Flora of North America species comparison

Zostera marina is adapted to the cold waters of the North Atlantic and North Pacific. It extends southward to North Carolina in the Atlantic and Baja California in the Pacific. At the southern limits of its range, active growth mostly is in the cooler months of autumn and spring, with flowering and fruiting mostly in the spring and the plants dying in the hotter summer months, the vegetation becoming dislodged from the substrate and floating to the water surface. The fruits apparently remain in the floating vegetation for a period of time, eventually falling from the shoots to the substrate. Movement in dislodged vegetative material is the only adaptation the fruits have for dispersal (C. den Hartog 1970).

The species is found mostly in the sublittoral region, only rarely being exposed at low tide. It occurs in more or less sheltered areas on soft mud or firm sand. Plants of sandy substrates had narrower leaves than plants growing on muddy substrates (C. H. Ostenfeld 1905). Fruits fall from the floating vegetation to the substrate and settle on the substrate ripple marks, which run more or less perpendicular to the direction of current. Seedling establishment is parallel with the ripple marks, forming vegetated ridges separated by depressions, which gradually fill with sediments, and the plants then grow laterally into them, forming a meadow (C. den Hartog 1970). The vegetation lowers the velocity of current flow, causing some suspended particles to settle out and accumulate around the base of the plants, slowly building the substrate. As more particles accumulate, the substrate gets deeper over the rhizomes, since the rhizomes grow horizontally, not vertically. Eventually, the rhizomes are too deep, and the plants begin to die back, a phenomenon followed by erosion.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Zostera, especially Z. marina, is very possibly the most important taxon of marine angiosperms in the Northern Hemisphere. Much has been written about the biology and economic importance of Z. marina, and the literature was summarized by C. den Hartog (1970). The species functions in sediment deposition, substrate stabilization, as substrate for epiphytic algae and micro-invertebrates, and as nursery grounds for many species of economically important marine vertebrates and macro-invertebrates. In addition to those functions, the species once was the principal material for the Dutch dikes, and it has been utilized as packing material and as stuffing for mattresses and cushions. Finally, the species has been used for food and recreation by the Seri tribe of Native Americans (R. Felger and M. B. Moser 1973).

During the 1930s, Zostera marina of North America began to develop large brown spots on the leaves and rhizomes. Slowly the plants of a population died, the dieback eventually spreading throughout the North Atlantic, until there was very little eel-grass remaining there. This slow dieback of eel-grass became known as "wasting disease." Biologists began studying the disease and eventually determined the causative agent to be Labyrinthula zosterae, a slime mold. That dieback resulted in huge decreases in the population sizes of most fauna that depended on Zostera. Slowly, eel-grass returned to most areas that it had occupied prior to the disease.

Species 12 (2 in the flora).

(Discussion copyrighted by Flora of North America; reprinted with permission.)