Activity and Homing Behavior of Two Species of Acanthopleura (Mollusca: Polyplacophora) on a Subtropical Shore in Japan
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 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. Low tide comes at midnight on the day of spring tide (Fig. 4B), and ‘outside/active’ occurred in the ‘emerged’ condition after evening high tide and sometimes the ‘outside/active’ continued before becoming ‘submerged’ in the morning. In the daytime low tide, occurrence of ‘outside/active’ was recorded only 11 times in 2743 observation, four of which were observed in or just after rain, three of which were just after an ‘awash’ period, and the other two of which were just before sunset. Almost all ‘outside/active’ events in the early dawn (84 cases in 85 observations) stopped before sunrise and the chitons returned to ‘home’. Of the evening ‘activities’, 16 cases in 30 observations started the ‘moving’ just after sunset. At night in some cases, the beginning of ‘moving’ in the ‘emerged’ condition shifted day from to day following the retardation of tide condition (Figs. 3-1G, H; 3-2I, M, O). Some chitons showed active ‘moving’ day after day, and others stayed ‘home’ for up to 10 days. The frequency of ‘outside/active’ states varied between the chitons. For comparing the activity patterns of A. gemmata and A. tenuispinosa, observed cases and ratios of activities in each immersion state in the daytime and the night are shown in Table 1. Chitons of both species moved actively at night in the ‘emerged’ and ‘awash’ condition. The difference in night activity between two species in the 5 water conditions is examined in Table 2. A. gemmata is significantly more active than A. tenuispinosa in ‘awash’ conditions at night. Behavior of Acanthopleura in Okinawa 127 Fig. 3-1. Examples of daily activity patterns. Three individuals of A. gemmata (A, G, H) from 21 September to 5 October. 128 E. Yoshioka & E. Fujitani Fig. 3-2. Examples of daily activity patterns. Three individuals of A. tenuispinosa (I, M, O) from 21 September to 5 October. Table 1. Activities of chitons on each water condition. Data from the two periods of days are unified. A A. gemmata - Daytime B A. gemmata - Night Emerged Awash I Awash II Awash III Submerged Emerged Awash I Awash II Awash III Submerged Outside/Active 3 (0.2) 30 (6.6) 131 (27.8) 68 (19.6) 5 (0.7) 380 (20.2) 185 (42.2) 222 (49.5) 43 (14.3) 3 (0.4) Home/Turning 21 (1.5) 32 (7.1) 62 (13.2) 38 (11.0) 20 (2.7) 46 (2.4) 21 (4.8) 34 (7.6) 26 (8.7) 7 (0.9) Outside/Stopping 2 (0.1) 1 (0.2) 2 (0.4) 7 (2.0) 3 (0.4) 97 (5.1) 15 (3.4) 13 (2.9) 6 (2.0) 7 (0.9) Outside/Missing 1 (0.1) 0 (0) 1 (0.2) 2 (0.6) 0 (0) 0 (0) 0 (0) 0 (0) 3 (1.0) 1 (0.1) Home/Stopping 1366 (98.1) 389 (86.1) 275 (58.4) 232 (66.8) 723 (96.2) 1362 (72.3) 217 (49.6) 179 (40.0) 222 (74.0) 745 (97.7) Total 1393 (100) 452 (100) 471 (100) 347 (100) 751 (100) 1885 (100) 438 (100) 448 (100) 300 (100) 763 (100) C A. tenuispinosa - Daytime D A. tenuispinosa - Night Emerged Awash I Awash II Awash III Submerged Emerged Awash I Awash II Awash III Submerged Outside/Active 8 (0.6) 35 (8.8) 108 (20.8) 27 (8.5) 1 (0.3) 473 (29.1) 105 (22.4) 91 (20.7) 7 (2.4) 0 (0) Home/Turning 21 (1.6) 25 (6.3) 55 (10.6) 34 (10.7) 13 (3.5) 49 (3.0) 20 (4.3) 38 (8.7) 19 (6.5) 14 (3.8) Outside/Stopping 6 (0.4) 3 (0.8) 4 (0.8) 0 (0) 1 (0.3) 126 (7.8) 21 (4.5) 7 (1.6) 4 (1.4) 0 (0) Outside/Missing 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 7 (0.4) 1 (0.2) 0 (0) 0 (0) 0 (0) Home/Stopping 1315 (97.4) 336 (84.1) 353 (67.8) 258 (80.8) 361 (95.9) 968 (59.7) 322 (68.6) 303 (69.0) 261 (89.7) 356 (96.2) Total 1350 (100) 399 (100) 520 (100) 319 (100) 376 (100) 1623 (100) 469 (100) 439 (100) 291 (100) 370 (100) Behavior of Acanthopleura in Okinawa 129 A B Fig. 4. Schematic drawings of ‘activities’ of A. gemmata (striped) and A. tenuispinosa (dotted) depending on typical neap tide (A) and spring tide (B). Table 2. Comparison of activity on each water condition