In the early spring of 2015 I found numerous specimens of Polychaos dubium. They all had a normal nucleus. Some weeks after, both the amoebae in the samples as and in the wild showed an enlarged nucleus, irregular of shape, granulated en with some larger more or less homogeneous pieces. This might be caused by a parasite.
Polychaos dubium, with abnormal nucleus
Normal nucleus, 29 µm, and abnormal nucleus, c. 43 µm
Polychaos dubium with large nucleus (for detail see below)
Nucleus, c. 43 µm in diameter; to the left of the nucleus is the contractile vacuole
n = nucleus
Trophozoit and its nucleus, c. 58 µm in diameter, somewhat compressed by the cover glass.