The distributions and growth patterns of Symmetroscyphus intermedius, Tridentata tumida and Ventromma halecioides on leaves of the seagrass Thalassia testudinum from South Water Cay, Belize were examined. Colonies of S. intermedius and T. tumida smaller
than 3 cm and 4 cm respectively grew up and down the leaves, but larger colonies of both species grew only downward. Downward growth has three main advantages; it maximizes the residence time on the leaves, which grow upward from a basal meristem and disintegrate or are grazed from the tip,
it reduces the potential for interspecific competition with other epiphytes, and it places the hydroids in proximity to short young leaves, which may be colonized by vegetative stolon T. tumida was relatively rare on the older leaf surfaces because of a progressive degeneration of the
distal hydroid tissue. V. halecioides showed no orientation of hydrorhizal growth and was most abundant on the older leaf surfaces, a distribution typical of facultative epiphytes. There are close similarities between the morphologies, growth patterns and distributions of S. intermedius,
T. tumida and V. halecioides on Thalassia, and Campanularia asymmetrica, Sertularia perpusilla and Aglaophenia harpago respectively on Posidonia oceanica in the Mediterranean.
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