between two species in the night. Considered only the individuals observed in both two periods of days (see Fig. 8). A. gemmata is significantly more active on ‘awash’ conditions. Individuals of A. gemmata Individuals of A. tenuispinosa Water condition A B C D E F I J K L M t value (Welch test) df Emerged 23/322 87/303 17/245 80/229 25/236 44/226 34/318 138/300 80/257 67/255 79/250 1.50 ns 8.61 Awash I 27/75 39/67 23/58 25/61 28/60 25/54 9/65 22/73 29/96 24/92 13/84 4.50 p<0.002 8.77 Awash II 23/66 35/62 47/74 25/60 46/69 34/65 15/57 3/59 25/84 27/85 13/82 4.34 p<0.002 8.94 Awash III 2/41 6/46 11/42 5/49 9/44 10/41 2/55 1/48 0/48 0/51 1/44 4.23 p<0.005 8.92 Submerged 0/28 0/54 1/113 0/133 0/123 2/146 0/37 0/52 0/47 0/49 0/72 --- 130 E. Yoshioka & E. Fujitani A B C D E F Fig. 5. The distance and the duration of excursion on both species. A. A. gemmata: all data. B. A. gemmata: daytime. C. A. gemmata: night. D. A. tenuispinosa: all data. E. A. tenuispinosa: daytime. F. A. gemmata: night. Distance and duration of excursion, and velocity In the present study, an excursion is defined as the period from leaving ‘home’ to returning. The distance and the duration of excursions by both species are shown in Fig. 5. The distance of an excursion was calculated by summing up the sides of the polygons of positions observed every 30 minutes. The duration of the excursion is estimated on the basis that one ‘outside’ observation corresponds to 30 minutes’ excursion. The average distance and the duration of excursions by A. gemmata in the daytime are 23.7 ± 29.7 cm and 1.15 ± 1.10 h (n = 97, except in cases where the moving period included sunrise or sunset). Excursions at night are 37.2 ± 32.4 cm and 2.17 ± 1.7 h (n = 178, except in cases where the moving period included sunrise or sunset). The longest distance of one excursion in A. gemmata was 184 cm observed at night, and the longest duration of excursion was 9.5 h, also at night. The average of distance and duration of excursions by A. tenuispinosa in the daytime are 13.7 ± 10.9 cm and 0.95 ± 0.61 h (n = 75). Excursions at night are 34.0 ± 31.9 cm and 2.64 ± 1.91 h (n = 134). The longest distance of one excursion by A. tenuispinosa was 169 cm, observed at night, and the longest duration of any excursion was 8 h, also at night. The distance and the duration of excursions in the daytime and at night were compared within each species. In both species, the Behavior of Acanthopleura in Okinawa 131 cm/min A B C cm/min D E F Fig. 6. The range and the average velocity of movement in each individual. A. A. gemmata: all data. B. A. gemmata: daytime. C. A. gemmata: night. D. A. tenuispinosa: all data. E. A. tenuispinosa: daytime. F. A. gemmata: night. distance is significantly longer at night (Welch’s t-test: A. gemmata, t = 3.468, ndaytime = 98, nnight = 178, p < 0.001; A. tenuispinosa, t = 6.709, ndaytime = 75, nnight = 134, p < 0.001), and the duration of excursion is also significantly longer at night (Welch’s t-test: A. gemmata, t = 5.992, ndaytime = 98, nnight = 178, p < 0.001; A. tenuispinosa, t = 9.418, ndaytime = 75, nnight = 134, p < 0.001). Since the chitons sometimes stop during an excursion, the velocity can not be calculated by simply dividing the total distance by the total duration of one excursion. So the velocity was calculated for each 30 minute ‘moving’ period separately and summarized. The range and the average velocity for each individual are shown in Fig. 6. The average velocity of A. gemmata was 0.26 ± 0.24 cm/min. (n = 294) in the daytime, and 0.28 ± 0.24 cm/min. (n = 797) at night. The average of velocity of A. tenuispinosa was 0.17 ± 0.14 cm/min. (n = 218) in the daytime, and 0.24 ± 0.24 cm/min. (n = 628) at night. Comparing the velocities between the daytime and night within each species, only for A. tenuispinosa was the velocity higher at night than in the daytime. (Welch’s t-test: A. gemmata, t = 1.190, p > 0.05; A. tenuispinosa, t = 5.331, p < 0.001). The average of the distance and the duration of excursion, and the average of the velocity were compared between two species for daytime and night separately (Table 3). In these 6 comparisons, the distance in the daytime is significantly longer in A. gemmata and the velocity in the daytime is also significantly higher in A. gemmata. 