[2] The legs arising from the first three segments of the abdomen are expanded basally to form broad plates.

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.

M. affinis is itself the prey of Saduria entomon, Harmothoe sarsi (a polychaete) and fishes such as cod, herring and the fourhorn sculpin, Myoxocephalus quadricornis.

[5] After mating in the autumn 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.

It was moved to the new genus Monoporeia by Edward L. Bousfield in 1989, alongside two other species, "M. microphthalma" and "M. gurjanovae",[6] which are now considered synonyms of M. affinis.