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Idotea balthica is a species of marine isopod which lives on seaweed and seagrass in the subtidal zone of rocky shores and sandy lagoons.
The color of the body is extremely variable, ranging from muted greens to striking black-and-silver patternings; the female is usually darker. On the other hand, the males are frequently colored a light green. Color intensity is variable and can be altered through the restriction and dilation of . Additionally, 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 Camouflage in multiple microhabitats.
The species can be distinguished from other Idoteidae by the shape of the telson, which is dorsally keeled with straight sides in I. balthica, and has a distinct protrusion at the end.
Microhabitat choice is also impacted by the life stage of the organism, with the microhabitat selection of an adult being more heavily influenced by predator avoidance than that of a juvenile. However, microhabitat selection is not dependent on the color morph of the organism. Conversely, varying microhabitat preferences between different colorations of this isopod produce differing levels of predation that select for a color polymorphism.
Finally, their habitat selection is influenced by interspecific competition where the more dominant species will often outcompete the less dominant species in shared habitats and subsequently restrict the microhabitat choices of the less dominant species. This effect is displayed in a study of the competition between Idotea balthica and Idotea emarginata in shared environments. Idotea balthica appears to have to settle for migrating between suboptimal habitats while Idotea emarginata (the more dominant species) are able to settle permanently in habitats with better resources .
Once the males have secured a mate, they will attempt to initiate pre-copulatory mate guarding to limit other males' access to the mated female. These pre-copulatory pairs are often initiated around a week before parturial ecdysis because the male wants to monopolize copulations of the female. If a pre-copulatory pair is formed, the male will carry the female around on its back until it undergoes moulting in two phases (anterior/posterior) and becomes receptive.
Other males will sometimes attempt to take over a mating pair if it is larger than the paired male. This form of intrasexual competition is part of the reason why paired males tend to be larger on average than solitary males.
On the other hand, female resistance to mate guarding attempts is a way for females to engage in female mate choice. Female resistance is a mechanism of selecting for larger sizes (or other phenotypic traits) in males because males with certain phenotypes are more effective in preserving pre-copulatory pairs, and therefore have a higher fitness. Female resistance also selects for additional aggressiveness in males. Certain phenotypes are also selected for in males by enabling them to be more successful at male-male competition.
Female resistance to mate guarding typically consists of the female writhing strongly to throw off the male. 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. Males may kick back in response to these resistance measures. Either this leads to the male being kicked off, the female escaping, or, if the female is close to parturial ecdysis and the male perseveres through the resistances, a pre-copulatory mating pair formation. Evidence has shown that the resolution of this conflict is frequently decided more by the female interest.
The outcome of this conflict is dictated by the level of aggressiveness displayed by the male and female. The level of aggressiveness displayed by both sexes is dependent on the benefits of a pre-copulatory pair compared to the costs of engaging in pre-copulatory mate guarding at a specific stage in the female's reproductive cycle. 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. The level of aggressiveness is also dependent on other characteristics, such as degree of size dimorphism, which influence the likelihood of winning the conflict. Females have been found to display more resistance, and apply certain resistance maneuvers more frequently, when there is less size dimorphism in the population because the males are not as large and there is a lower of cost of resistance for the female and a greater chance for success.
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