132 E. Yoshioka & E. Fujitani Table 3. Comparison of the duration and distance of excursions, and the velocity. Considered only the individuals observed in both two periods of days (see Fig. 8). Individuals of A. gemmata Individuals of A. tenuispinosa A B C D E F I J K L M t value df Duration Daytime 1.63 1.67 1.04 1.05 1.07 0.89 1.17 0.75 0.83 0.82 1.06 1.89 9.00 ns (hr) Night 2.00 3.18 1.84 2.17 1.70 2.38 1.84 2.82 3.91 2.46 2.25 1.08 7.74 ns Distance Daytime 21.5 39.8 20.2 24.5 18.6 24.2 13.6 8.6 11.0 11.8 20.6 3.12 8.99 * (cm) Night 48.2 60.7 18.5 36.4 27.4 52.4 25.5 32.6 54.1 23.8 33.6 0.79 8.92 ns Velocity Daytime 0.22 0.29 0.23 0.28 0.21 0.30 0.14 0.11 0.16 0.15 0.23 3.83 8.50 ** (cm/min.) Night 0.35 0.32 0.19 0.27 0.24 0.34 0.24 0.20 0.30 0.19 0.27 1.38 8.92 ns *: p < 0.025, **: p < 0.01 Fig. 7. Examples of the trace of three chitons for 5 days from 17 to 21 July 2000. In one excursion without ‘stopping’, velocity of movement immediately after leaving home was calculated for a period of 120 minutes after the chiton started the excursion. Velocity in returning home was estimated for a period of 120 minutes directly before it came back home. In A. gemmata, the average velocity was 0.29 ± 0.14 cm/min. when leaving home (n = 104) and 0.33 ± 0.17 cm/min. when returning home (n = 104), so the velocity while returning was higher (Welch’s t-test: t = 1.98, p < 0.05). In A. tenuispinosa, the average velocity was 0.28 ± 0.15 cm/min. when leaving home (n = 46) and 0.30 ± 0.17 cm/min. when returning home (n = 46), so the velocity was not different between these situations (Welch’s t-test: t = 0.67, p > 0.05). The bedrock temperature ranged from 15.2-64.8 (average 31.7 ℃), and the sea water temperature ranged from 24.1-31.7 ℃ (average 27.4 ℃). The bedrock temperature fluctuated sharply and sometimes exceeded 50 ℃ in the daytime on clear days. The sea water temperature changed gently, but during typhoons (28 June, 7 August, 29 August and 29 September) it dropped sharply. Behavior of Acanthopleura in Okinawa 133 Occupation and vying for ‘pits’ Examples of the tracks of three chitons over 5 days are shown in Fig. 7. Each chiton went out in every direction, but some chitons visited the same places frequently The tracks of chitons sometimes overlapped even within the same day or within a few hours. In almost all cases, the tracks of chitons returning home overlapped with the tracks made leaving home. In a few cases, however, the tracks made returning did not overlap the leaving tracks. No difference in track features was detected between the two species. Occupation of ‘pits’ by chitons throughout the survey periods is shown in Fig. 8. Almost all chitons in both species occupied only one or two ‘pits’ throughout the 31 observed days, and they showed strong homing behavior. In the area where the number of ‘pits’ exceeded the number of chitons, some of them frequently changed their ‘home’. The duration of occupation ranged from only a few hours to entire days during the present study. Only two cases of vying for a pit were observed, and they are illustrated in Fig. 9. In the case shown in Fig. 9A, an A. gemmata [‘A’] and an A. tenuispinosa [‘I’] occupied the adjoining ‘pits’ 9 and 10. They sometimes swapped homes (see Fig. 8). 1. ‘A’ occupied pit 10 and ‘I’ occupied pit 9 as ‘home’ for 12 days . 2. In the morning of 2 October, ‘A’ parked itself in pit 9 after ‘outside/moving’, at 9:30. 3. 10 :20-10:22. ‘I’ returned to pit 9 and touched it, then ‘I’ moved back before mounting on ‘A’. 4. 10 :22-10:39. Putting ‘I’ on its back, ‘A’ shook its body for long time in the hollow around pit 9. 5. 