Seagrasses are flowering plants from one of four plant families (Posidoniaceae, Zosteraceae, Hydrocharitaceae, or Cymodoceaceae), all in the order Alismatales (in the class of monocotyledons), which grow in marine, fully saline environments.

Ecology [edit]

Floridian seagrass bed

Seagrass bed with several echinoids, Grahams Harbour, San Salvador Island, Bahamas

Seagrass bed with dense turtle grass (Thalassia testudinum) and an immature queen conch (Eustrombus gigas), Rice Bay, San Salvador Island, Bahamas

White-spotted puffers are often found in seagrass areas.

Evolution of seagrasses

These unusual marine flowering plants are called seagrasses because in many species the leaves are long and narrow, and these plants often grow in large "meadows" which look like grassland: in other words many of the species of seagrasses superficially resemble terrestrial grasses of the family Poaceae.

Like all autotrophic plants, seagrasses photosynthesize so are limited to growing in the submerged photic zone, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. Most species undergo submarine pollination and complete their entire life cycle underwater. There are about sixty species worldwide.

Seagrasses form extensive beds or meadows, which can be either monospecific (made up of a single species) or in mixed beds where more than one species coexist. In temperate areas, usually one or a few species dominate (like the eelgrass Zostera marina in the North Atlantic), whereas tropical beds usually are more diverse, with up to thirteen species recorded in the Philippines.

Seagrass beds are highly diverse and productive ecosystems, and can harbor hundreds of associated species from all phyla, for example juvenile and adult fish, epiphytic and free-living macroalgae and microalgae, mollusks, bristle worms, and nematodes. Few species were originally considered to feed directly on seagrass leaves (partly because of their low nutritional content), but scientific reviews and improved working methods have shown that seagrass herbivory is a highly important link in the food chain, with hundreds of species feeding on seagrasses worldwide, including green turtles, dugongs, manatees, fish, geese, swans, sea urchins and crabs.

Some fish species that visit/feed on the seagrass raise their young in adjacent mangroves or coral reefs. Also, seagrass traps sediment and slows water movement, causing suspended sediment to fall out. The trapping of sediment benefits coral by reducing sediment loads in the water. ^[1]

Genera of seagrasses [edit]

Cymodoceaceae family

• Amphibolis
• Cymodocea
• Halodule
• Syringodium
• Thalassodendron

Hydrocharitaceae family

• Enhalus
• Halophila
• Thalassia

Posidoniaceae family

• Posidonia

Zosteraceae family

• Phyllospadix
• Zostera

Environmental services [edit]

Seagrasses are sometimes labeled ecosystem engineers, because they partly create their own habitat: the leaves slow down water-currents increasing sedimentation, and the seagrass roots and rhizomes stabilize the seabed. Their importance for associated species is mainly due to provision of shelter (through their three-dimensional structure in the water column), and for their extraordinarily high rate of primary production. As a result, seagrasses provide coastal zones with a number of ecosystem goods and ecosystem services, for instance fishing grounds, wave protection, oxygen production and protection against coastal erosion. Seagrass meadows account for 15% of the ocean’s total carbon storage. Per hectare, it holds twice as much carbon dioxide as rain forests. Yearly, seagrasses sequester about 27.4 million tons of CO₂. Due to global warming, some seagrasses will go extinct – Posidonia oceanica is expected to go extinct, or nearly so, by 2050. This would result in CO₂ release. ^[2][3]

Uses [edit]

Historically seagrasses were collected as fertilizer for sandy soil. This was an important use in the Ria de Aveiro, Portugal, where the plants collected were known as moliço.

In the early 20th century, in France, and to a lesser extent the Channel Islands dried seagrasses were used as a mattress (paillasse) filling, and it was in high demand by French forces during World War I. It was also used for bandages and other purposes.

Currently seagrass has been used in furniture, and woven like rattan.

Disturbances and threats [edit]

Natural disturbances such as grazing, storms, ice-scouring, and desiccation are an inherent part of seagrass ecosystem dynamics. Seagrasses display an extraordinarily high degree of phenotypic plasticity, adapting rapidly to changing environmental conditions.

