Gregarious aggregative behavior in the marine isopod Cirolana harfordi

Abstract

Gregarious huddling behavior where animals seek the close company of conspecifics is thought to have facilitated the evolutionary transition from sea to land in some species of isopods, the theory being that closely apposed bodies decrease surface area and reduce desiccation. In this study, specimens of the marine isopod Cirolana harfordi were investigated to determine whether they displayed gregarious aggregative behavior. Animals were placed in a 20‐cm‐diameter acrylic cylinder arena that contained two 5‐cm‐diameter circular acrylic shelters that were tinted red (to reduce light transmittance), one at either end of the arena. Specimens of C. harfordi chose one of the two identical shelters at random, and the majority of animals congregated under this winning shelter. When an arena with one shelter tinted red and one clear shelter was used, the majority of animals congregated under the red shelter. These data indicate that the aggregative behavior displayed by this marine isopod species is driven by gregarious, conspecific attraction as well as reactions to environmental heterogeneity and negative phototaxis. Gregarious behavior in this marine isopod may afford the species advantages other than avoiding desiccation.

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

“…The spaces under the acrylic disc shelters in the present study are analogous to a rock cover or crevice in the natural environment because they provide a hard overhead surface. Similar to other species of isopod, C. harfordi is negatively phototactic and is attracted to light‐reducing acrylic disc shelters (Abbott, ; Devigne et al ., ; Broly et al ., ; Salma & Thomson, ), although the present study shows that C. harfordi individuals are attracted to the space under acrylic shelters with a diameter of 5 cm even when these shelters do not offer a reduction in light. Little is known about the acuity of eyesight in C. harfordi .…”

“…The presence of more conspecifics in a group may amplify positive feedback between individuals and help the aggregation process (Broly et al ., ). Group size (between 4 and 16 individuals) is reported to have no effect on social aggregation of C. harfordi specimens under the winning shelter in an arena with two red shelters, although a small but significant positive effect of increased group size exists with respect to the total percentage of the specimens positioning themselves under the two shelters collectively, as opposed to staying out in the open (Salma & Thomson, ). By contrast, there is no significant amplification of the attractiveness of the two clear shelters with increased group size in the present study.…”

“…). Similar arenas have been used for both C. harfordi (Salma & Thomson, ) and the terrestrial isopod Porcellio scaber , with the exception that the shelter discs were tinted red and markedly reduced the light penetration through the shelter (Devigne et al ., ; Broly et al ., ; Broly et al ., ). A video camera (Sony handicam, product number HDR‐CX190E; Sony Corp., Japan) was attached to a tripod so that it pointed straight down over the arena.…”

“…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 ., ). Comparable results were obtained with C. harfordi using a similar arena with red tinted shelters (Salma & Thomson, ). The reason for using red tinted shelters in that arena system was that isopods are negatively phototactic and the red tinting reduces light transmittance through the shelter.…”

“…In support of the proposal that aggregative behaviour in isopods occurred before their transition to land, social aggregative behaviour has been demonstrated in the marine isopod Cirolana harfordi (Lockington, ) where specimens were in water and in no danger of desiccation (Salma & Thomson, ). Cirolana harfordi is a common cirolanid (belonging to the family Cirolanidae) isopod found in the rocky intertidal zones of the Pacific coast of North America, Australia and Japan (Johnson, ; Bruce, ).…”