Marine Seagrasses

There are four prerequisites for a seagrass: (1) It must be adapted to saline water; (2) it must be able to grow completely submersed; (3) it must be securely anchored with perennial rhizomes or "holdfasts" to withstand wave action and tidal currents; and (4) it must be water pollinated. At least ten aquatic species in the Pacific states satisfy the above criteria, although only five are native to rocky shores and sandy or muddy bays: Phyllospadix torreyi, P. scouleri, Zostera marina, Z. pacifica, and Z. nana. Four additional species, including Ruppia maritima, Zannichellia palustris, Potamogeton pectinatus, and Najas marina live submersed in brackish water of salt marshes, ditches and estuaries. These latter species have cosmopolitan distributions and can survive very sudden and large fluctuations in salinity. In spite of their enormous salt tolerance they seldom penetrate the purely marine habitat. A tenth species that certainly ranks as a true marine seagrass, Halodule wrightii, was naturalized in the Salton Sea and may occur in the Gulf of California.

Assuming the seagrasses gradually evolved from freshwater plants, one of their biggest hurdles to overcome was adapting to saltwater. Like celery and carrot sticks placed in saltwater, the roots of most plants rapidly lose water if they are emersed in seawater. Salt-loving plants (halophytes), such as the seagrasses and mangroves, generally have a lower concentration of water molecules (lower water potential) in their root cells so they can take in water. They maintain lower water potentials in their roots by having higher internal salt concentrations than seawater and by losing water at the leaf surface. Since high internal salt concentrations can be lethal to plant cells, some halophytes can excrete excess salt through their leaves and stems.

Rocky intertidal zone of San Diego County at low tide revealing emerald green meadows of surfgrass (Phyllospadix torreyi).

Peculiar one-seeded fruits of surfgrass (Phyllospadix torreyi). The leathery covering (exocarp) on inner side of claws on right fruit has been eroded away, exposing the dense, stiff bristles. The horseshoe-like claws and inwardly pointing bristles enable the fruits to cling to algae and other attached forms in the wave-battered intertidal zone.

Flowering surfgrass (Phyllospadix scouleri) from the rocky California coast. Note the thickened rhizome, slender, flattened leaves, and female flower stalks (spadices). In this photo the flowers have already been pollinated and are forming one-seeded fruits that will soon be released and dispersed by the waves. This species can withstand pounding surf along wave-battered rocky shores.

Aerial view of Scammons Lagoon in Baja California showing dark patches of eelgrass meadows (Zostera marina) on the bottom. Note the California gray whale and her calf at lower middle of photo.

Seagrasses often form luxuriant submarine meadows teeming with life. Marine meadows in the Gulf of Mexico and Caribbean are composed of five different seagrasses: Thalassia, Halodule, Syringodium, and usually two species of Halophila, but no temperate Zostera or Phyllospadix. According to G. Thorson (Life in the Sea, 1971), turtlegrass beds (Thalassia testudinum) in the tropical Atlantic may contain 30,000 individual animals per square meter. In addition, more than 100 species of algae are known to grow epiphytically on its leaves. Eelgrass meadows (Zostera) are among the richest and most productive of all biotic communities. Numerous crustaceans, mollusks, small fish, diatoms, nematodes, sea urchins, sponges, bryozoans, brittle stars, annelids and flatworms find shelter and food in eelgrass meadows of bays and deeper sublittoral waters. Dead, decaying leaves serve as nutrients for blooms of bacteria and protozoans which provide food for the larvae of oysters and other animals. Eelgrass beds also provide a staple winter food supply for migratory waterfowl, including sea brant, Canada geese and black ducks. In addition, the densely matted rhizomes play an important role in maintaining the biological productivity of bays and estuaries by trapping nutrient-rich silt, thus preventing it from washing out to sea.

To appreciate the ecological importance of seagrasses, consider the sudden disappearance of eelgrass beds along the Atlantic coast during the 1930s. An epidemic infestation of the parasitic slime fungus (Labyrinthula), called "wasting disease," literally destroyed the rich eelgrass meadows, the results of which were catastrophic. Populations of cod, shellfish, scallops and crabs were greatly diminished, and the oyster industry was ruined. There was also a serious decline in overwintering populations of Atlantic brant. Areas formerly covered by dense growths of eelgrass were completely devastated and beaches which had been protected from heavy wave action were now exposed to storms. Without the stabilizing effects of eelgrass rhizomes, silt spread over gravel bottoms used by smelt and other fish for spawning. This resulted in a decline in waterfowl populations that fed on the fish. Without the filtering action of eelgrass beds, sewage effluent from rivers caused further water pollution, thus inhibiting the recovery of eelgrass.

Compared to terrestrial flowering plants the seagrasses are not well-known to most naturalists, and yet they play a major role in marine ecosystems. They are an intriguing and marvelously adapted group of seed plants. What they lack in showy blossoms and fragrant scents they more than make up for by their picturesque habitats, exposed only at low tides when sunlight reveals their emerald green masses.