Oct. 14, 1997
Seagrasses:
The seagrasses evolved much like the mangroves in the Tethys Sea during the Cretaceous period. They evolved from terrestrial grasses. Several genera were able to evolve before the Atlantic and Pacific formed. There are now 12 genera with 58 species. All are rooted in soft mud or sand substrates and are found offshore of salt marshes and mangroves. Here they are protected from the full force of wave action. All seagrasses have horizontal stems and regular roots. Seagrasses have very high productivity, up to 600 g/m2/ yr. (A good amount of this biomass is epiphytic, with algae growth on the blades.) Seagrasses are not very nutritious, but the organisms that eat it are probably getting nutrients from the epiphytic growth. When seagrasses die, the detritous matter becomes nutritious.
In the temperate regions (20 C or less) there are 5 genera, and 7 genera are found in the tropical regions (greater than 20 C). If a species is euryhalic, it can take a wide variety of salinities.
Manatees and turtles are associated with the seagrasses, as well as mollusks and crustaceans.
In the division Anthophyta there are 3 families.
1. Hydrocharitacea
Thallasia (turtlegrass) - this is the most common seagrass, found
in the tropics. This is the prefered food of turtles.
Halophila - deepest growing, up to 85 m.
Enhalus - can grow into the intertidal zone with Zostera spp.
Usually they are subtidal.
2. Potamogetonaceae
Zostera - second most common seagrass, found in temperate and tropic
regions, there are 10 species.
Heterozostera - close relative to Zoestra.
Philospandix - 5 species are found in the temperate zones.
Ruppia (widgeon grass) - tropical.
Posidonia - not very common, but found in temperate areas.
3. Zannichellaceae
Cymodaceae - tropical.
Thalasodendron - 3 tropical species, all in Indonesia.
Halodule (shoal grass) - 3 tropical species. Found by reef flats
and in front of mangrove areas.
Syringodium - manatee grass.
Some uses of seagrasses are as dolls, mats, and baskets (especially Zostera and Thallasia by native Indians). The Misquito Indians also ate the seeds of Thallasia. Seagrasses are also beds for predatory gastropods as such helmet shells, cones, and conchs looking for smaller molluscs. Echinoderms like sand dollars, sea biscuits, and big star fish are also found here. This is also a good nursery area for smaller fish because then the bigger fish come in to find them.
Flowering plants: angiosperms: seagrasses
-some will live shallow enough to put a stalk out above the water with
a flower on it. This will have wind blown pollination.
-most release pollen into the water and allow it to get carried away by
tides and current.Usually they have a timing mechanism to reproduce all
at the same time
-Monecious: only one sex on a plant
at a time (male or female)
-Diecious: both sexes on one plant
-advantageous:
fertilize self and no need to wait for pollination
Zostera is an example of a seagrass that pollinates via the air. This is because this species can tolerate low tides and is found growing in areas with a lesser amount of water than usual.
Seagrasses go through 3 morphs: simple blade, tubluar stem, and spoon or ovid shaped. they link themselves together, and the epiphythic growth occurs between sections.
How big are these areas?
-3,000,000 acres in Florida
-1/3 Gulf Coast
-1/3 Florida Bay
-1/3 Atlantic Side
-Still losing a lot of this to development. Sewage outfalls kill off seagrasses
because of turbitity and extra nutrients create algal blooms ( these either
take over or cover up plants).
Diseases in seagrass beds: 1931 - East coast, Zostera marina hit with wasting disease. 1932 - Wasting disease showed up in Europe. 1933 - 90-99% of this species gone, took decades for Zostera to recover. Still don't know what this disease was, bacterial or fungial disease?, warming of waters?, El Nino?
The biggest problem now for seagrasses seems to be dredging. Also freshwater
runoff.
Restoration and Revegetation Projects:
It is important to reestablish seagrass beds because they are imporant
nurseries for fisheries, they help with erosion, and help with nutrient
pollution.
There are several methods for revegetation: transplanting new beds - estimated
costs are $2000/acre in 3 ft. of water, and $200,000/acre in 8 ft. of water.
Non-anchoring methods include:
1)plugs: dig out seagrass with a shovel and replant it somewhere else.
2)cans with bottoms cut out: put plant in can and transplant cans.
3)direct seeding: take seeds directly from plant and plant the seeds in
a good spot.
4)transplanting mats of material: try and pull up the whole mat and transplant
it.
Anchoring methods include: use of ties or rubber bands weaving into plastic
or paper mesh ( effective but expensive).
R. Felger observed the Seri Indians in the Gulf of California and wrote two papers on their practices. The first described how Zoestra was washed to the beach and then collected and used as a food source, for dolls, and baskets. The second discussed the indigenous practice of turtle hunting, which was passed on to the Mexican people . This sharing of knowledge quickly wiped out the turtle population living in seagrass beds as they overfished and overhunted.