BULLETIN OF MARINE SCIENCE, 28(4): 659-666, 1978 ORIENTATION AND SOCIAL BEHAVIOR IN THE SUPRALITTORAL ISOPOD LIGIA EXOTICA (CRUSTACEA: ONISCOIDEA) James A. Farr ABSTRACT Ligia exotica is aggregative, tending to seek out conspecifics. It is hypothesized that Ligia must seek new shelters repeatedly with changing tides because of their restricted water requirements, and that attraction to con specifics already in a suitable habitat is an additional orientational mechanism to those external environmental factors influencing microhabitat selection. Experiments on object orientation demonstrated a strong tendency to move toward contrasting landmarks, the result being occupation of rocky areas on the shoreline. Brief descriptions of sexual and agonistic behavior are included. It is generally held that terrestrial isopods, ties to facilitate location of wet, but not suborder Oniscoidea, are poorly adapted submerged, habitats. Because Ligia must physiologically for an existence on land, and occupy very wet areas, behavioral mecha- that their ability to minimize desiccation is nisms for water conservation may not be as a consequence of several behavioral adapta- prominent as those for finding water. tions resulting in their occupying cool, The present study was initiated to investi- humid areas (Edney, 1954, 1968; Warburg, gate the tendency of Ligia exotica to 1968). Three oniscoid genera, Porcellio, aggregate. They often cluster in large num- Oniscus, and Armadillidium, have been bers under rocks in the supralittoral zone, studied extensively in this regard. These but it is unknown if they orient to preferred animals are hygropositive and photonegative, microhabitats directly or if they actively and thus are able to find favorable micro- seek conspecifics. In the experiments on habitats (Abbott, 1918; Gunn, 1937; Wal- aggregation, oriented responses to environ- off, 1941; Warburg, 1968). A classic study mental stimuli were revealed, and these by Allee (1926) and subsequent work by were investigated further. Additionally, ob- Friedlander (1965) demonstrated that the servations on sexual and agonistic behavior tendency of land isopods to aggregate is an of L. exotica are also summarized herein. important mechanism for reducing desicca- tion. Orientation to external stimuli and MATERIALS AND METHODS attraction to conspecifics are both essential Ligia exotica were captured in rocky areas in counteracting the harshness of the emer- on the shore of Santa Rosa Sound at the gence from water. U.S. Environmental Protection Agency The oniscoid genus Ligia is much more Laboratory on Sabine Island, Escambia restricted to a wet environment than those County, Florida. Movements and abun- genera mentioned above. Of those genera dance were observed in the natural habitat. studied to date, Ligia is the most susceptible Most orientation work and some experiments to desiccation (Edney, 1951, 1954; Mayes on the tendency to aggregate were conducted and Holdich, 1975), and yet prolonged sub- outdoors at the EPA Laboratory in a mersion in seawater is also fatal (Hewitt, circular fiberglass tank, 2.4 m in diameter 1907; Tait, 1917; Barnes, 1932). Ligia, and 0.9-m high. The tank was placed in thus restricted to the immediate supralittoral an open area away from buildings and trees, zone, inhabit rocky areas at the water's edge. and the bottom was covered with 4.0 cm One would expect its orientational capabili- white beach sand kept moist with 15-20 %0 659 660 BULLETIN OF MARINE SCIENCE, VOL. 28, NO.4, 1978 seawater. Other experiments on aggregation In the second test, small glass aquaria were conducted in the laboratory in glass were used in the laboratory to examine aquaria, 50 cm X 30 cm, with 2.0 cm white further the hypothesis that L. exotica seek beach sand moistened with 20 %0 seawater. conspecifics. For each run, two animals I observed social behavior of animals in a were placed into an aquarium containing large glass aquarium, 1.2 m X 0.3 m with two clay saucers cleaned in tapwater. I re- sand and several rocks for shelter. Labora- corded whether the same or different saucers tory animals were maintained on a 12: 12 were selected as shelters. The probability photoperiod at 22-24°C and fed a dried that both animals would choose the same flake tropical fish food. saucer is p = 0.5. If the animals tend to Three experiments were conducted to be socially attracted to each other, a binomial determine if L. exotica aggregate because test would show the mean to be greater than they actively seek conspecifics or aggregate 0.5 N. Forty-eight pairs of animals were by chance in preferred microhabitats. In tested: 16 runs used two males, 16 used the first experiment 60 10.0-cm diameter two females, and 16 tested heterosexual red-clay saucers were arranged in a circle pairs. Different animals were used in each inside the circular outdoor tank so that an run. edge of each saucer rested on the lip of the Finally, a third