10 :39-10:47. ‘A’ moved to the edge of the hollow with ‘I’ on its body. ‘I’ dismounted and left the hollow, as if urged by ‘A’. 10:47-10:58. ‘A’ went back to pit 9. ‘I’ moved to just above the hollow and stayed there for 2 minutes. 6. 10 :58-11:02. ‘I’ quickly moved to the hollow around pit 10. 11:02-11:05. ‘I’ turned its body and parked itself in pit 10. It is very interesting that vying for a pit was shown by two chitons, and that one chiton moved smoothly to another ‘pit’ after steady occupation of its ‘home’ by a competitor. In the case shown in Fig. 9B, there were four ‘pits’, 2, 3, 4, and 5, in a big hollow in the bedrock and two A. gemmata [‘E’ & ‘C’] and one A. tenuispinosa [‘O’] were sharing these ‘pits’ as ‘home’. 1. ‘E’ occupied pit 2 and ‘C’ occupied pit 3 for 3 days, and ‘O’ occupied pit 5 for 12 days. 2. At 11:40 in the morning of 2 October, ‘E’ parked itself in pit 3 while ‘C’ was out. 3. When ‘C’ returned, it rubbed ‘B’ with its body and went toward pit 5. 4. ‘O’ had parked in pit 5 and pushed ‘C’ back just after they contacted. 5. ‘O’ returned to pit 5 quickly. ‘C’ left the hollow and moved leftwards. 6. ‘ C’ bumped into a protuberance and went downward by rubbing the edge with its foot, and moved to the inside of the hollow and set itself in pit 4. It is remarkable to notice that the rejected chiton smoothly moved toward another ‘pit’, and it moved to the third ‘pit’ when the second ‘pit’ was occupied already. This suggests that chitons have rudimentary cognition of geographical features. 134 E. Yoshioka & E. Fujitani Fig. 8. Occupation of ‘pits’ by chitons throughout the surveyed periods. The number 1 to 24 indicate ‘pits’, and ‘A’ to ‘O’ indicate chitons. ‘A’ to ‘H’ are A. gemmata and ‘I’ to ‘O’ are A. tenuispinosa. White areas in the bars of both sides show the daytime and the black areas show the night. Black dots indicate ‘home/stopping’ in the high tide. Small dots indicate ‘home/stopping’ after changing the home in the low tide. Solid line means moving in the low tide, and thin line means no moving. Dotted line means unidentified. Discussion The majority of chitons are nocturnal (Glynn, 1970; Schmidt-Effing, 1980; Smith, 1975; Chelazzi et al., 1983a). Nishihama et al. (1986) reported that chitons are active both in daytime and at night, and Demopulos (1975) reported within the same species only the inhabitants of the lower zone are active both in the day and at night. Behavior of Acanthopleura in Okinawa 135 A B Fig. 9. Vying of ‘pit’ by chitons. Ovals encircled by broken line indicate ‘pit’, and the dotted ovals are chitons. Details are written in the text. Chelazzi et al. (1983a) noted “Since their activity (feeding excursions) occurs only during nocturnal low tides...” in a study of Acanthopleura gemmata in Somalia, and Thorne (1967) noted “Nocturnal excursions are made only when uncovered by the tide. …Most individuals therefore are in their home sites during daylight or high tide.” in a study of A. gemmata in Queensland. In the present study, both A. gemmata and A. tenuispinosa were active in ‘awash’ periods in the daytime and in ‘awash’ and ‘emerged’ periods at night. It is considered that these differences of activity due to the specific ecological conditions in Japan, Australia and Africa. Chelazzi et al. 136 E. Yoshioka & E. Fujitani (1983a) did not express the frequency of observation clearly, so they might not have continuously observed in the daytime and might have failed to notice the diurnal activity. The Gastropod Nerita funiculata takes shelter in the cracks of rocks during high tide and daytime low tide. Nerita scabricosta takes a rest by crowding together during high tide and daytime low tide. Levings & Garrity (1983) demonstrated by the field experiment that Nerita funiculata and Nerita scabricosta die in high rates due to heat and desiccation during ebb tide, and they also demonstrated in the laboratory that Nerita scabricosta suffered a fatal wound when exposed to a piscine predator on an unsheltered rock in a