Why are spermatic ducts and urinary ducts connected in some fish and amphibians?

In many fish and amphibians, the connection between spermatic and urinary ducts exists because these structures share a common developmental origin within the urogenital system.1

In these anamniotes, the male duct system is closely linked to the excretory system. Specifically, the seminiferous tubules of the testes connect to the kidney tubules of the mesonephros. Consequently, the mesonephric duct—which originally functions to drain the embryonic kidney—is utilized to transport both urine and sperm to the exterior.231

While this shared drainage is a basic pattern in many fish and amphibians, there is a general evolutionary tendency toward the separation of these functions. In some species, this has led to the development of independent ducts, such as the formation of separate ureters to drain the adult kidney while the original duct remains dedicated to sperm transport. The degree of separation varies significantly across different species, ranging from fully combined systems to those where the spermatic and urinary ducts empty independently.2