Idotea balthica is a species of marine isopod[1] which lives on seaweed and seagrass[2] in the subtidal zone of rocky shores and sandy lagoons.

[5] The color of the body is extremely variable, ranging from muted greens to striking black-and-silver patternings; the female is usually darker.

[6] 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.

[6][7] 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.

[13][14] The isopod selects their habitat based on a trade-off between limiting predator risk and maximizing food intake.

During the day, both sexes of the isopod generally are less active in their search for food than at night [5] and favor concealing habitats to limit the risk of detection by a predator.

[5] Researchers have found that males tend to move between microhabitats more often than females and are more inclined to be reckless in regards to predation risks.

Not much is known about the mechanism of male-male competition, but sexual selection favors large appendages in males like larger secondary antennae.

[21][20] Idotea balthica have a long pre-copulatory phase where the male will attempt to begin mate guarding well before the parturial ecdysis of the female.

[19] On the other hand, the struggles before pre-copulatory pair formation and mate guarding can also result in females incurring fitness costs in the form of decreased fecundity and stored energy compounds.

[19] If that mechanisms fails, then the female may bend their body ventrally into a round configuration to make it difficult for the male to remain situated on top of them.

[22] The net result of engaging in a pair varies by population based on factors such as the operational sex ratio and the synchrony of reproduction.

[22] The level of aggressiveness is also dependent on other characteristics, such as degree of size dimorphism, which influence the likelihood of winning the conflict.