Jonathan Lowrie
Walk into a local fish store, and take a peek at the tanks.  You would see a variety of freshwater fish, plants, saltwater fish, corals, and inverts, supplies, etc.  But no saltwater plants.  You might find a bag of Caulerpa, most likely some excess from a local hobbyist who traded it in for a new fish. The freshwater hobby abounds with diversity in the plant arena.  In fact, veteran freshwater hobbyist consider a well-planted biome to be the saltless reef equivalent.  This seems to be true, as my planted freshwater tank is full of a diverse array of invertebrate life, as well as fish.  But for those of us with saltwater aquariums as well, don’t despair.  Plants too live in the ocean.
Seagrasses are the sole marine representatives of the angiospermae family of plants.  All seagrasses belong to order Helobnidae, and are in either family Potomogetonaceae, or family Hydrocharitaceae.  Currently, there are 58 species of seagrasses recognized.  The great land down-under, Australia, is home to over half, 30, of the species.  These species belong to only 12 genera.  Grasses that could tolerate the high salt environment and the rigors of ocean life have been around for many millions of years.  Fossil records indicate that early seagrasses were abundant in the Cretaceous period, and even before around the ancient Tethys Sea.  Geological history and archaeological history show that the habitat once occupied by seagrasses has been declining the past 50 million years. Rather than being early ancestors of terrestrial plants, seagrasses were thought be a migration back to the sea by terrestrial plants. 
Seagrasses spend the majority of their existence submerged, although they may be exposed to air with tidal changes.  Their flowers, when produced, are pollinated underwater.  They remain anchored to the substrate via a network of underground root/rhizomes.  Because they are photosynthetic, they tend to be found in shallow water, like coastal areas, salt marshes, estuaries, and in the tropics, associated with mangrove forests.
Seagrasses are unique plants, adapted to live entirely beneath the water.  However, they share many characteristics with their terrestrial cousins the grasses and flowering plants, or angiosperms.  As true vascular plants, they utilize the rhizome to obtain nutrients in the sediment, and use the blades and sunlight for photosynthesis.  Most of these grasses are located in soft, silty sediments, but some species like Phyllospadix do attach to rocks. This adaptation back to the saline environment involves some complex physiological adaptations and anatomical features to allow for a submerged lifestyle. 
As fully aquatic plants, they blades of the grasses must be buoyant to catch the suns light.  They have single layer of tissue (cholorphyllous) called the epidermis, below which is a thick colorless layer with large air canals running the length of the blade.  In the sea, food sources via light may be limited, because of silty conditions, so these plants utilize the rhizome, the large submerged root as a source of nutrient.  Like many plants- potatoes for example, seagrasses store starches in their rhizome. The roots also contain root hairs, which allow for absorption of nutrients from the substrate.  When they flower, the do so underwater.  And the pollination also occurs underwater.  With sexual reproduction, many seeds are produced and are spread via water currents.  Seagrasses may also reproduce asexually, using the rhizome to extend new blades out along the seafloor. 
As aquarists why should we even care about seagrass?  Simply because they form some of the most biologically productive habitats in the world.  They perform a vital function in the physical and biological relationships between plants and animals.  Seagrass meadows provide a nutrient source for fish, waterfowl and other wildlife.  The epiphytic communities living upon the blades of seagrasses provide a rich nutrient source for many other species, as well as a nursery ground for thousands of species. These nursery meadows provide protection from predators for juvenile fish, shrimp, crab and other animals.  They also serve as a major contributor of organic material for nutrient recycling in estuarine waters.  And dense crops of grasses help prevent soil erosion and stabilize sediment loss via their dense root system.
Widespread and dramatic changes to the terrestrial ecosystem often bring corresponding changes to the human populations which have to depend on this natural resource as a means of food and fuel.  In the past decades, increases in human populations and the closer pairing of man and the infringement on the ecosystem have impacted the balance of the ecosystem.  These changes are most noticed not in the loss of a few acres of seagrass, but in the change to the commercial fisheries community.  Because of the destruction of habitat, and the fact that many seagrasses now live only in protected areas, they have been tough to acquire for the hobby.
Some of the best seagrasses for the home aquarium are now available through limited channels.  Turtle grass, Thalassia testudinum is a grass that colonizes the deeper waters of many regions.  It can tolerate higher salinities, and is quite tolerant of salinity flux.  Eelgrass, Zostera marina is another coastal grass. Its blades are considerably thinner, and it appears as a much more wiry grass. Syringodium filiforme is yet another species that can make it into the hobby.  As for lighting, these all like high intensity light.  I have kept them all successfully in shallow tanks under 2 48-inch gro lux or vitalights.  I have found they do best with natural sunlight, or compact fluorescent lighting.  They have a constant growth cycle, and look healthy.
The most crucial aspect to success with seagrasses in the home aquarium has to do with transplantation.  These grasses utilize their roots for nourishment, and stability. The rhizomes are also covered with fragile root hairs, and if these are severely damaged or torn, the acclimatization of the grass to the aquarium can be rough.  As with all plants that utilize roots, seagrasses have root tips with an apical meristem.  This region contains some undifferentiated cells and a high concentration of hormones.  If at all possible, make sure you plant grasses with the apical portion of the rhizome intact.  This will facilitate new shoots to grow in your tank.  I have found that the transport of these grasses should be semi- wet.  If the rhizomes were collected with the natural sediments, then these should be packed in wet newspaper and transported that way.  This will also ‘seed’ the tank with the associated fauna.
Besides creating habitat and providing décor for the lagoon system, seagrasses also play a role in nutrient conversion.  In nature, seagrasses are able to take light energy and store it as sugars starches, and fibers many grazers take advantage of this source of nutrients.  Since they also uptake nutrients from the sediment, they can provide a form of nutrient export and conversion.  In a closed system, they grasses will also provide a food source to the tank animals.  And they will also uptake nutrients from their root structures.  As for significant nutrient uptake in a closed system, you will have to look elsewhere.  Much like mangroves, seagrasses do uptake primary nitrogenous wastes from the sediment and water, but not in the quantities needed to perform sole denitrification in an aquarium.  Of all the algaes that could accomplish this task only the fast growing turf algaes have the growth needed to uptake the vast quantities of nutrients produced in a typical reef.
Seagrasses are gaining popularity with home aquarists, and a few facilities have begun to cultivate them for sale.  Not only is this beneficial for the natural grass beds, but it allows for a grass that is already adapted to typical lighting in an aquarium, and the mixture of sands and gravels commonly used in today’s aquariums.  Of course, not all systems are candidates for seagrasses.  If you have a reef crest tank, with many Acropidae species, then you would not find any seagrasses associated with this habitat.  But if you have a more lagoonal, or reef trough type of habitat, with corals that require slower currents, then seagrasses would make an excellent addition to your captive eco-system.