Activity and Homing Behavior of Two Species of Acanthopleura (Mollusca: Polyplacophora) on a Subtropical Shore in Japan
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VENUS 65 (1-2): 123-139, 2006 Activity and Homing Behavior of Two Species of Acanthopleura (Mollusca: Polyplacophora) on a Subtropical Shore in Japan Eiji Yoshioka1 and Erika Fujitani2 1Department of Humanities, Kobe Yamate College, 6-5-2, Nakayamate-ave., Chuo-ku, Kobe 650-0004, Japan; [email protected] 2Department of Fisheries, School of Agriculture, Kinki University, 3327-204 Naka-machi, Nara 631-8505, Japan Abstract: Moving patterns and homing behavior in Acanthopleura gemmata and A. tenuispinosa were investigated on the rocky shore of Sesoko Island, Okinawa, Japan. In the daytime, both A. gemmata and A. tenuispinosa moved only when washed by sea water, while at night they moved not only when washed by water but also when they were exposed to the air. Almost all chitons rest in a fixed ‘home’ site in the daytime and during periods when the rocks are submerged. They do not move when strong sunlight heats the rocks in their habitat either. Surface temperatures of dry rocks under such conditions have been measured as high as 64.8 C. When under water, they suffer the risk of predation by fish or other carnivorous invertebrates. Their movement patterns can therefore be explained as avoidance of heat and desiccation of rock surfaces and predation. Homing behavior in these species was observed throughout the period of study. Homing and moving patterns of A. gemmata and A. tenuispinosa were studied to compare daytime and nighttime activity, including when they go out and when they come back ‘home’. The nighttime activity was longer than the daytime activity, and activity patterns were slightly different between these two species. The speed of movement was not significantly different between when they go out and when they return. Vying for use of locations as ‘home’ and cognitive behavior were observed, suggesting that cognition of geographic features is found among chitons. Keywords: Acanthopleura, homing behavior, geographic cognition Introduction Chitons belonging to the genus Acanthopleura are a main component of the intertidal fauna in the southern part of Japan. The intertidal zones are characterized by periodicities of 24 h daily light-dark cycle and ca. 12.4 h tidal ebb-flow cycle. Most chitons move and graze in the night, and intertidal chitons cling to rock surfaces or lodge under stones during daytime low tides. In several species, “homing behavior” has been reported ̶ these animals have a fixed ‘home’ and come back to that place every day (Thorne, 1967; Glynn, 1970; Demopulos, 1975; Chelazzi et al., 1983a, 1987; Mook, 1983; Nishihama et al., 1986; Nishihama & Nojima, 1990). Nishihama & Nojima (1990) estimated the activity patterns of Acanthopleura japonica by observing the behavior of chitons in a tank in the laboratory. Takenoshita (1998) observed chitons in the field during low tide periods, and estimated their activity pattern by surveying their excrement. It is very hard to observe intertidal chitons continuously, because the environment there changes severely and sometimes involves heavy wave action. The only continuous data of chiton activity from the spring tide to the neap tide were reported by Smith (1975) for Mopalia muscosa. Hence, our knowledge about activity patterns of intertidal chitons is scarce. Around the Okinawa Islands in the southern part of Japan, four species of Acanthopleura, 124 E. Yoshioka & E. Fujitani Fig. 1. Location and profile of Sesoko Island. Study site was indicated by the cross point of the meridian and latitude lines. A. gemmata, A. tenuispinosa, A. loochooana, and A. miles, inhabit the upper intertidal zone. These four species inhabit nearly the same areas, but they are dispersed in different places both horizontally and vertically. So, each species has a unique habitat range (Yoshioka & Nakashima, 1996). The purpose of the present study is to reveal and compare the activity patterns of the larger two species of Acanthopleura, A. gemmata and A. tenuispinosa, on Sesoko Island, Okinawa. We surveyed the activity of the individual chitons continuously for two periods from the spring tide to the neap tide, and recorded the water and tidal conditions. Materials and Methods Study area The survey was carried out on the intertidal rocky shore of Sesoko Island, Okinawa, near the Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus (Fig. 1), for two periods from 17 July to 1 August 2000 (16 days), and from 21 September to 5 October 2000 (15 days). The habitat observed is composed of bedrock Ryukyu limestone with many small projections (Fig. 2A). When both Acanthopleura gemmata and A. tenuispinosa are resting, they stay in cracks or small hollows. The resting places are indented a few millimeters to make a footmark of the chiton, and the base is smoothly polished. This is defined as a home ‘pit’ . Various animals inhabit the bedrock surveyed, including herbivorous mollusks (A. loochooana, Siphonaria laciniosa, Cellana