Baltic Isopod

The male is larger than female, and can reach 4 centimetres (1.6 in) long, while the female reaches a maximum length of around 1.8 cm. The male has most likely evolved to be larger than the female on average because larger size is more vital for the reproductive success of the male compared to the female. The color of the body is extremely variable, ranging from muted greens to striking black-and-silver patternings; the female is usually darker. The males are frequently colored a light green. Color intensity is variable and can be altered through the restriction and dilation of chromatophores. Males and females display different coloration (sexual dimorphism) due to selection for different behaviors in the two sexes. Researchers also believe that the primary cause of selection for this color polymorphism within localized populations is predation (from carnivorous fish, crabs) in conjunction with gene flow and varying microhabitats. The color polymorphism allows for the isopod to display cryptic coloration in multiple microhabitats. The species can be distinguished from other idoteids by the shape of the telson, which is dorsally keeled with straight sides in I. balthica, and has a distinct protrusion at the end.

Idotea balthica has a broad geographical distribution, having been recorded from the Belgian Exclusive Economic Zone, the British Isles, Cobscook Bay, Dutch Exclusive Economic Zone, European waters, Greek Exclusive Economic Zone, Gulf of Maine, Knokke, North West Atlantic, Red Sea, Voordelta, West Coast of Norway, Wimereux and the Black Sea.

In 2022, I. balthica was discovered to help Gracilaria gracilis reproduce – the first known case of an animal helping algae reproduce.