MARINE MEADOW AND SURFWEED COMMUNITIES

Most people, in fact, most biologists, are unaware that there are two submarine, totally submerged habitats for flowering plants, i.e., hidden beneath the waves, along tropical and temperate coastlines, where one expects to find only marine algae. The habitats are called marine meadow and surfweed. The surfweed community is more commonly known as kelp forest. Both habitats and three species of special seed-bearing halophytes occur submerged below tide along the California coastline.

In all, 58 species of monocotyledons, collectively termed seagrasses or marine phanerogams, live in such habitats, especially in marine meadows, where they are the primary producers of, probably, the most productive community on Earth. Marine meadows are especially famous as the feeding grounds of grazing, herbivorous dugongs and sea turtles, which is why certain species are called turtlegrass.

Seagrasses thrive in shallow sedimentary environments of mainly warm tropical waters, especially in flats of mud or sand ("meadows") with depths of less than ten meters. These habitats must be sheltered from intense surf and oceanic swell, and often the best habitats occur behind offshore coral reefs and atolls, or in lagoons with small tidal effects and very clear water. In places where rock outcrops occur, seagrasses typically are replaced by algal macrophytes (seaweeds), although a few seagrass species occur in the surfweed community, attached to rocks below the level of lowest tide. Eelgrasses, Zostera marina and Z. pacifica, are species that occur in intertidal estuarine mud flats, but have also been located at a depth of 30 meters off the California coast; Zostera occurs as well in New England and northern Europe bordering cordgrass saltwater marsh (Spartina). Around Caribbean islands, turtlegrass (Thalassia testudinum) commonly occurs on the outer edge of Rhizophora, red mangrove. The remarkable record for depth is held by Halophila, found once at 90 meters in exceedingly clear water.

Taxonomy of Seagrasses

There are 12 genera of seagrasses, belonging to four monocotyledonous families.

1. Family Cymodocaceae (17 species)
• Amphibolis
• Cymodocea
• Halodule
• Syringodium
• Thalassodendron
2. Family Hydrocharitaceae (15 species)
• Euhalus
• Halophila
• Thalassia
3. Family Posidoniaceae (9 species)
• Posidonia
4. Family Zosteraceae (16 species)
• Heterozostera
• Phyllospadix
• Zostera

In external design, the halophytic seagrasses from different families can appear very similar, but evolution of these groups of genera has taken place several different times, quite likely early in the differentiation of monocotyledons, and likely from ancestors that inhabited brackish water of coastal salt marsh and estuary.

Some authors also list Ruppia maritima (Family Ruppiaceae) as a seagrass, whereas this species is generally not found beyond the margin of salt marsh.

Marine phanerogams have been observed along all continents except Antarctica (even though ironically one species is named Amphibolis antarctica!). Thirty of the 58 species are found around Australia, where much of the experimental work on seagrasses has been done. The largest seagrass meadow in the world occurs in the hypersaline Shark Bay of western Australia, where 12 species occur, but 85% of that coverage is by A. antarctica, which forms expansive monospecific stands. Shark Bay has very few rocky substrates, and the entire inlet is dominated by seagrasses. There are as many as 17 species in the general region of Perth, Australia in nearshore environments.

The rich assemblage of seagrasses around the continent of Australia produces a great amount of variation between localities. This differs greatly from the Neotropics, where there are only four major species, Halophila decipiens, Halodule wrightii, Thalassia testudinum, and Syringodium filiforme. Remarkably, there is only one tiny population of a seagrass, Heterozostera tasmanica, on the entire western coastline of South America, at Coquimbo, Chile. Absence of seagrass is attributed in large part to the lack of a protected, shallow, flat site with clear water.

