Vegetative growth

Sexual reproduction: flowers and seeds

Population structure

Eeelgrass growth is seasonal and closely related to environmental temperature. In Britain, growth generally occurs during the spring and summer, from April to September. In Danish waters, leaf biomass has been found to quadruple and rhizome mass to double during this period (Sand-Jensen & Borum, 1983).

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. 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 (Olesen & Sand-Jensen, 1994b). Eelgrass populations can therefore expand either by the vegetative growth of shooting rhizomes that have survived the winter, or sexually, by production of seed. Subtidal Z. marina beds in the UK are perennial and are believed to persist almost completely as a result of vegetative growth rather than by seed production.

In intertidal populations of Z. noltii and Z angustifolia, new leaves appear in spring and the eelgrass meadows develop over the intertidal flats during the summer. Leaf growth ceases around September or October (Brown, 1990), and leaf cover begins to decline during the autumn and over the winter. Intertidal plants may experience a complete loss of foliage, dying back to the buried rhizomes. Natural leaf-fall, grazing by wildfowl and a few specialized invertebrates and removal by wave action are the major factors contributing to this seasonal disappearance of the leaves. In perennial populations, the rhizomes survive the winter to produce new leaves the following spring, while in annual populations, both the leaves and rhizomes die. In contrast to the two intertidal species, sublittoral Z. marina beds can remain green throughout the year, as summer leaves that are shed in the autumn are generally replaced with smaller winter leaves.

In all three species, flowers and seeds are generally produced between early/late summer (May/July) and early autumn (September) (Brown, 1990; Tubbs & Tubbs, 1983). Zostera flowers are highly adapted to optimize pollination efficiency in an aquatic environment (Ackerman, 1983, 1986). 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. After fertilization, the seed develops within a green membranous wall which photosynthesises, 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 15 oC) 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. In comparison, the Z. angustifolia and Z. noltii intertidal beds in the UK rely on a combination of vegetative growth and seed set. Zostera angustifolia appears to rely more on seed set while Z. noltii appears to rely more on vegetative growth (Cleator, 1993; Rae, 1979; van Lent & Vershuure, 1994a,b).

Zostera patches resulting from vegetative growth will be composed of plants with an identical genetic composition. Beds formed largely by this process will as a result be less genetically diverse than those arising from sexual reproduction. This may have a major impact upon the resilience of a bed to anthropogenic impacts. In intertidal eelgrass beds, the genetic composition can be complex as both Z. angustifolia and Z. noltii rely on a combination of vegetative growth and seed set. The situation can be made more complex still as often both species often co-occur in mixed beds (Cleator, 1993).

Molecular-genetic techniques can be used to assess the relative importance of vegetative growth and sexual reproduction in determining population structure. Alberte et al. (1994) used DNA fingerprinting to assess the genetic similarity of three geographically and morphologically distinct populations of Z. marina from central California. They found that the within- and between-population genetic diversity were both higher than expected for largely vegetatively-reproducing populations, indicating that some sexual reproduction was occurring. In addition, they found that the genetic diversity of an intertidal population in a disturbed habitat was lower than that of one occurring in a more pristine habitat 30 km away. This research suggests that there may be significant differences between populations of Zostera in the relative importance of sexual and vegetative reproduction.

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