Effects of group size on aggregation against desiccation in woodlice (Isopoda:Oniscidea)

Abstract

Aggregation in terrestrial isopods, a behaviour that results in the formation of dense clusters, is readily accepted as a mechanism of resistance to desiccation. Thus, aggregation is considered to be an adaptation to terrestrial life in this fully terrestrial suborder of crustaceans. In the present study of Porcellio scaber Latreille, a cosmopolitan species, individual water loss is investigated experimentally as a function of the size of the aggregates and, for the first time, over a large range of group sizes (groups of 1, 10, 20, 40, 60, 80 and 100 individuals). From the perspective of an isolated individual, aggregation behaviour is effective in reducing the rate of water loss whatever the group size, and reduces the individual water loss rate by more than half in large groups. However, the water loss rate of an individual follows a power law according to group size. Accordingly, if the addition of individuals to small groups strongly reduces the water losses per individual, adding individuals to large groups only slightly reduces the individual water losses. Thus, the successful reduction of the water loss rate by this aggregation behaviour is confirmed, although only up to a certain limit, particularly if the number of individuals per aggregate exceeds 50–60 under the experimental conditions used in the present study. Moreover, the individual surface area exposed to the air, as a function of group size, follows a similar pattern (i.e. a similar power law). Thus, a geometrical explanation is proposed for the nonlinear water losses in woodlice aggregates. These results are discussed in relation to the group sizes observed both in the laboratory and the field.

Mixed‐species aggregations in arthropods

“…A striking example is provided by terrestrial crustaceous woodlice, for which desiccation is a major concern as they are very sensitive to water loss. In response to this environmental stress, aggregation has been shown to offer protection against drying (Broly et al, 2014) (Fig. 1D).…”

Gregarious aggregative behavior in the marine isopod Cirolana harfordi

“…; Broly et al. ). More recently the presence of aggregative behavior in many species of terrestrial isopod led to the proposal that aggregative behavior in isopods had a more ancient origin and evolved in marine isopods (Broly et al.…”

“…American individuals of C. harfordi have often been found in wet sand under rocks exposed at low tide and emerge at high tide to swim, scavenge, and hunt (Johnson 1976). Despite the common occurrence of C. harfordi and other cirolanid isopods in the Pacific region, little is known about their behavior; for example, prior to this study, it was not known whether they display gregarious aggregative behavior as do terrestrial isopods (Devigne et al 2011;Broly et al 2012Broly et al , 2013aBroly et al , 2014.…”

“…It has been proposed that a primary reason that terrestrial isopods evolved gregarious and closely aggregative behavior was to reduce evaporative surface area and reduce desiccation (Allee ; Edney , ; Broly et al. ). More recently, however, it has been suggested that gregarious aggregation in terrestrial isopods brings other advantages such as reduction of danger from predators and making it easier for individuals to find a good place to shelter (Broly et al.…”

Social aggregation of the marine isopodCirolana harfordidoes not rely on the availability of light‐reducing shelters

“…That study employed a cylindrical arena (diameter 19.3 cm) made from poly(methyl methacrylate) (i.e. acrylic) with two circular red tinted acrylic shelters (diameter 3.5 cm) raised 5 mm from the floor at either side of the arena (Devigne et al ., ; Broly et al ., ). When specimens of P. scaber were placed in the arena, the majority of the group of specimens randomly picked one of the two shelters and aggregated under this shelter, thus demonstrating that they actively seek each other's company (Devigne et al ., , Broly et al ., ).…”