Pollen limitation is relatively common in plants pollinated by animals, but whether pollen availability may limit the reproduction of water-pollinated plants is much less known. We investigated pollen production and possible limitation of pollen availability in the hydrophilous dioecious seagrass Thalassia testudinum in an area where flowering was abundant (20 to 33% of the shoots produced flowers annually). Fruit production decreased from 86 to 60% after experimental removal of half of the male flowers (estimated pollen:ovule ratio of 6.7 × 10 4 :1) during April 2008. In late April to May 2009, when male flowers were less abundant than in the previous year (at a pollen:ovule ratio of 3.7 × 10 4 :1), artificial pollination resulted in an increase in fruit production from 65% in control flowers to 91% in the treated flowers in the same plot. This study is the first experimental evidence of pollen limitation in a dioecious plant with hydrophilous pollination.

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