Reproductive Patterns

Patterns in the reproduction of seagrasses may provide insight into the cues that induce floral development, trigger flowering, and control the flowering process (i.e. phenology) within individuals and populations. These patterns will vary with biotic factors related to the genetics, physiology (growth regulators), and conditions of the local population, and with abiotic factors related to light (photope-riod, red to far red ratios), temperature, and latitude (Pettitt, 1984; Rathcke and Lacey, 1985; see Smith and Walker, 2002). Lastly, geographic location, ease

Fig. 2. Microsporogensis and microgametogenesis with reference to successive pollen development in seagrass genera (modified from Iwanami et al., 1988).
Fig. 3. Pollen development involves an elongation of the microspores prior to reductive division in the Zosteraceae and post reductive division in the Cymodoceae. Scale bar = 20 |m (after Rosenberg, 1901a; Harada, 1948; Stewart and Rüdenberg, 1980).

of access to sites, and societal factors such as interest, and number of scientific personnel has also influenced the understanding of local and regional conditions (Walker et al., 2001). Consequently, there are geographic regions, such as the North Atlantic, Mediterranean, and parts of the North Pacific coasts, where phenological patterns are well described, but the majority of coasts, including the regions of high diversity, are less well known (Walker et al., 2001).

Coincident with large-scale phenomena is the spatial and temporal variation in flowering phenology that exists on local scales. This is most evident in taxa in which flowering is periodic, rare and/or in which reproductive material has not been described (e.g. Cymodocea angustata Osterfeld, Halodule beaudet-tei (den Hartog) den Hartog, Halodule bermuden-sis den Hartog, Halodule emarginata den Hartog, Halophila johnsonii Eiseman; Kuo and den Hartog, 2001).

Notwithstanding these gaps in understanding, there are taxa in which the phenological patterns are well known. For example, the phenology of Zostera marina L has been described (de Cock, 1980), and reproductive shoots can represent from 0 to 100% of a given population with variation existing due to photoperiod and water temperature (Phillips et al., 1983; Olesen, 1999). Walker et al. (2001) provide a detailed review of this topic from a geographic perspective, which reveals that most genera are reproductive in the spring and summer (i.e. increase in day length and warmer water temperatures) (e.g. Tha-lassia, Halophila, Syringodium, Halodule, Cymodocea, Thalassodendron, Phyllospadix, Nanozostera, Zostera), a number flower in the fall and winter (i.e. decrease in day length and cooler water temperatures) (e.g. Amphibolis, Posidonia, Heterozostera), and one genus is reproductive throughout the year (Enhalus), although there are exceptions especially for tropical species.

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