General Features of Seagrasses

• Plants clone by creeping rhizomes, buried under sediments, and form adventitious roots at nodes along the lower surface of the rhizome.
• Leaves are narrow and strap-shaped (linear), a design that would tend to limit damage via wave action.
• As in typical monocotyledons, leaf growth occurs only from a basal (intercalary) meristem. This is beneficial in particular for seagrasses, which have leaves colonized by epiphyllous (epiphytic) algae, so that growth at the base may reduce shading on new tissues by epiphytes. Leaves are protected when young by sheaths, which may shield the immature leaf region from desiccation by the saline solution.
• No leaf of a seagrass has stomates, and yet the leaf is typically covered with a thin, waxy cuticle. At first glance, this would appear to restrict carbon dioxide from entering the leaf, because gases do not diffuse well through wax. However, studies have shown that this cuticle is relatively porous in texture, so that carbon dioxide enters at a relatively rapid rate through the leaf surface.
• Leaf photosynthesis takes place almost exclusively within the epidermis, the outermost layer of leaf cells, not the central mesophyll (as in typical land plants). Epidermal cells have well developed chloroplasts and many organelles associated with active metabolism, whereas mesophyll cells of seagrasses possess very few chloroplasts and appear to store starch grains.
• The roots and rhizomes occur in an anoxic (anaerobic) environment, so that oxygenation is required for proper and rapid growth of anchoring organs. In seagrasses, oxygen generated inside chloroplasts during the light reaction of photosynthesis diffuses most easily to the center of the leaf, where there are large intercellular air chambers (lacunae), in a tissue type called aerenchyma. Use of oxygen by the roots creates a sharp oxygen gradient, from high levels in leaf aerenchyma (greater than 30% in these lacunae) to near zero in growing roots; hence, via mass flow oxygen diffuses from leaf to stem to root and rhizomes through aerenchyma air spaces. At each node there occurs a diaphragm, a wall of cells that appears to block passage of air molecules into the next segment (internode), but this is merely an illusion, because there are air gaps large enough between cells of the diaphragm that no blockage likely occurs in the pathway (based on calculated resistances).
• Seagrass organs seem to have scanty development of xylem, water-conducting tissues, in comparison with related land plants with equal leaf size.
• Flowers on all seagrasses are unisexual, i.e., either male or female, and in some species there are separate male and female plants (dioecious). For sexual reproduction to be successful, a pollen grain from a male flower must be transported to the tip of the female flower, in the ocean without the aid of an animal vector. Female structures and some male structures appear to be waterproof, so that tissues to not desiccate in the saline solution.
• Pollen grains of numerous seagrasses are extremely long and thread-like (filament-like or filiform). In fact, the longest one of any seed plant is of a seagrass, three millimeters in length with forked ends (Amphibolis antarctica)! These thread-like pollen grains often have no germination apertures (sulci or colpae) and have considerably reduced wall layers, even lacking certain zones. Researchers have analyzed possible effects of these features on pollination in the submarine environment, and speculated, with some corroborative evidence, that the filiform grains, which may be bow-shaped and are lighter per volume by having less wall, have a slower sinking velocity that spherical pollen grains. This would allow the pollen grain to be suspended in the water column longer, therefore have longer time to come in contact with the female flower and to take a longer path and use a greater search area.
• Vivipary, germination of the seed and formation of a seedling while still attached to the mother plant, has been found in some seagrasses, and is well documented in Amphibolis and Thalassodendron. Vivipary in plants is most common among halophytes (mangrove swamp and salt marsh plants in particular), and is thought to be a way to minimize some of the harmful effects of chloride ions during seed germination.

Because seagrass communities are highly productive, exceeding that of most crop plants, plant biologists have attempted to study in detail how carbon dioxide is delivered to the leaf chloroplasts across the waxy cuticle. Investigators know that at a typical ocean water pH of 8.2, as much as 99% of the carbon dioxide is present instead in the form of bicarbonate, HCO3. Speculation has been that bicarbonate, because it is so abundant, could be the carbon-containing molecule used. The best models to date, however, have revealed that carbon dioxide, a much smaller molecule, diffuses much more rapidly across the unstirred layer next to the leaf and through cutinized cell walls than does bicarbonate, so carbon dioxide is now taken to be the chief gas used to enter the leaf, even if present at a very low concentration.

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