tank. They concluded that the activity of these species was suppressed by the physical stress of heat and desiccation, and by predatory pressure. Chelazzi et al. (1983b) noted “It is evident that resting in a hollow strongly reduces the high predation pressure due to the toad fish (Arothron immaculatus).” on A. gemmata in Somalia. Even though no experiment has been done in the present study, the temperature of the bedrock rose to 50 ℃ in the daytime, and Diodon holocanthus and other piscine predators were swimming around the habitat of the chitons. So it is considered that these two species of chiton shelter in their ‘home’ to avoid the fatal heat and desiccation in the daytime low tide, and shelter to avoid predation by fish during both daytime and night high tides. The presence of an internal clock (synchronized with tidal periodicity or a timer measuring the period since the previous high tide) was suggested by the fact that the start time of ‘moving’ during nighttime low tides shifted from day to day along with the tide. It is also suggested that their activity is governed by light conditions from the fact that almost all ‘moving’ chitons go back ‘home’ just before sunrise and start ‘moving’ just after sunset. ‘Home/turning’ behavior mainly occurred in the same conditions as ‘outside/active’; moreover, ‘home/turning’ occurred in ‘submerged’ conditions whether during the daytime or at night and in ‘emerged’ conditions in the daytime. Sometimes, ‘home/turning’ occurred several times consecutively. Chitons move forward, then return to the initial position by turning their bodies 180 degrees in less than 5 minutes, and they can do this more than twice in 30 minutes (Note: if they did it an even number of times between the observations, the behavior would not be recorded. So the actual frequency of this behavior should exceed the recorded number.) This behavior cannot be explained if the chiton clings to the bedrock only to avoid heat, desiccation and predation. Schmidt-Effing (1980) supposed that Ceratozona angusta was abrading the rock to make a home, because the chiton rests in a tight-fitting hollow of about 1 cm depth in the bare intertidal bedrock. In the study of the diet and the movement of Acanthopleura japonica (referred to as Liolophura japonica), Takenoshita (1998) compared the stomach contents of the moving chitons with the resting chitons. She reported that 70 percent of chitons had full stomachs in both cases. She said that the chiton moved and turned even when at rest in the home, and she considered that the stomach contents of the resting chiton derived from such behavior. Even if the ‘home/turning’ behavior is regarded as a short case of ‘moving’, it is not likely that they do it to get food, because the algae or other food materials would become very scanty in the area around home by repeated grazing. Considering the circumstances mentioned above, a reasonable interpretation of the ‘home/turning’ behavior is ‘maintenance’ of their ‘home’. Chitons graze and polish the base of the ‘pits’ to maintain a comfortable fit for their bodies. To confirm this hypothesis, we have to determine the effect of frequent ‘turning’ on the base of the ‘home’. Takenoshita (1996) confirmed grazing behavior by applying paint and quick-drying glue as a marker; the grazed paint and glue were confirmed in the excretion 10-14 hours after moving. This method will be useful for confirming the maintenance of the ‘home’ by applying the marker to the base of the pit. The purpose of the present study is to reveal and compare the activity pattern of two species of chiton by surveying the activity of the chitons individually and continuously. Chelazzi et al. (1987) studied the activity pattern of two species of Acanthopleura in Somalia. In the study, the two species showed the same activity pattern, but they showed differences in the stability of the Behavior of Acanthopleura in Okinawa 137 feeding area, in the height of the resting site, and in