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

The effects of emergence stress and morphological constraints on the distribution and growth of 3 subtropical seagrass species,Cymodocea rotundata,C. serrulataandThalassia hemprichii, were examined in a series of transplantation experiments and a laboratory desiccation experiment.C. rotundataandT. hemprichiioccur in both the intertidal and subtidal zones off Ishigaki Island, southwest Japan, whereasC. serrulatais found only in the subtidal zone. Within the same species, intertidal shoots for bothC. robundataandT. hemprichiiare 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 ofC. serrulata(2% of the initial density) was much lower than that ofC. rotundata(48%) andT. hemprichii(48%). In subtidal-to-intertidal transplants, the size ofC. rotundataandT. hemprichiileaves decreased, approaching that of intertidal shoots, indicating high morphological plasticity.In situobservations revealed that transplanted seagrasses with large leaves were exposed to air for longer than were intertidal controls. In the desiccation experiment,T. hemprichiileaves had higher water contents than those ofC. rotundataandC. serrulata. The morphological traits ofC. rotundataandT. hemprichiiand their ability to change shoot size appear to be related to the better survival of these species in the intertidal area as compared toC. serrulata. This is consistent with the observed variation in the species distribution in the intertidal zone.