Texas’ Seagrasses

Texas seagrass beds are vital in maintaining vibrant and healthy coastal marine ecosystems. Seagrass meadows provide critical nursery habitat for many coastal fauna including flounder, redfish, speckled trout, blue crab and shrimp. These are harvested both commercially and by sports fisherman. These species, along with their primary food sources (microscopic algae, polychaete worms, small crustaceans, and other small invertebrates), are abundant in seagrass meadows. Although more than 50 seagrass species can be found around the world, six species are common within the Gulf of Mexico. Five of these six species can be found along the Texas coast, including the two most common species, shoal grass (Halodule wrightii) and turtle grass (Thalassia testudinum), and three others, manatee grass (Syringodium filiforme), widgeon grass (Ruppia maritima), and star grass (Halophila engelmannii).

Genetic Diversity of Texas Seagrasses

The genetic diversity and background as noted by the composition (or lineage) of four distinct genetic types in populations of Halodule wrightii sampled along the south Texas coast as compiled by Patrick Larkin (TAMU-CC).

Shoal grass (Halodule wrightii)

Left: modified from Phillips and Menez (1988); Right: photo by Ken Dunton

Shoal grass leaves are thin, flat and tinsel-like, typically 10-30 cm in length. Shoal grass occupies a wide range of salinities, and is often characterized by having large, dense beds (meadows). Shoal grass is an efficient colonizing species, engineered to colonize and proliferate on disturbed, wave dominated, or shallow water areas. Shoal grass roots and rhizomes are consumed extensively by migrating waterfowl during the winter months, including redhead ducks (Aythya americana). Floating wracks of senescent Halodule leaves are a major consequence of these grazing activities. When Halodule is pollinated, tiny, egg-like black seeds become buried within the sediment acting as a seed reserve. The roots and rhizomes of this pioneer species aid in sediment stabilization, ultimately prompting the succession of climax species such as turtle grass or manatee grass.

Turtle grass (Thalassia testudinum)

Left: modified from Phillips and Menez (1988); Right: photo by Ken Dunton

Turtle grass can be easily recognized by its distinctive blade structure. Blades are wide, flat, and ribbon-like, typically 20-50 cm in length. Leaves may be brightly colored green or appear brown, encrusted with tiny organisms called epiphytes. Turtle grass prefers high salinity water in areas buffered from intense wave action. Successful pollination results in buoyant green fruits, which are dispersed by currents. A dense network of carbon-rich roots and rhizomes provide sediment stabilization and accretion. This extensive network creates continuous, carpet-like meadows. Turtle grass received its common name from the herbivorous green sea turtles (Chelonia mydas) that graze prolifically on newly emerging leaves.

Manatee grass (Syringodium filiforme)

Left: modified from Phillips and Menez (1988); Right: photo by Ken Dunton

At first glance, manatee grass appears to resemble shoal grass. Further inspection yields blades that are tough and cylindrical in shape, culminating in a rounded and blunt tip. Syringodium blades can be upwards of 50 cm in length. The roots and rhizomes of this species do not penetrate deeply into the sediment and are easily uprooted. Manatee grass is usually found in mixed beds alongside turtle and/or shoal grasses, or may be found in monospecific patches. Manatee grass received its common name from the manatee, which is known to graze extensively on seagrass, particularly Syringodium.

Star grass (Halophila engelmannii)

Left: modified from Phillips and Menez (1988); Right: photo by Ken Dunton

Star grass (sometimes called clover grass) is unique in the way that the blades are oriented into a star-like whorl. These whorls generally consist of 4-8 elliptical and finely serrated leaf blades. Star grass is a relatively small plant, rarely exceeding 10 cm in length. It is generally found in sandy or muddy substrates within sheltered and deeper waters. This species is more shade tolerant than most seagrasses, often occurring as an understory species within mixed seagrass beds.

Widgeon grass (Ruppia maritima)

Left: modified from U.S. Fish and Wildlife Service (https://www.flmnh.ufl.edu/fish/southflorida/seagrass/profiles.html)
Right: photo by Ken Dunton

Widgeon grass grows in a tremendous range of salinities, including fresh, brackish and saline waters. It has been observed in Baffin Bay, Texas at salinities exceeding 100. Ruppia is generally found in the soft sediments of wind-protected lagoons and bays. This species closely resembles shoal grass but can be distinguished at maturity by its morphology. Widgeon grass has multiple slender, threadlike blades that protrude in an alternating fashion along the stem. These blades appear bushy and fan-like. Root and rhizome structures do not form thick, extensive mats making them easy to uproot. Ruppia rhizomes are also consumed by migrating waterfowl.