the direction and the distance to the feeding area. The authors proposed that the difference reflected differences in the algae eaten. In the present study, A. gemmata and A. tenuispinosa showed approximately equivalent activity patterns. There is no difference in the feeding area or the height of the ‘home’. Both species moved in all directions from the ‘home’. Slight differences were shown in the activity pattern; A. gemmata is mainly active in ‘awash’ conditions at night and A. tenuispinosa is active in ‘awash’ and ‘exposed’ conditions at night. The distance of ‘moving’ in the daytime was longer in A. gemmata, and the velocity was faster in A. gemmata than A. tenuispinosa. It is supposed that these differences reflect differences in tolerance of desiccation, heat, and low osmotic pressure. In the present study, very tight homing behavior was shown in both species. As noted by Chelazzi et al. (1987) “Homes proved to be not strictly individual and periodically interchangeable.” More ‘pits’ for ‘homes’ were prepared in some place and chitons sometimes changed ‘home’. The tracks of movement overlapped between chitons of different species. They did not show the territorial behavior of the chiton Mopalia muscosa as reported by Smith (1975). Although Chelazzi et al. (1983b) noted “Instead, A. gemmata only defends its resting hollow, showing an aggressive behavior which in the extreme can be followed by temporary suppression of the feeding activity of the rival”, competitive behavior was only shown regarding the occupation of ‘homes’ in the present study. The food resources in the area might be so abundant that the competition is unnecessary. Some experimental studies on the physiological mechanisms of homing behavior in chitons have proposed physiological cues for homing. Thorne (1967) considered one clue to be the complex of the chemical compounds of trail and the memory of topological profiles. Mook (1985) also considered cited the complex of trail following and memory of the mobile senses. Chelazzi et al. (1987, 1990) emphasized the importance of retracing the departure track. Even though no experimental manipulation was done in the present study, we can theorize about the physiological mechanism of homing behavior. In a few cases, the courses of ‘departure’ and ‘arrival’ did not overlap at all. At the very least, therefore, retracing the departure track is not the only key for homing. In the continuous observation, a chiton moved smoothly to another ‘pit’ when its ‘home’ had been occupied by another chiton, and when the second chosen ‘pit’ was also occupied by the other chiton, it subsequently moved to another ‘pit’. And when it moved to the other ‘pit’, it took a detour and bumped into a protuberance before went to the other ‘pit’. From this behavior, it seemed as if the chiton made some sort of geographic cognition in checking the protuberance as a landmark. It is our belief that the geographic cognition in the chiton is supported by this observed homing behavior. Acknowledgements Our special thanks are expressed for the helpful support of Prof. Koichi Ueno (School of Agriculture, Kinki University), who is the supervisor of EF on the graduation thesis, with the main part of this article, and thanks are also due to Dr. Toru Kobayashi (School of Agriculture, Kinki University) for advice on the presentation and computer operation. Thanks are also expressed to Dr. Shiro Nishihama (Seikai National Fisheries Research Institute, Fisheries Research Agency) and Dr. Yoshiaki J. Hirano (Marine Biosystems Research Center, Chiba University) for introducing literature and sending reprints, to Mr. Kinya Ota (School of Agriculture, Kinki University) for supporting making programs of data conversion and making figures, and to the staff and the students in Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus and the students in Laboratory of Aquatic Biology, Department of Fisheries, School of Agriculture, Kinki University for helpful support in the field and in the laboratory. 