. Thus, it is essential that the fishing industry effectively monitors fish tanks and water in the surrounding environments for the presence of zoosporic parasites, including I. chabelardi, so as to take steps to prevent large losses in these fisheries.