Although large perennial brown algae are known to show great morphological plasticity responding to specific environmental factors such as depth and wave exposure, there are few reports showing this morphological variability taking into account age or size composition. Here, we conducted age- and size-based morphological comparison between the perennial brown alga Sargassum macrocarpum C. Agardh from their upper depth limit, middle depth, and lower depth limit at an exposed coast in northern Kyoto, Japan. Model II regression was fitted for the relationships between age or stipe diameter (SD) and morphological variables including SD, holdfast weight (HW), number of main branches (MBN), total weight of main branches (MBW), thallus weight (TW), and thallus length (TL) of 30 specimens collected seasonally from each depth zone. The differences between depths in these regression equations were analyzed using SMATR. Although SD, HW, and MBN of the specimens did not differ between depths under both age- and SD-basis, there were significant differences between depths in MBW, TW, and TL, suggesting that the differences in TW between depths are resulted from differences in MBW. Whereas TW at the lower depth limit was lower than that at the upper depth limit or middle depth in many months, TL at the upper depth limit was shorter than that at the middle depth or lower depth limit in some months. These results suggest that S. macrocarpum at shallower depths tend to have short and bushy morphology, whereas those at deeper depths have long and less bushy morphology.
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