Marine Phyla Pages -- Coastal Carolina University
SEAGRASSES
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Systematics 1
Worldwide, there are about 12 major divisions of seagrasses, consisting of approximately 57 species. They are called 'seagrass' because most have ribbon-like, grassy leaves, but none is a true grass. There are many different kinds of seagrasses and some do not look like grass at all. For example, they may have oval leaves. Seagrasses are true plants with roots, stems and leaves. They are angiosperms, and thus form tiny flowers, fruits and seeds. Most seagrasses reproduce by pollination - the pollen is transported to other plants by water- but much of their expansion is simply due to vegetative growth and spreading.
The roots and horizontal stems (rhizomes), often buried in sand or mud, anchor the grasses and absorb nutrients. Leaves, usually green, are produced on vertical branches and also absorb nutrients. The stems and leaves of seagrasses contain veins and air channels so they can carry fluid and absorb gases. Seagrasses rely on light to convert carbon dioxide into oxygen (photosynthesis). The oxygen is then available for use by other living organisms.
Along the East Coast of the United States, Zostera marina (eelgrass) is the dominant species from the Chesapeake Bay to Canada, and the important tropical species of Florida and the Caribbean include Thallassia (turtle grass) and S ( manatee grass). Seagrasses are generally absent from the coastal waters of the Carolinas and Georgia.
Habitat 1
Seagrasses are mainly found in bays, estuaries and coastal waters from the mid-intertidal (shallow) region down to depths of 50 or 60 meters. Most species are found in shallow inshore areas.
Seagrasses inhabit all types of ground (substrates), from mud to rock. The most extensive seagrass beds occur on soft substrates like sand and mud. Seagrasses cover areas in coastal waters from tropical (hot) to temperate (cool) regions. The number of species is greater in the tropics than in the temperate zones. Only two species, Halophila ovalis and Syringodium isoetifolium, occur in both regions.
The most diverse seagrass communities are in the waters of north-eastern Queensland and are an important part of the flora in the Great Barrier Reef region. Over 30 species can be found within Australian waters.
Other Information 1
Seagrass communities are one of the most productive and dynamic ecosystems. They provide habitats and nursery grounds for many marine animals. In many areas, seagrass meadows are important as they provide sheltered refuges and feeding areas for shrimp and juvenile fish. In some coastal areas, entire fisheries may depend on the productivity of these seagrass beds.
Seagrasses also act as substrate stabilizers. The rhizomes and roots of the grasses bind sediments on the bottom, where nutrients are recycled by microorganisms back into the marine ecosystem. The leaves of the grasses slow water flow, allowing suspended material to settle on the bottom. This increases the amount of light reaching the seagrass bed and creates a calm habitat for many species.
Few animals directly graze on seagrasses as a major component of their diet, relying on them more as shelter and substrate. DOM and POM from seagrasses, however, are very important contributors to the microbial loop. Seagrass meadows are a major food source for a few important species of marine mammals and marine reptiles, including manatees (Trichechus sp.), the dugong (Dugong dugon), and the green turtle (Chelonia mydas). An adult green turtle eats about two kilograms of seagrass a day while an adult dugong eats about 28 kilograms a day.
Links
1. http://www.aims.gov.au/pages/research/project-net/seagrass/apnet-seagrasses01.html