The ectosymbiont fauna associated with crustaceans in South American inland waters is characterized by a marked microendemism, as the species concerned appear to show high host specificity. The aim of the present study is to present a review of ciliated Protozoa reported as ectosymbionts on South American inland water malacostracans. The literature revealed the existence of three genera, Chilenophrys with one species, Lagenophrys with five species, and Operculigera with seven species. The main limiting factor is that many species descriptions require further validation, as the reports have not been confirmed. The literature reports the presence of Lagenophrys lenticulata associated with Hyalella curvispina from Uruguay. H. curvispina is widespread throughout southern South America, including Argentina and Chile, and it may be possible to find epibionts on other populations of H. curvispina , as well as on other Hyalella populations. Lagenophrys aegleae has been recorded on the genus Aegla in eastern and western South America; L. antichos on crayfishes in east and west South America; and L. lenticulata on amphipods in North America and Uruguay. Ecological and biogeographical topics are also addressed.

The amphipods of the genus Hyalella have many endemic species throughout South America, and these may have poorly studied associated species as epibionts. The aim of the present study was to review the literature on the presence of the protozoon Lagenophrys lenticulata (Kellicott, 1885) on species of Hyalella and contrast the results with the geographical distribution of the reported hosts. The literature records the presence of L. lenticulata on populations of H. azteca from Canada, Mexico and the United States of America, and on populations of H. curvispina from Uruguay. The literature also reports that H. curvispina is widespread throughout southern South America, including Argentina and Chile, and that it would be possible to find the presence of epibionts on other populations of H. curvispina. The corresponding biogeographical and evolutionary topics are discussed.

Hyalella Smith, 1874, is a diverse and ecologically important genus of freshwater amphipods widely distributed throughout the Americas, occurring from temperate North America to southern South America. Here, we provide a comprehensive overview of the current state of knowledge of Hyalella, integrating historical developments in its study with advances in taxonomy, systematics, biogeography, ecology, reproductive and population biology, and molecular research. Despite substantial progress in recent decades and an accelerating rate of species descriptions, several regions of the Americas remain poorly sampled, while cryptic diversity, unresolved species complexes, and limited molecular information continue to challenge estimates of the actual diversity and evolutionary history of the genus. Future research should prioritize integrative taxonomy combining morphological and molecular evidence, expanded geographic and habitat-based sampling, phylogenomic approaches, standardized genetic reference libraries, ecological and physiological studies, and the application of emerging tools such as environmental DNA and species distribution modelling. By integrating more than a century of research, this review identifies major advances and persistent knowledge gaps and provides a framework for future studies aimed at understanding the diversity, evolution, ecology, and conservation of one of the most distinctive freshwater amphipod radiations in the Americas.