Which animal lives in mutual association with algae that supply it with oxygen and food?

The River Niger is Africas third-longest river (4,180km). It enters Nigeria from the northwest, flows through the country, and empties into the Atlantic through the Niger Delta.

Importance: transportation route, fishing, irrigation, hydroelectric power (Kainji Dam), and fertile floodplains for agriculture. Major cities like Lokoja and Onitsha sit along it.

The Niger-Benue confluence at Lokoja divides Nigeria geographically. The Niger Delta is Africas largest wetland and Nigerias oil-producing region.