Social aggregation of the marine isopod Cirolana harfordi does not rely on the availability of light-reducing shelters: Social aggregation in a marine isopod

Social aggregation under shelters can afford benefits to animals such as protection from predators. Many isopods and insects are negatively phototactic and this may help them gravitate towards shelter. Previous studies show that, when placed in an arena with two red shelters, specimens of the marine isopod Cirolana harfordi and the terrestrial isopod Porcelio scaber pick one of the two shelters at random and aggregate under it, demonstrating social aggregation under light-reducing shelters. In the present study, an arena with two clear shelters was used to determine whether group sizes of 4, 8, 12 and 16 C. harfordi specimens display social aggregation when the shelters do not accommodate negative phototaxis. In all group sizes, C. harfordi specimens picked one of the shelters at random and significantly more animals aggregated under this shelter compared with the other. Cirolana harfordi also displayed aggregation in an arena with no shelters. Accordingly, C. harfordi specimens do not require shelters that reduce light to display social aggregation. The ability to locate suitable shelter under which there is no substantial reduction in light could benefit the animal in a natural environment comprising heavily shaded areas, as well as at night.