Biology of Eelgrass
Eelgrass growth is seasonal and closely related to environmental temperature. Zostera invests a large proportion of its resources in the maintenance of rhizomes and roots. The underground mat of horizontal rhizomes branches during growth, producing vertical leaf shoots, which are responsible for the lateral expansion of patches. In this way, eelgrass spreads quickly over a small area, just like the grass in our lawns. Short pieces of rhizome that break off the parent plant and are carried away by currents may generate new plants if deposited on a suitable substratum. Eelgrass populations can therefore expand either by the vegetative growth of shooting rhizomes that have survived the winter, or sexually, by production of seed.
Sexual Reproduction: Flowers and Seeds
eelgrass seeds
Flowers and seeds are generally produced between early/late summer (May/July) and early autumn (September). Zostera flowers are highly adapted to optimize pollination efficiency in an aquatic environment. The male flowers release long filamentous strands of pollen into the water. The density of these pollen filaments enables them to remain at the depth at which they were released for periods of up to several days, so increasing the likelihood of the pollen filaments encountering receptive stigmas.
sprouts
eelgrass flower
photo credit: Fred Short
click on image for larger version
After fertilization, the seed develops within a green membranous wall which photosynthesizes, producing a small bubble of oxygen that is trapped inside the seed capsule. Eventually this forces the capsule wall to rupture, releasing the mature seed. The seeds generally sink and are dispersed by currents, waves and, possibly over short distances, on the feet of birds. However, Churchill et al. (1985) found that the bubble can adhere to the seed's coat, increasing its buoyancy and consequently its likelihood of dispersal.
Relatively high temperatures (above 15ƒC) appear to be required for flowering and seed germination, suggesting that sexual reproduction does not play a major role in the life history of Z. marina in northern latitudes.
Balance Between Humans and Nature: Eelgrass