Seagrasses are in global decline, with some 30,000 km² (12,000 sq mi) lost during recent decades. The main cause is human disturbance, most notably eutrophication, mechanical destruction of habitat, and overfishing. Excessive input of nutrients (nitrogen, phosphorus) is directly toxic to seagrasses, but most importantly, it stimulates the growth of epiphytic and free-floating macro- and micro-algae. This weakens the sunlight, reducing the photosynthesis that nourishes the seagrass and the primary production results.

Decaying seagrass leaves and algae fuels increasing algal blooms, resulting in a positive feedback. This can cause a complete regime shift from seagrass to algal dominance. Accumulating evidence also suggests that overfishing of top predators (large predatory fish) could indirectly increase algal growth by reducing grazing control performed by mesograzers such as crustaceans and gastropods through a trophic cascade.

When humans drive motor boats over shallow seagrass areas, sometimes the blade propeller can tear out or cut the sea grass.

The most-used methods to protect and restore seagrass meadows include nutrient and pollution reductions, protection using marine protected areas, and restoration using seagrass transplantation.

See also [edit]

• Alismatales
• Blue Carbon
• Salt marsh
• Mangrove

Bibliography [edit]

References [edit]

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009)

Hydrocharitaceae is a flowering plant family that includes a number of species of aquatic plant, broadly called the Tape-grasses, and includes the well known Canadian Waterweed and Frog's Bit.

The family includes both fresh and marine aquatics. They are found throughout the world in a wide variety of habitats, but are primarily tropical.

The species are annual or perennial, with a creeping monopodial rhizome with the leaves arranged in two vertical rows, or an erect main shoot with roots at the base and spirally arranged or whorled leaves. The leaves are simple and usually found submerged, though they may be found floating or partially emerse. As with many aquatics they can be very variable in shape - from linear to orbicular, with or without a petiole, and with or without a sheathing base.

The flowers are arranged in a forked, spathe-like bract or between two opposite bracts. They are usually irregular, though in some case they may be slightly irregular, and either bisexual or unisexual. The perianth segments are in 1 or 2 series of (2-)3 free segments; the inner series when present are usually showy and petal-like. Stamens 1 - numerous, in 1 or more series; the inner ones sometimes sterile. Pollen is globular and free but in the marine genera (Thalassia and Halophila) - the pollen grains are carried in chains, like strings of beads. The ovary is inferior with 2 - 15 united carpels containing a single locule with numerous ovules on parietal placentas which either protrude nearly to the centre of the ovary or are incompletely developed. Fruits are globular to linear, dry or pulpy, dehiscent or more usually indehiscent and opening by decay of the pericarp. Seeds are normally numerous with straight embryos and no endosperm.

Pollination can be extremely specialised.

The most recent phylogenetic treatment of the family recognizes four subfamilies - Hydrocharitoideae (Hydrocharis, Limnobium), Stratiotoideae (Stratiotes), Anacharidoideae (Apalanthe, Appertiella, Blyxa, Egeria, Elodea, Lagarosiphon and Ottelia) and Hydrilloideae (Enhalus, Halophila, Hydrilla, Maidenia, Najas, Nechamandra, Thalassia, Vallisneria).^[1]

Hydrilla verticillata in Hyderabad, India.
Ottelia alismoides in Hyderabad, India.

Uses

Some species have become established ornamental plants, and subsequently serious weeds in the wild (especially Egeria, Elodea and Hydrilla).

Genera

References

1. ^ Les DH, DH; Moody, ML; Soros, CL (2006), "A reappraisal of phylogenetic relationships in the monocotyledon family Hydrocharitaceae (Alismatidae)", Aliso 22: 211–230
2. ^ Tanaka, Norio; Setoguchi, Hiroaki; Murata, Jin (1997), "Phylogeny of the family hydrocharitaceae inferred fromrbcL andmatK gene sequence data", Journal of Plant Research 110 (3): 329, doi:10.1007/BF02524931
3. ^ Les, DH; Cleland, MA; Waycott, M (1997), "Phylogenetic studies in Alismatidae, II: Evolution of Marine Angiosperms (Seagrasses) and Hydrophily", Systematic Botany 22: 443
4. ^ Genera of Hydrocharitaceae, GRIN Taxonomy for Plants
5. ^ Angiosperm Phylogeny Group (2003). "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II". Botanical Journal of the Linnean Society 141(4): 399-436. (Available online: Abstract | Full text (HTML) | Full text (PDF))