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Page 1 VENUS 71 (3-4): 199–207, 2013 ©Malacological Society Of VENUS 71 (3–4): 199–207, 2013 ©Malacological Society of Japan Swimming Ability of Juvenile Mactra chinensis by Pedal Flapping Yoshitake Takada1*, Yuko Ito2 and Ikuo Hayashi3 1Japan Sea National Fisheries Research Institute, Fisheries Research Agency Japan, Suido-cho 1-5939-22, Niigata 951-8121, Japan 2Musa 1-28-20, Kushiro, Hokkaido 085-0806, Japan 3Kumano Biological Research Institute, Atashika-cho 1320, Mie 519-4206, Japan Abstract: Swimming behaviour of juvenile Mactra chinensis was observed in the laboratory. Clams of 7–15 mm shell length swam by pedal flapping after launching from the bottom by leaping. The average swimming speed of 7–9 mm clams was 6.0 cm s–1 and that of 9–15 mm clams was 7.8 cm s–1. The average distance swum reached 17.4 cm. Larger individuals of 31–35 mm shell length class did not show swimming behaviour but they performed leaping. Swimming by pedal flapping extended the range of locomotive distance through the water column at least 4.1 times over that achieved by leaping. Therefore swimming is thought to be an adaptive behaviour to reduce the risk of predation and increase the chance to find a better place to burrow. Keywords: behaviour, swim, jump, foot flapping, size, Mactridae Introduction Bivalves inhabiting soft sediment areas are not always bound to the seafloor bottom. The occurrence of post-metamorphic bivalves in the water column has often been observed from plankton samples (Williams & Porter, 1971). Some bivalve species are known to produce byssus threads and to be dispersed passively by water currents (Sigurdsson et al., 1976; Beukema & de Vlas, 1989). Other groups of bivalves actively propel themselves into the water column by swimming and leaping (Ansell, 1969; Stanley, 1970). According to Stanley (1970), for bivalves swimming is defined as a mode of self-propulsion through the water. Swimming is observed only in a few groups of bivalves (Morton, 1964; Stanley, 1970). Scallops (Pectinidae) and file shells (Limidae) are notable in swimming by expulsion of water (Morton, 1980; Donovan et al., 2004). Propulsion of these groups arises from clapping the shells. Occasional swimming is observed in the Solenidae and Solemyidae (Stanley, 1970), which eject water to propel themselves by rapidly clapping the shell and retracting the foot. On the other hand, leaping is defined as launching the shell by kicking the substratum with the foot (Ansell, 1969; Stanley, 1970) and is not counted as swimming behaviour. This definition stresses the locomotive mechanism. Feder (1967) illustrated the leaping behaviour of Cardium [= Acanthocardia] as an escape response from starfish predation. Another species of Cardiidae, Laevicardium [= Fulvia] laevigatum can swim by pedal movement after leaping (Stanley, 1970), but the behaviour was only briefly described as “Swimming is accomplished by kicking against the water with the foot and simultaneously clapping the thin valves together to expel water ventrally”. Fraser (1967) reported that Tagelus divisus (Solecurtidae) swim by pedal movement after leaping, but the description was also very brief. To our knowledge, swimming by pedal movement has been regarded as a very rare behaviour and only minimal attention has been paid to * Corresponding author: [email protected] 200 Y. Takada et al. its biological significance. In this study, we observed swimming behaviour of Mactra chinensis Philippi 1846 (Veneroida: Mactridae) by pedal movement. The focus of this study is to evaluate the frequency of the swimming behaviour in the laboratory. This behaviour was recorded by a video camera and the influence of shell size on the behaviour was investigated. Materials and Methods Dredge sampling was carried out at 7 m depth on a sandy bottom off Niigata (138°54´E, 37°52´N) on August 25th, 2008. The temperature of the bottom seawater was 25.6°C and the salinity was 33.0. Juvenile Mactra chinensis were sorted into four size classes by their shell length (7≤–<9, 9≤–<11, 11≤–<13, and 13≤–<15 mm, measured to 0.1 mm precision) and kept in a reserve tank (30 L, cylindrical transparent polycarbonate tank) with running natural seawater for 1 to 3 days before the experiments. An additional sampling was carried out at 6 m depth at the same area on October 22nd, 2008. At this time, the seawater temperature was 21.4°C and the salinity was 33.5. Clams of 31–35 mm size class (shell length 31≤–<35 mm, 0.1 mm precision) were sorted out and kept in the same tank as before. Clams of the other size classes were rare and could not be used for the following experiment. Swimming