testudinaria, Nerita plicata, Nerita helicinoides), carnivorous mollusks (Mancinella hippocastnum, Morula granulata, Purpura persica), crabs (Grapsus tenuicrustatus), blenny fishes, flatworms, and errantian polychaeta. Sessile animals such as barnacles, mussels, and sedentarian polychaeta are scarcely observed and the surface of the limestone is bare. Behavior of Acanthopleura in Okinawa 125 A B Fig. 2. Profile of the bedrock and objective chitons. A. Acanthopleura gemmata. B. A. tenuispinosa. Materials and methods The two species of Acanthopleura that are the subjects of the present study are shown in Fig. 2. In the first period (from 17 July to 1 August), 6 individuals of A. gemmata (A-F) and 6 individuals of A. tenuispinosa (I-N) were observed. In the latter period (21 September to 5 October), 2 individuals of A. gemmata (G, H) were added to the first six, and one individual of A. tenuispinosa (O) was added as a replacement for a disappeared chiton (N). All chitons observed were 4-6 cm in length, and marked by colored reflective tape individually according to the method introduced by Yoshioka (1992). Throughout the two periods, the location and direction of the chitons and tidal conditions were recorded every 30 minutes as thoroughly as possible, except during daytime low tide when the chitons never move. Movement was judged by comparing the location with the previous observation. The level of immersion was classified as ‘emerged’ – no water splashed, ‘submerged’ – covered with sea water, and ‘awash’ – washed by waves. The ‘awash’ condition was subdivided into the following 3 conditions by the ratio of underwater period (U); ‘awash I’ – 0<U<1/3, ‘awash II’ – 1/3<U<2/3, ‘awash III’ – 2/3<U<1. In the present study, ‘home’ was defined as the ‘pit’ occupied by a chiton at least once regardless of whether that occurred in the daytime or at night. (Note: one place was added as ‘home’ even though that place is not polished like a typical ‘pit’, because the place was stayed in by a chiton beyond a sequential ebb-flow cycle in the daytime.) The locations of chitons were divided into ‘home’ and ‘outside’. Movement was judged by comparing locations with the previous observations and categorized as ‘moving’ – the location of chiton changed; ‘turning’ – the orientation of the chiton was changed while it remained in the same place; ‘stopped’ – neither location nor orientation of the chiton changed, and ‘missing’ – the chiton was not found in the surveyed area. So, the situations of the chitons are categorized into the following patterns: ‘home/moving’, ‘home/turning’, ‘home/stopped’, ‘home/missing’, ‘outside/moving’, ‘outside/turning’, ‘outside/stopped’ and ‘outside/missing’. ‘Home/moving’ and ‘home/missing’ are impossible, and ‘Outside/moving’ and ‘outside/turning’ cannot be distinguished, so they are unified as ‘outside/active’. 126 E. Yoshioka & E. Fujitani Temperatures of the bedrock and sea water ware continuously recorded with a temperature data logger “Ondo-tori Jr. TR-51 (T&D Co.)”. When interaction between chitons occurred, their behavior was observed and recorded as thoroughly as possible. Statistical examination With regard to the movement of the chitons, the data for distance, duration of excursion and velocity of movement were obtained in the daytime and at night separately for each species. Naturally, such data have to be examined via two-way analysis of variance. But in the present study, only 6 individuals of A. gemmata and 5 individuals of A. tenuispinosa were observed throughout the study period and the activity shows great diversity between individuals. So, there is the likelihood that the characteristic difference between individuals has an effect on the comparison between species. For the reasons mentioned above, the data were examined by Welch t-test, and the individual units of data were examined for the comparison of their species. Results Activity pattern For explaining via examples of daily activity patterns, 3 individuals of A. gemmata (A, G, H) from 21 September to 5 October are shown in Fig. 3-1, and 3 individuals of A. tenuispinosa (I, M, O) from 21 September to 5 October are shown in Fig. 3-2. In the daytime ‘outside/active’ behavior mainly occurred in the ‘awash’ condition, and at night it mainly occurred in the ‘awash’ and ‘emerged’ conditions. ‘Outside/stopped’ often occurred for long time in the ‘emerged’ condition at night. ‘Home/turning’ mainly occurred in the same conditions as ‘outside/active’; moreover ‘home/turning’ occurred in the ‘submerged’ condition both in the daytime and at night and in the ‘emerged’ condition in the daytime. Sometimes, ‘home/turning’ occurred several times successively. Schematic drawings of ‘activities’ depending on tidal conditions are shown in Fig. 4. Whether in the day of spring tide or neap tide, ‘activities’ occurred in the ‘awash’ condition in the daytime. At night, high tide comes at midnight on the day of neap tide (Fig. 4A), and ‘outside/active’ occurred in the ‘emerged’ condition after sunset before ‘submerged’ and in ‘awash’ or ‘emerged’ conditions before sunrise.