The survival of coral reef ecosystems and their menagerie of rainbowed residents relies on seldom seen, historically overlooked cryptobenthic reef fishes - the smallest of marine vertebrates. According to a new study, these tiny fish, no larger than five centimeters in length, play a vital - yet previously unrecognized - role in maintaining reef diversity and productivity in what would otherwise be an oceanic desert. Vast expanses of Earth's tropical oceans are lonely, barren places largely bereft of productivity or life. Coral reefs punctuate these desolate regions with pockets of exceptional biological diversity. However, how coral reef ecosystems manage to survive and flourish in low-productivity oceans - a puzzle often termed "Darwin's Paradox" - has baffled scientists for nearly 200 years. Using data on larval and adult reef-fish dynamics and population modeling, Simon Brandl and colleagues revealed the hidden productivity of cryptobenthic reef fishes, which, despite countless hungry predators living around them, manage to maintain their populations in reefs because their larvae stay close, avoiding the perils of the open ocean. This results in a steady stream of larvae to rapidly replace each adult fish that is devoured by hungry predators. Brandl and colleagues show that this process contributes nearly 40% of reef-fish biodiversity and accounts for nearly two thirds of the near-reef larval fish pool. Critically, it also provides nearly 57% of the total fish flesh consumed by larger reef fish, organisms on which millions of people depend. Brandl et al. note that the massive contributions of these tiny fish to coral reef production have been rarely perceived because they are eaten almost as quickly as they are produced. However, due to their extraordinary and unique life histories, cryptobenthic fishes are the "dark productivity" underpinning the paradoxically high productivity of coral reefs. As reefs are currently undergoing dramatic changes due to climate change and manmade stressors, understanding the role of cryptobenthic reef fishes in reef survival may help protect what underpins reef-fish communities, and their value to humanity.