Propagule dispersal of the SE Asian seagrasses Enhalus acoroides and Thalassia hemprichii

Dispersal rates of seeds and fruits of the 2 common SE Asian seagrasses Enhalus acoroides and Thalassia hemprichii were quantified under field conditions during flotation and the subsequent period of bottom dispersal. For F. acoroides, traveling speeds of buoyant seeds were timed for 70 different experimental batches of 10 to 30 propagules in seagrass beds across 9 different sites on the Bolinao reef flats, and during different seasons and tidal conditions. For T hemprichii, only 3 experiments could be done due to the limited availability of propagules in this less frequently flowering species. Overall, fruits and seeds of E. acoroides floated at a speed of 0.26 +/- 0.02 km h(-1) (mean :L standard error), whereas those of T. hemprichii traveled at 0.47 +/- 0.15 km h-1. Median fruit and seed flotation times were 158 and 0.5 h, respectively, for E. acoroides, and 55 and < 0.5 h, respectively, for T. hemprichii. Dispersal rates were similar to in situ measured flows due to tides and wind together, Neither the difference between species nor that between propagule types was significantly different, which is in agreement with their similar sinking velocities in still seawater but in contrast with a marked difference in seed volume and weight. Estimating the distance that could be covered by floating fruits led to medians of 41 and 23 km for E, acoroides and T. hemprichii, respectively, pointing to d considerable dispersal capacity of these propagules. Floating seeds traveled only a limited distance (< 5 km at most for E, acoroides and < 13 km for T hemprichii), In situ bottom dispersal of E, acoroides seeds stopped after 2 to 5 d, Le, after the germinating seedlings had formed a hairy mass at the base that anchored them to the sediment. Maximum distance traveled over the sediment by observed seeds was 204 cm, and mean rates ranged between 10 and 29 cm d(-1). In 2 of the 3 experiments, seeds of T hemprichii moved much faster over the sediment (around 100 cm d-1) than those of E. acoroides, disappearing from the experimental tracks within 2 to 3 d, probably largely due to herbivory or burial by invertebrates.