Seagrass biology

In the early Silurian, approximately 430 million years ago, the first plants started to colonize land. These organisms slowly evolved into the plants we know today, and after more than 300 million years on land, during the Cretaceous era, a few angiosperms gradually started to recolonized the seas and become the marine angiosperms or seagrasses.

Seagrasses are flowering plants, angiosperms, which live in marine habitats and are water pollinated. Among the three hundred thousand species of angiosperms only 0.02%, or 60 species, measure up to these requirements and can be called seagrasses. They are all monocotyledons and belong to the subclass Alismatidae. Phylogenetic analyses have reveled hydrophily to evolve many times during the history of seagrasses and that today's species comes from three separate lineages. The 60 species (example) are divided into 5 families Zosteraceae, Cymodoceaceae, Posidoniaceae, Ruppiaceae and Hydrocharitaceae in the traditional classifications mainly based on characters. A more modern classification based on molecular analysis shows that seagrasses may be classified in three different separate lineages: Hydrocharitaceae (primarly tropical species), Cymodoceae and Zosteraceae (primarly temperate species).

All seagrasses are rhizomatous plants with a clonal nature, well adapted to a life in high-energy marine environment, with stems extending horizontally below the sediment surface. All parts of the seagrass plant have highly adapted channels (lacunae) for transportation of gases, e.g. oxygen from the photosynthetic parts to the roots. The flow of oxygen is essential to support root respiration and to avoid accumulation of harmful anaerobic metabolites in the sediments. The aerial channels in the leaves also confer buoyancy, allowing them to remain upright while being flexible in the currents.

Like the rest of the angiosperms, seagrasses bear flowers and produce seeds and fruits. Flowers are however relatively unusual, for some species no more than 10% of the shoots in a meadow produce them in any one year. Seagrasses generally rely on asexual reproduction instead of sexual reproduction when it comes to colonization of new habitats and recovery from disturbances. The role of seed dispersal in reproduction is still poorly understood. Seagrass flowers are often inconspicuous and very simple, because they do not rely on animals for pollination. All species except one (Enhalus acoroides) are pollinated under water, and they seldom produce more than one seed per plant.

Most seagrass meadows are monospecific, particularly those in the temperate zone. In the tropics, monospecific meadows are abundant, however mixed meadows with up to 10 coexisting species are common in certain locations, for example in East Africa and South East Asia. Even though the species richness is significant (number of seagrass species per meadow) the species diversity (the evenness of their contribution to the community) is typically low. The meadows with the richest species diversity are found in the Indo-Pacific area and the Red Sea, where mixed meadows are abundant, containing up to 12 co-occurring species.

Distribution and habitat requirements of seagrasses

Seagrasses occur in all coastal areas of the world, except along the Antarctic shores, probably due to ice scouring, which greatly damage seagrasses. They are mainly found in bays, estuaries and coastal waters from the mid-intertidal (shallow) region down to depths of about 50 metres, but most species are however found in shallow inshore areas. Since they are marine phototropic plants with roots, the most obvious requirements of seagrasses are a marine environment, adequate rooting substrate, sufficient immersion in seawater and illumination to maintain growth.

When it comes to water quality, most seagrass species can accept a wide range of salinity, from brackish (10‰) up to hypersaline (45‰). Seagrasses also appear to be relatively resistant to contaminants, but they are unlikely to inhabit highly polluted habitats, which would probably have rather reduced transparency as well. High levels of nutrients, promoting algae blooms that shade the seagrass, have also been reported to have negative effect on Seagrasses.

Where space is available, seagrass populations can only develop if the substrate and the hydrodynamic conditions are suitable. Most seagrass species are confined to sandy to muddy sediments, which are easily penetrated by the roots, although some species can grow over rock. There are for example large extensions of seagrass Phyllospadix sp. covering the Californian rocky shores. Habitats with strong currents and wave action can be unsuitable for seagrass plats, since highly mobile sediments can cause successive burial and erosion, eventually leading to mortality.

Seagrasses, flowering plants with a marine life cycle, are confined to a life in seawater. Some species are however able to withstand exposure to air, and can under the right circumstances develop large intertidal populations. Exposure to air nevertheless always involves some form of stress for the seagrass plants, usually through desiccation and photodamage by high irradiance.

The depth limit of seagrasses is set by the light requirements for growth and depends on the turbidity of the water. The light requirements for seagrass is typically defined as percentage of surface irradiance (the irradiance immediately below the water surface) that needs be received by the plants to grow, which ranges between 4% and 29%, with an average of about 11%. The light requirements of seagrasses are generally higher than for other marine phototrophs, such as macroalgae and microalgae.