18.2.1  Evolution of seagrasses

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Primitive plants evolved in the Silurian (about 450 million years ago) from green algae, particularly the Charophytes (Figure 18.7). A variety of terrestrial plant groups evolved over the next several hundred million years, for example lycopods, bryophytes, ferns, gymnosperms, yet it was not until 100 million years ago that angiosperms evolved and reinvaded the sea. Mangroves and saltmarsh plants colonise intertidal zones of marine environments (Section 17.3) but only seagrasses live in total submersion. Curiously, all vascular plants inhabiting marine environments are angiosperms.

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Figure 18.7 Evolution of seagrasses from algae. Algal evolution in the Silurian was followed by appearance of the first land plants which diversified by developing higher plant characteristics such as woodiness and sexual reproduction. During the Cretaceous, marine angiosperms evolved, characterised by mangroves and salt-marsh plants in intertidal zones and seagrasses as the dominant submerged macrophyte. (Courtesy W.C. Dennison)

Limited seagrass fossils combined with taxonomic and evolutionary studies indicate at least four taxonomically distinct seagrass families: Hydrocharitaceae, Posidoniaceae, Cymodoceaceae and Zosteraceae. These families are so unrelated that they probably arose from at least four separate reinvasions of the sea. However, each seagrass family shares a common environment, rooting into soft substrates of shallow oceans. Characteristic conditions are low and unpredictable light, nutrient reserves in anaerobic sediments, slow diffusion of inorganic carbon to the photosynthetic apparatus and, of course, high salinity. Yet the relatively limited number of seagrass species adapted to these stringent environmental conditions have colonised all the major oceans and form extensive meadows along the world’s coastlines. Not surprisingly, sea-grasses exhibit a spectacular suite of survival mechanisms to deal with submergence.

 

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