Understanding fundamental life history and ecological attributes of declining endemic fishes is essential for developing effective strategies for their conservation and recovery. In the Great Plains and desert rivers of North America, numerous imperiled leuciscids belong to a unique reproductive ecotype with drifting eggs and larvae (pelagophils). Herein, we synthesize three decades of research on the embryology and early life history of Rio Grande Silvery Minnow Hybognathus amarus, a federally endangered member of this ecotype, and explore how our findings can guide management and conservation of these sensitive taxa. We investigated three early developmental aspects of Hybognathus amarus through aquarium and laboratory studies: 1) egg morphology, development, and density, 2) larval development, growth, and behavior, and 3) morphologic and meristic analysis of larvae and early juveniles. Eggs nearly doubled in size at 10 min post-fertilization, were nonadhesive and nearly neutrally buoyant (specific gravity: 1.0011–1.0024), and hatched within 30 hrs in water about 23°C. Recently emerged protolarvae first transformed to mesolarvae after about one week, to metalarvae after about three weeks, and to juveniles after about six weeks at 20–24°C. Based on six candidate models, larval fish development (i.e., from protolarvae to early juveniles) was best explained by a cubic polynomial growth curve. While most protolarvae developed a gas bladder and began to feed within a week, the complete complement of fin rays (i.e., required for proficient swimming) had not fully formed until about one month post-hatching. Early developmental characteristics (e.g., egg specific gravity and larval fin ray formation), combined with river fragmentation, flow regulation, and habitat loss, can profoundly affect the upstream retention and recruitment of Hybognathus amarus and other native pelagophils. Long-term recovery of these highly imperiled species will depend on restoring sufficient seasonal flows, river and floodplain connectivity, and habitat complexity to promote their successful spawning, growth, and survival.
