Shoal Grass, Halodule wrightii
Shoal Grass, Halodule wrightii. Underwater photograph taken in Bahia de La Paz, Baja California Sur, July 2019. Photograph, identification and related productive discussions courtesy of Abigail Cannon, La Jolla, California.
Phylogeny: Shoal Grass Halodule wrightii (Ascherson 1868), is a member of the Cymodoceaceae Family of Manatee Grasses. The genus Halodule is one of six genera in the Cymodoceaceae Family and there are six species in the Halodule Genus. Plants in the genus grow as long flattened blades. The apex of each blade has two or three teeth. They grow from underground, horizontal, rhizomes. The name Halodule is derived from the Greek word Halo, meaning salt. Plants in this genus are highly salt tolerant and are prevalent in marine environments. This species was named in honor of the American botanist Charles Wright. They are also known as Shoalweed.
Morphology: Shoal Grass has stiff, flattened, blades, which are dark green to bright green in color. They resemble flowering land grasses. The inflorescences are solitary. The fruits are egg-shaped and very small (2 mm). The blades are narrow, only about 2 mm (.07 inches) in width. The length of the blades is determined by water depth, and can reach a maximum of 40 cm (16 inches). The blades are sheathed at their base. Shoal Grass is often found growing in dense meadows.
Habitat and Distribution: Shoal Grass grows in mud to coarse sand substrates, and even shell hash. It is primarily found in shallow bays and other protected waters. It lives in the low intertidal zone, and to depths of 12 m (40 feet). Shoal Grass is uniquely suited to tolerate a range of salinities and exposure to air. It is one of the first sea grass species to colonize disturbed areas. Shoal Grass is a subtropical to tropical Atlantic, Indian, and Eastern Pacific Ocean species. In Mexican waters Shoal Grass is found along the entire east coast of the Gulf of Mexican and the Caribbean; it has a discontinuous distribution on the west coast in the Pacific Ocean, being found around La Paz, Baja California Sur and Puerto Arista, Chiapas. At one time it was found in the Salton Sea of Southern California but has since been extirpated.
Predators: Shoal Grass is preyed upon by ducks, fish, manatees, and sea turtles.
Reproduction: Shoal Grass most commonly reproduces asexually, through extending their rhizomes or by fragmentation. They can also reproduce sexually though flowering and seeding. They are gonochoric, with each plant producing either male or female flowers. The frequency and abundance of flowering and seed production varies by location.
Ecosystem Interactions: Shoal Grass provides important shelter and nursery habitat for various species of crustaceans, fish mollusks, and other invertebrates.
Human Interactions: Shoal grass has no direct impact on human activities. Humans impact Shoal Grass through dredging, coastal development, sedimentation, pollution, and boating activity. From a conservation perspective Shoal Grass is currently considered to be of Least Concern, however regional populations are often considered threatened.
Synonyms: Diplanthera beaudettei, Halodule beaudettei, and Halodule brasiliensis.
Request for Help. Our friend and strong contributor Abigail Cannon has asked us try to locate additional seagrass beds in the tropical waters of the Tropical Eastern Pacific which includes Mexican waters. We would greatly appreciate being advised of any sea grass beds that you might come across or are aware of in this area.