138 E. Yoshioka & E. Fujitani References Chelazzi, G., Focardi, S. & Deneubourg, J. L. 1983a. A comparative study on the movement patterns of two sympatric tropical chitons (Mollusca: Polyplacophora). Marine Biology 74: 115-125. Chelazzi, G., Focardi, S., Deneubourg, J. L. & Innocenti, R. 1983b. Competition for the home and aggressive behavior in the chiton Acanthopleura gemmata (Blainville) (Mollusca: Polyplacophora). Behavioral Ecology and Sociobiology 14: 15-20. Chelazzi, G., Santina, P. D. & Parpagnoli, D. 1987. Trail following in the Chiton Acanthopleura gemmata: operational and ecological problems. Marine Biology 95: 539-545. Chelazzi, G., Santina, P. D. & Parpagnoli, D. 1990. The role of trail following in the homing of intertidal chitons: a comparison between three Acanthopleura spp. Marine Biology 105:445-450. Demopulos, P. A. 1975. Diet, activity and feeding in Tonicella lineata (Wood, 1815). The Veliger 18 Supplement: 42-47. Glynn, P. W. 1970. On the Ecology of the Caribbean Chitons Acanthopleura granulata Gmelin and Chiton tuberculatus Linne: Density, Mortality, Feeding, Reproduction, and Growth. Smithsonian Contributions to Zoology (66): 1-21. Levings, S. C. & Garrity, S. D. 1983. Diel and tidal movement of two co-occurring neritid snails; differences in grazing patterns on a tropical rocky shore. Journal of Experimental Biology and Ecology 67: 261-278. Mook, D. 1983. Homing in the West Indian chiton Acanthopleura granulata Gmelin, 1791. The Veliger 26: 101-105. Nishihama, S., Nojima, S. & Kikuchi, T. 1986. Distribution, diet and activity of a chiton Liolophura japonica (Lischke), in Amakusa, west Kyushu. Publication of Amakusa Marine Biological Laboratory, Kyushu University 8: 113-123. Nishihama, S. & Nojima, S. 1990. Laboratory experiment on the activity rhythm and the homing of the chiton Acanthopleura japonica. Publication of Amakusa Marine Biological Laboratory, Kyushu University 10: 135-144. Schmidt-Effing, U. 1980. Beobachtungen zum Heimfinde-Verhalten und zum Aktivitäts-Rhythmus von Chiton stokesii Broderip, 1832 mit Anmerkungen auch zu Ischnochiton dispar (Soweby,1832) (Polyplacophora). Zoologischer Anzeiger 5/6: 309-317. Smith, S. Y. 1975. Temporal and Spatial activity patterns of the intertidal chiton Mopalia muscosa. The Veliger 18 Supplement: 57-62. Takenoshita, S. 1996. The foraging behaviour of chitons. vi+50 pp. Graduate thesis in Faculty of Science, Shizuoka University. (in Japanese with English abstract) Takenoshita, S. 1998. The ecology of Acanthopleura japonica (Lischke) on the intertidal rocky shore of Nabeta Bay, Izu Peninsula. vi+68 pp. Master thesis in Graduate School of Science, Shizuoka University. (in Japanese with English abstract) Thorne, M.J. 1967. Homing in the chiton Acanthozostera gemmata (Blainnville). Proceedings of the Royal Society of Queensland 79: 99-108. Yoshioka, E. 1992. Methods for marking on intertidal shells by using color reflect tape. (Hansha te-pu wo mochiita choukantai-karirui no hyoushiki hou.) Kuroshiwo 11: 15-16. (in Japanese) Yoshioka, E. & Nakashima, Y. 1996. Distribution of four species of Acanthopleura (Polyplacophora: Chitonidae) in Sesoko Island, Okinawa. Venus (Japanese Journal of Malacology) 55: 41-49. (Received June 11, 2004 / Accepted November 11, 2005) Behavior of Acanthopleura in Okinawa 139 沖縄産ウニヒザラガイ属 2 種の活動パターンと帰家習性 吉岡英二・藤谷絵里加 要 約 沖縄島瀬底島の潮間帯岩礁で 2000 年 7 月から 8 月，9 月から 10 月の 2 期間にウニヒザラガイ属 2 種 オニヒザラガイ Acanthopleura gemmata とキクノハナヒザラガイ A. tenuispinosa の活動パターンおよび帰 家習性について調査をおこなった。 両種ともに昼は波打ち際状態の時に，夜は波打ち際状態に加え干出状態の時に家の外へ出て活動した。 両種ともに移動距離・外出時間とも昼より夜の方が長かった。2 種間では，夜の活動でオニヒザラガイ はより波打ち際状態で活動するのに対し，キクノハナヒザラガイは干出状態と波打ち際状態で一様に活 動することや，昼の活動でオニヒザラガイよりキクノハナヒザラガイの移動距離が短く，移動速度も遅 いことなど若干の差が見られた。岩盤の表面温度は，昼の晴天時には 50 度を超え，水中の捕食者となり うる動物も観察した。したがって，乾燥適応・捕食適応が活動パターンに関与していると考えられた。 帰家行動についても，両種で同様に見られ，2 期間（通算 31 日間）を通じてほとんどの個体は 1 また は 2 カ所の家だけを利用していた。また，家の奪い合いの観察例より，他個体に家を占拠されてもすみ やかに近隣の家（pit）に入る行動などから，ヒザラガイは周辺の地形をよく認識していることが示唆さ れた。