Sea Grass

This topic submitted by Richard Fetters ( at 9:32 AM on 6/10/07.

Susan found a "Treasure Chest" in Fernandez Bay, San Salvador, Bahamas. See other beautiful phenomena from the Bahamas.

Tropical Field Courses -Western Program-Miami University

Rich Fetters
Tropical Marine Ecology
Dr. Hays Cummins
Grass-bed Ecology

To begin, there are a few things you should know right away about grass beds and their ecology. First of all to be considered a “sea grass” they must grow in a saline environment, which is an environment that has salt water. Another condition for something to be a sea grass is that it has to live in the photic zone. The photic zone is also known as the euphotic zone, this zone is the area of any body of water where enough sunlight can penetrate for photosynthesis to occur. Finally the basic definition of a sea grass according to is: “are flowering plants from four plant families that grow in the marine environment.” The four plant families that the definition is talking about are: Posidoniaceae, Zosteraceae, Hydrocharitaceas, and Cymodoceaceae.
The most common sea grass in the world is the Thalassia type, which is also know as “turtle grass”. This is because this is the main food for most sea turtles. “Green sea turtles can eat an enormous amount of sea grass. IN fact, they have to: as cold-blooded reptiles they need to burn a lot of fuel to maintain their body temperature. Each mature female needs approximately one thousand square feet of turtle grass per year for her grazing plot.” (Davidson, P. 109)
Sea grass beds are one of the most diverse ecosystems on the planet. Sea grass beds are filled with many different types of animals; some of these animals are hundreds of species of fish, shellfish, microalgae, macroalgae, and manatees.
Sea grass beds are also very important to coral reefs and islands themselves. Sea grass beds act as a shield from incoming waves. This shield helps prevent against erosion. Also sea grass beds also help to stabilize the ocean floor since their roots grow horizontally. This process is called rhizome. “…and asexually, by an underground stem, or rhizome, which grows horizontally. The rhizome sends thick roots down and new shoots up at regular intervals. These roots form a mat extending down several feet, adding tremendous stability to what otherwise might be shifting sands. (Davidson, P. 66)
Back now to the four main types of sea grasses, we will first start of with the Posidoniaceae type. This type of sea grass is said to contain two- nine different species of sea grass. This type of sea grass is typically found in the waters off southern Australia and in the Mediterranean. About a year ago, a huge “clonal colony” of this type of sea grass was found off the coast of Ibiza, which is located in the Mediterranean. This grass bed was over 8 km long, and is thought to be over 100,000 years old.
The second main family of sea grasses is the Zosteraceae family. This family of sea grass is normally found in sub-tropic waters. Many taxonomist believe that this family of sea grass is monophyletic, which means that all come from a common descendent. This family of sea grass is also pretty easy to spot, since its leaves are ribbon like, and it has very prominent rhizome, which are the underground roots that I talked about earlier. One type of the sea grass from this family is the zostera type, which is monoecious, is always covered by a spathal sheath, and has elongated roots.
The third group of the sea grasses is the Hydrocharitaceaes. This family includes a wide range of species. The one main difference from this family from the others is that, this family includes both fresh water and salt water species. Most of these sea grasses are found in tropical waters. These sea grasses also are an annual or sometimes perennial species. Some of the genera in this family are the Thalassia, Hydrilla, Elodea, Egeria, and Najas to name a few. The thalassia when it is flowering has a single flower in the male inflorescence, while the female has 6 – 8 different styles divided into two stigmata.
Finally the last major family of the sea grasses is the Cymodoceaceae family. This family is also known as the “manatee-grass family”. Many taxonomists still have not yet recognized this family. There are over a dozen species that are included into this family. This family of sea grass is normally found in sub-tropical to tropical waters.
The sea grasses that we should expect to see in San Salvador are a large amount of the Thalassia also known as the turtle grass. We should also see manatee and shoal grasses, but these are less abundant since there is such a large amount of the turtle grass.
Since sea grasses have been around for hundreds of millions of years they have gone from tiny one celled plants, to now huge sea grass beds. Their evolution has taken some time. At the beginning sea grasses were once algae, but over time this algae moved to land and evolved into land plants, such as normal grasses and trees. After a while these plants evolved into mangroves, which are essentially trees and shrubs that grow in salt water coastal habitats in the sub tropics and tropics. This evolution process started around the Silurian period which was around 443 and 416 millions these mangroves started to evolve into sea grasses around the Cretaceous period which was around 145 to 65 million years ago. Sea grass is the only land plant to this day to make the trip from the ocean to land, back to the ocean.
Therefore sea grasses are a very key part to any island or coastal regions ecosystem. Sea grasses can help prevent erosion of coral reefs, beaches, and most land masses. Sea grasses also play an important role to a coral reefs ecosystem. These sea grass meadows can provide an excellent home for many types of animals from fish all the way to invertebrates. These sea grass beds are also an excellent food source for many sea creatures, such as the sea turtle, which sea grass is their primary diet. Finally the classification of all the sea grasses can become pretty complex, but its nice to know that there is only 4 main families of sea grass, so once you find the family s in, they are pretty easy to identify.

Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 1st June 2007.


Davidson, Osha Gray . The Enchanted Braid. New York: John Wiley & Sons, Inc., 1998.

"Sea Grass." Wikipedia. 28 May 2007. 10 Jun 2007 .

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