experiment was designed adjacent saucer. For each of 10 replicates, to determine if chemoreception might be used 59 to 81 animals were released in the morn- in locating conspecifics. In each run a single ing in the center of the arena. After 24 h, animal was released into a small glass the number of animals residing underneath aquarium with two saucers. One saucer was each saucer was recorded. Following each placed in the stock aquarium with approxi- run, all animals were returned to the en- mately 75 adult animals for 24 h prior to its vironment and new ones captured for the use, and the other was cleaned in tapwater next run. prior to its use. In each run I recorded Two statistical tests were performed on whether the animal selected the clean or the distribution of animals in each replicate. the conditioned saucer; 25 animals of un- First, I tested whether they tended to determined sex were used. Preference for aggregate. If shelter seeking is random, the the conditioned saucer would demonstrate distribution of animals under the saucers an attraction to a chemical factor associated should be Poisson-distributed; deviation with the stock cage and hence, with con- from a Poisson distribution would show that specifics. The previous experiment with two they aggregated. However, merely demon- animals and two cleaned saucers, and earlier strating aggregation would not necessarily observations of single animals with one or demonstrate a tendency to seek can specifics, two cleaned saucers indicated no aversion to because the aggregations could result from cleaning with tapwater; all animals occupied orientation to an external stimulus rather a shelter. than to conspecifics. Therefore, I used In preliminary tests of aggregation in L. Rayleigh's test (Batschelet, 1972; Zar, exotica, it was discovered that they tended 1974) to test for a preferred directional to aggregate in the direction of a large over- orientation. Significant aggregation in the hanging tree visible from the floor of the absence of a preferred direction would sup- tank, and the circular tank had to be moved port the hypothesis that they are attracted to a more open location. Herrnkind (1968, to conspecifics. If the preferred direction 1972) discussed the importance of orienta- varied from test to test, then one cannot tion to areas of sharp optical contrast in separate orientation to a changing array of shore-living arthropods, particularly fiddler external stimuli from orientation to con- crabs of the genus Uca. Therefore, my tests specifics in different sections of the tank. were designed to discover if such object FARR: BEHAVIOR OF LlCIA EXOTICA 661 Table 1. Results of experiments on aggregation and directional preference in Ligia exotica. Significant X' values (d.f. = 5) indicate runs in which the animals tended to aggregate. Significant values of Rayleigh's r indicate preference for a single direction X' for comparison Rayleigh's r No. animals with Poisson for length of Mean angle Run tested distribution mean vector of orientation 1 81 54.46** 0.1590 2 78 11.00 0.0719 3 67 17.76** 0.2302 4 58 312.13** 0.3368** 5 63 48.83** 0.2972** 6 73 94.21 ** 0.2286* 7 71 185.39** 0.1337 8 59 31.44** 0.2033 9 76 42.07':"~ 0.0109 10 75 27.03** 0.2688** • significant at the 0.05 level. •• significant at the 0.005 level. orientation is exhibited by Ligia. Two ex- Ligia to aggregate. Runs 4, 5, 6, and 10 periments were conducted in the circular showed significant directional preferences, outdoor tank with 60 clay saucers arranged the preferred compass bearings ranging from in a circle as shelters. In each experiment, 124° to 330°. In four runs I observed ag- a piece of black vinyl plastic was used to gregation in a preferred direction, indicating cover a 40° section reaching from top to orientation to conspecifics only, to a variable bottom of the light blue walls of the tank. set of external stimuli, or both. In runs 1, A group of 54 to 81 animals was released 3, 7, 8, and 9, aggregation was exhibited in the center of the circle of saucers, and without any significant unimodal directional their distribution under the shelters was preference, and from this I conclude that analyzed to determine directional preference L. exotica actively seek conspecifics. In one with respect to the center of the black run, I observed neither aggregation nor pre- plastic. One experiment tested the location ferred direction. It should be noted that I of animals relative to the black plastic 15 tested only for preference to orient in a single to 30 min after their release. A second ex- direction. Conclusions that no directional periment examined their location 24 h after preference was exhibited could actually be their release. Each experiment consisted of false; different individuals could orient to eight runs, the piece of black plastic being different external stimuli, but the statistical placed in a different 45° segment of the test would show no preference.