Monoporeia affinis, formerly referred to as Pontoporeia affinis (Greek: pontos = sea, poreia = to travel), is a small, yellowish benthic amphipod living in the Baltic Sea, the Arctic Sea and the lakes of the Nordic Countries, measuring ca. 1 cm in length when fully grown. It is one of the Baltic glacial relicts. Originally a freshwater species, it also exists in lakes. M. affinis lives on soft bottoms, sometimes even as densely as 10,000 – 20,000 but usually hundreds to thousands of individuals per square metre. The amphipod has an important role in bioturbation (mixing and oxidating the bottom sediment).
After mating in the fall and bearing over the winter, the female M. affinis gives birth to 20 – 30 offspring, which only happens once during its 2 – 4-year lifespan.
The increasing loss of oxygen in the Baltic Sea bottoms – especially in the Gulf of Finland – has lately been affecting the M. affinis population, since its eggs and embryos are very sensitive to lack of oxygen. Thus M. affinis is often used as an indicator species of the state of the Baltic Sea.
Monoporeia affinis is closely related to another benthic amphipod Pontoporeia femorata, which can be distinguished from M. affinis by its light red eyes.