behaviour of the M. chinensis in the reserve tank was recorded by a hand-held video camera. Detailed observation was carried out in small cylindrical PVC experimental tanks (Fig. 1) of 30 cm diameter and 13 cm depth (9.2 L). The experimental tanks were placed in a darkroom with fluorescent lights (13–15 μmol m–2 sec–1, about 1000–1100 lux). Filtered seawater (25°C) flowed into the tank at a rate of 1.0 L min–1 from four tubes at the periphery of the tank and drained from the centre of the tank. Each batch of size sorted clams was placed gently in the experimental tanks: 100 individuals of 7–9, 9–11, 11–13 mm classes, 50 individuals of 13–15 mm class, and 20 individuals of 31–35 mm class. The clams were placed directly on the PVC bottom of the tank with one side of their shell down and they were spaced without contact with one another at the beginning of the experiment. The behaviour of the clams was recorded by a video camera right above the tanks for two hours starting directly after they were introduced into the tanks, and analyzed later. Occurrence of three types of behaviour (Tap, Leap, and Swim; see Results for details) at ten- minute interval was recorded. The duration and the length of each swimming bout were estimated from scanning consecutive frames of the video records that had 0.033 second intervals. The length Fig. 1. PVC experimental tanks (diameter 30 cm, depth 13 cm) from a view of the video camera. Swimming of Juvenile Clam Mactra 201 of the swim was measured by projection onto the bottom of the tanks. Some of the swimming bouts were terminated when the clams collided with the wall of the tank, but most clams continued swimming after they hit the wall. Data for these clams were included in the following analysis. Effects of size on swim distance and period were analysed by one-way ANOVA. Before the ANOVA, the distance data were square-root transformed and homogeneity of variances was tested by Bartlett’s test (P > 0.05). In order to analyse the size effects on the regression between the period and distance of the swim, the most fitted regression model was obtained by calculating AIC of all the combinations of the size classes. Results Three types of behaviour in Mactra chinensis were recognized in the laboratory: Tap, Leap, and Swim. Tap involved rotating or sliding the shell on the bottom by extending and pushing the foot down to the bottom of the experimental tank. Leap involved launching the shell into the water column after a rapid push of the foot onto the bottom. The foot did not flap during the Leap. Swim involved launching the shell by the initial rapid push of the foot just like in the Leap, and then flapping the foot to propel the shell in the water column. During the Swim, the shell valve slightly opened but did not clap (Fig. 2; Supplemental material). The siphons and the foot extended from the shell, and the clam moved in a posterior direction (to the direction of the siphons). The ventral margin of the shell was upward and the shell swung from side to side with the flapping of the foot. In the 48th frame (in Fig. 2), the foot was bent perpendicular to the direction of the movement, and it was straightened in the next frame. The foot was kept straightened for the next three frames (0.1 seconds). Next, the foot was bent slowly to the opposite side of the shell and the shell rotated. It took nine frames (0.3 seconds) from one bend of the foot to the next. In the experimental tank, the clams showed Swim behaviour a number of times during the 2-hour observation period, except those in the 31–35 mm class (Table 1). Among the four size classes (7–9, 9–11, 11–13, and 13–15 mm), the maximum Swim distance was 42.0 cm in the 9–11 mm class, and the maximum Swim period was 4.07 seconds in the 13–15 mm class. Statistically significant differences were detected in the average Swim distance (one-way ANOVA, P < 0.05) and the average Swim period (one-way ANOVA, P < 0.05). Clams of smaller size classes tended to swim for a shorter distance and over a shorter period (Table 1: Tukey- Kramer, P < 0.05). The most fitted regression model between the period (x, second) and distance (y, cm) of the Swim (Fig. 3) was obtained when the four size classes were combined into two size groups. The regression line for the smallest size class (7–9 mm) was y = 3.528 + 5.972x, and that for the other size classes (9–15 mm) was y = 2.915 + 7.848x. These slope parameters demonstrate that the clams in the smallest class swim more slowly than the larger clams. The average Leap period and distance for 13–15 mm clams were 0.69 ± 0.08 sec and 4.20 ± 1.39 cm (± SD, n = 5), respectively.
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