Morphogeographic variation under controlled conditions in five seagrasses, Thalassia testudinum, Halodule wrightii, Syringodium filiforme, Halophila engelmannii, and Zostera marina

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The morphogeographic evidence udner controlled conditions demonstrated variation in leaf width that correlated with the leaf patterns in the indigenous habitats. In Thalassia testudinum Banks ex König, Halodule wrightii Aschers., and Syringodium filiforme Kütz., the narrow-leaved variants from shallow bays in the northern Gulf of Mexico continued to produce narrow leaves in laboratory culture. Broader-leaved variants in the same three species that originated from seagrass beds associated with coral reefs in the southern Gulf of Mexico and the Carribbean continued to produce broad leaves. The turbid water and variable salinity and temperature of the shallow northern bays correlate with the presence of narrow-leaved plants, and the clear water and relatively constant salinity and temperature of the southern regions correlate with the broader-leaved populations. Although an ecotypic status for the narrow- and wide-leaved populations was suggested, the selective role of these habitat conditions needs further investigation. Clonal variation in leaf width was demonstrated within Halodule populations under uniform conditions. Plants of Halophila engelmannii Aschers. from deep seagrass beds in the Gulf of Mexico along the western coast of Florida continued to produce narrower leaves than those produced by plants from shallow Texas bays. Three collections of Zostera marina L. from Washington and Alaska produced leaves of significantly different widths under each of three temperature regimes. The experimental evidence suggests that the width of a seagrass leaf is dependent on its immediate environmental surroundings but that the limits of its ecoplasticity vary geographically depending on the genotype.