Can floral consumption by fish shape traits of seagrass flowers?

Seagrasses are marine flowering plants with hydrophilous pollination. This abiotic pollination by water assumes absence of flower-animal interaction, but animals can interfere in this process through consumption of reproductive structures. We studied predation on male flowers by fish for three dioecious seagrass species (Thalassia testudinum, Syringodium filiforme and Halodule wrightii) in the Mexican Caribbean. Seagrass flowers have a highly reduced or absent corolla and florivores directly consumed the anthers with pollen. The foliar structures (tepals, bracts or sheaths) protecting the male flower buds were removed by hand in situ. The floral buds were followed by videos or taking pictures at regular intervals and most (56–100 %, depending on seagrass species and experimental setting) artificially denuded male flower buds were consumed within 24 h by juvenile fish of various species. Histochemical analysis showed that the pollen and embedding mucilage were rich in polysaccharides and proteins, thus potentially nutritious. The seagrasses had copious production of pollen (between 0.2 and 1.2 × 10⁶ pollen per flower, depending on the species). But T. testudinum and S. filiforme were often pollen limited, and the probability of fruit set was reduced ~50 % when the females were at the distance of 1 and 5–6 m from the males flowers, respectively. Under natural conditions, depredation on pre-anthesis male flowers in the three species was low because flower bud emergence (few hours) and pollen release (1–4 h) were ephemeral processes. In addition, the release of pollen of T. testudinum occurred at dusk when herbivorous fish became inactive. These life-cycle characteristics aid to avoid excessive pollen consumption by fish, however, whether they are anti-predator strategies or mere adaptations for submarine pollination remains to be established.