MEPS 284:117-131 (2004)  -  doi:10.3354/meps284117

Emergence stress and morphological constraints affect the species distribution and growth of subtropical intertidal seagrasses

Yoshiyuki Tanaka1,*, Masahiro Nakaoka2

1Ocean Research Institute, University of Tokyo, Minamidai 1-15-1, Nakano, Tokyo 164-8639, Japan 2Graduate School of Science and Technology, Chiba University, Yayoi-cho 1-33, Inage, Chiba 263-8522, Japan

ABSTRACT: The effects of emergence stress and morphological constraints on the distribution and growth of 3 subtropical seagrass species, Cymodocea rotundata, C. serrulata and Thalassia hemprichii, were examined in a series of transplantation experiments and a laboratory desiccation experiment. C. rotundata and T. hemprichii occur in both the intertidal and subtidal zones off Ishigaki Island, southwest Japan, whereas C. serrulata is found only in the subtidal zone. Within the same species, intertidal shoots for both C. robundata and T. hemprichii are significantly smaller than subtidal shoots. A subtidal-to-intertidal transplantation experiment was conducted on 3 occasions: July and September 2001 and January 2002. In July and September, when the intertidal sites were exposed to air for a shorter period, shoot densities of the transplanted 3 species did not decrease. In contrast, the shoot densities of all 3 transplanted species decreased significantly in February, when the exposure to air was longer. The survival of C. serrulata (2% of the initial density) was much lower than that of C. rotundata (48%) and T. hemprichii (48%). In subtidal-to-intertidal transplants, the size of C. rotundata and T. hemprichii leaves decreased, approaching that of intertidal shoots, indicating high morphological plasticity. In situ observations revealed that transplanted seagrasses with large leaves were exposed to air for longer than were intertidal controls. In the desiccation experiment, T. hemprichii leaves had higher water contents than those of C. rotundata and C. serrulata. The morphological traits of C. rotundata and T. hemprichii and their ability to change shoot size appear to be related to the better survival of these species in the intertidal area as compared to C. serrulata. This is consistent with the observed variation in the species distribution in the intertidal zone.

KEY WORDS: Seagrass · Emergence · Desiccation · Morphological plasticity · Transplantation · Ishigaki Island

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