Why do river fish have more thorns than sea fish?

The trend of biological evolution is that the number of bones decreases, and the fish with less spurs are more advanced in evolutionary sequence, such as perch and flounder. On the contrary, there are many bone spurs, such as 387 skulls of silver carp and many bone spurs of anchovies. Therefore, it can't be generalized that freshwater fish are spinier than marine fish. It can only be said that the river fish we usually eat are relatively low-level fish, while the marine fish we usually catch are relatively advanced. River fish also have fewer bone spurs, such as freshwater pomfret, mandarin fish and perch.