Size does matter, but not only size: Two alternative dispersal strategies for viviparous mangrove propagules

We studied the propagules of two widespread mangrove species, Ceriops tagal and Rhizophora mucronata, that are similar in shape but differ in other morpho-anatomical features (average length is 23.1 ± 2.2 cm and 44.4 ± 4.3 cm, resp., n = 180). We hypothesized the propagules of both species to have a different hydrodynamic behavior and establishing capacity, resulting in a different dispersal strategy. More specifically, we hypothesized that C. tagal propagules have a larger dispersal capacity than those of R. mucronata. The dispersal strategies of C. tagal and R. mucronata propagules were elucidated through a combination of a propagule tracking (n = 180 per species), predation (n = 20 per species) and root-growth experiment (n = 120 per species), carried out in the field. C. tagal and R. mucronata adopted two different dispersal strategies. C. tagal releases a large number of propagules and disperses fast, having a slender morphology and low density (average ρ: 985.29 ± 19.02 g L−1), as well as a high agility (smaller size) when dispersing through dense root systems. C. tagal propagules have a theoretical advantage to disperse over longer distances over the thicker, longer and denser R. mucronata propagules (average ρ: 1003.92 ± 8.52 g L−1; t = 8.90, p < 0.0001, n = 197). C. tagal have, however, lower establishment chances due to slower root-growth, desiccation sensitivity and smaller size. In contrast to Ceriops' tactic of releasing high numbers of propagules and fast dispersal, R. mucronata has adopted a dispersal tactic of survival. Fewer propagules are released, but they are more resistant to predators due to their larger size and they can anchor themselves faster due to quicker root-growth (Mann–Whitney U: p < 0.0001, nCt = 59, nRm = 57). Overall, propagule characteristics of both species result in different and alternative dispersal strategies on a local scale, contradicting our initial hypothesis. On a global scale, we hypothesize this might lead to a similar capacity for long-distance dispersal, ending in successful establishment.