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Adaptive mechanical tolerance of Laminaria japonica to water motion was examined by field transplant and laboratory flow-tank experiments. L. japonica attaching to plastic plates were obtained at mid-water platforms sheltered from and moderately exposed to wave action. The environmental water velocities were determined during the cultivation period and transplant experiment using ultrasonic flow meters and available offshore wave height records. When transplanted to 3 depth sites with different intensities of wave-induced water motion, almost all the 'sheltered' plants were dislodged from the plastic plates by waves within 5 d, while most of the 'exposed' ones remained attached even at the shallowest depth 72 d after transplantation. This difference in mechanical tolerance was quantified by a flow-tank experiment conducted in sinusoidal oscillatory flows: 50% of the 'sheltered' plants were dislodged at a velocity amplitude of 0.8 to 0.9 m s -1 , while all of the 'exposed' plants persisted even at 1.1 m s -1 . The maximum water velocities encountered by the transplants were estimated using ultrasonic flow meter records and the period-averaged water speeds, which were determined by correlating water velocity with decreased mass of gypsum blocks. These estimates coupled with the observed survivorships at the transplant sites were consistent with the results of the flow-tank experiment. Attachment strength increased significantly as the holdfast grew for the 'exposed' plants but was constant for the 'sheltered' ones. In addition, the 'sheltered' plants had a short, wide, undulated blade which resulted in higher drag while the 'exposed' plants had a long, narrow, flat blade and thereby lower drag. Despite the marked difference in tolerance to water motion, the frequency distribution of the root-mean-square water velocity experienced by plants during the growth period was only 0.05 m s -1 higher at the exposed culture station than at the sheltered one, suggesting the presence of a critical velocity in developing the adaptive mechanical tolerance. The observed dislodgement velocity for the 'sheltered' plants was considerably lower than predicted from general hydrodynamic theories describing attachment strength of a thallus stretched out in the direction of flow. This indicates that detailed dynamic behaviors of plants should be explored to predict the wave-induced mechanical failure in large, flexible algae such as L. japonica.

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