Danionella priapus

Danionella priapus is a diminutive transparent micro-fish from shallow streams of Bangladesh and Myanmar, known scientifically for males possessing an unusual ossified structure used in producing surprisingly loud acoustic signals — extraordinary for a fish barely 1 cm long. Like other Danionella, it is completely transparent, revealing the skeleton, organs, and reproductive state at a glance. In the aquarium it demands a stable, mature nano setup with extremely gentle water movement and a diet of micro-foods. The species is rarely encountered outside specialist circles but has attracted significant scientific attention. Husbandry parallels Danionella cerebrum: species-only or with equally tiny companions, ultra-gentle filtration, and infusoria-grade feeding for fry. Males defend small territories vocally and with brief displays but cause no physical harm.