An Overview of the Genus Hyalella Smith, 1874 (Amphipoda, Hyalellidae): Progress and Future Perspectives

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.