There are 58 different types of seagrass found around the world but the British Virgin Islands are home to only five different species. The most common seagrasses found in the BVI are turtle grass (Thalassia testudinum) and manatee grass (Syringodium filiforme). These grasses are found around almost every island in the BVI. Some of the densest areas include Anegada’s northern and southwestern shore; Fat Hog’s Bay, Tortola and Manchioneel Bay, Cooper Island. Other types of seagrasses found in selected areas in the BVI are: shoal grass (Halodule wrightii), paddle grass (Halophila decipiens) found in deeper waters and widgeon grass (Ruppia maritima) found only in salt ponds such as the Josiah’s salt pond.

Evolutionary History

The evolutionary history of seagrasses is not well known because of major continental shifts that occurred millions of years ago and the limited number of known fossil records. Fossil records that do exist indicate angiosperms (flowering plants) first colonized the marine environment during the Cretaceous Period (144 – 65 million years ago). By the Late Cretaceous, rich seagrass meadows existed worldwide and were closely associated with coral reefs and mangroves.

What are seagrasses?

Seagrasses are not a true “grass”, but are named so for their ribbon-like grass appearance. In order to be classified as a seagrass, there are four properties they most possess. First, the plant must grow in a saline (marine) environment, it must be able to grow completely submerged, it must be able to anchor itself well enough to withstand tidal action, and lastly, it must be able to complete its entire life cycle while fully submerged.

There are a few physical controls on where seagrasses can grow. They grow in a range of depths but it is largely determined by the depth at which there is enough light for the plant to grow. The deepest known seagrass to exist is off of Australia’s Great Barrier Reef in 58m (190ft) of crystal clear water. The tide and water movements also control where seagrasses live mostly by influencing how much light is available. Seagrasses are physically capable of a wide range of water movements but need water flow to seed other areas with seagrass. Another physical control is the salinity. Individual species of seagrass can tolerate different levels of salinity. If the salinity level of an area changes, some species may not survive and a different species may come in to compete for survival.

Seagrasses play a number of ecologically important roles. First, seagrasses reduce wave energy allowing sediments to settle out of the water column. Secondly, the horizontal stem and roots trap sediments and stabilize the bottom. Third, many types of algae and invertebrates are dependant on seagrass and live on their blades. Fourth, seagrass remove nutrients from the water. This is very important for water quality in shallow areas. Fifth, seagrasses are highly productive primary producers. Sixth, seagrass is a food source for many animals. Some studies have shown over 340 animals directly eat seagrass. Finally, seagrasses provide habitat, shelter, and nurseries for many commercially and ecologically important species such as fish and sea turtles.

Threats to seagrasses

The primary causes of seagrass loss in tropical areas such as the BVI is extensive sediment discharge into coastal waters due to unregulated and / or poor coastal development practices and pollution from agricultural and industrial sources. Other threats include anchor damage, propeller scarring, dredging, land reclamation. These threats are all caused by people but seagrasses also have a number of natural threats. A wasting disease thought to be caused by a slime mold has destroyed large areas in the US and could be a potential threat in the Caribbean region. Intense wave action caused by storms can also cause widespread damage to established seagrass beds by breaking or uprooting seagrass. In addition, a number of marine animals disturb seagrasses while foraging such as sea urchins and rays disturb the roots as they forage for buried prey. 

Uses of seagrass

Seagrasses in the BVI are not used for human purposes but in other parts of the world, seagrass has been found to have a number of useful purposes for people. Dried seagrass material is non-flammable and can be used for house insulation and even sound proofing buildings such as radio studios. Coastal areas in the UK & Europe have used seagrasses for thatch roofs and even some species of seagrass have been used as a stuffing material for pillows and mattresses. In Australia and Japan seagrasses have been used for making baskets, mats & rugs.

Seagrass Monitoring Page