Embryonic and early postembryonic development of the freshwater isopod crustacean Asellus aquaticus - staging based on morphology

The developmental biology of arthropods has advanced rapidly in recent decades, largely through the application of novel experimental techniques to a limited number of established model organisms. Extending comparative embryology beyond these traditional models is essential for a comprehensive understanding of arthropod evolution. Among crustaceans, the freshwater isopod Asellus aquaticus Linnaeus (1758) represents a promising emerging evo-devo model. This widespread European species inhabits diverse freshwater environments, including groundwater, resulting in pronounced phenotypic variation, most apparent in cave-adapted populations. It is easy to culture in the laboratory, reproduces year-round, and produces numerous accessible embryos with relatively fast development. Here, we present the most comprehensive developmental staging system for A. aquaticus to date. Using light microscopy, fluorescence microscopy with DAPI staining, and transmission electron microscopy, we integrate event-based and age-based landmarks to define nine embryonic stages (S1-S9) and a first postembryonic, premanca stage (S10). We further provide a coarse description of postembryonic development through mancae and juveniles to sexual maturity and provide a practical guide for non-invasive staging of embryos within intact marsupia of live gravid females. This refined staging framework resolves ambiguities in previous descriptions and provides a precise temporal backbone for comparative and functional evo-devo studies of arthropods. It enables detailed investigations of developmental divergence between surface and cave populations, facilitates field-based studies of reproductive phenology and environmental effects on development, with potential applications in climate-change biology. Together, these advances establish A. aquaticus as a powerful model for studying the evolution of development across environmental gradients and adaptive transitions.