Bull. Jpn. Soc. Fish. Oceanogr. 76(1), Page 18-23, 2012   Effect of temperature on swimming behavior of paralarvae of the Japanese common squid Todarodes pacificus Jun Yamamoto1†, Sachi Miyanaga2*, Shin’ichi Fukui1 and Yasunori Sakurai2 1 Field Science Center for Northern Biosphere, Hokkaido University, 3-1-1 Minato, Hakodate, Hokkaido 041-8611, Japan 2 Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1 Minato, Hakodate, Hokkaido 041-8611, Japan * Toyama Kobayashi Pharmaceutical Co., Ltd., 100-1 Naka-Okubo, Toyama, Toyama 939-2295, Japan † e-mail: yamaj@fish.hokudai.ac.jp The present study is to describe the effect of water temperature on the swimming behavior of Todarodes pacificus paralarvae between 16 and 22.5°C, the range known to be suitable for normal embryonic development, and at 23.5°C, known to be warmer than the optimal range. Paralarvae obtained through artificial fertilization were reared at different temperatures, and those capable of swimming were observed in a 29 cm-deep tank. The swimming rate of paralarvae reared at temperatures between 19.5 and 23.5°C increased at Stage 31, suggesting that this temperature range was optimal for normal swimming, and that swimming ability was developed at this stage. The swimming rates of paralarvae reared at below 18.5°C were low at all temperatures after hatching, implying that egg masses must be maintained above this temperature for successful swimming. However, the swimming rate of paralarvae at Stage 32 dropped at 23.5°C. Paralarvae placed in a 150 cm-deep cylindrical aquarium always swam upward and remained near the surface. This behavior supports previous results of field studies showing hatchlings of T. pacificus were active near the surface. Mantle length of paralarvae at developmental stages 28 and 31 was significantly different from those reared in water of 16-22°C, suggesting that low temperature negatively affects the growth of the paralarvae in regards to swimming ability. These findings may help us improve the predictability of stock fluctuation of T. pacificus affected by environmental changes such as global warming. Key words: Todarodes pacificus, paralarva, swimming behavior, temperature