''Halophila johnsonii'', or Johnson's seagrass, is a small, asexual

seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the or ...

in the family

Hydrocharitaceae Hydrocharitaceae is a flowering plant family including 16 known genera with a total of ca 135 known species (Christenhusz & Byng 2016), that including a number of species of aquatic plant, for instance the tape-grasses, the well known Canadian ...

(the "tape-grasses"). It occurs only on the southeastern coast of

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

, and was the first marine plant listed on the United States endangered species list, where it is listed as a threatened species. Female flowers have been observed, but even with decade long observational studies, neither male flowers nor seed have ever been observed.National Marine Fisheries Service. 2002. ''Recovery plan for Johnson's Seagrass (''Halophila johnsonii'')''. Prepared by the Johnson's Seagrass Recovery Team for the National Marine Fisheries Service, Silver Spring, Marylan

/ref> It may have the most limited distribution of all seagrasses. It occurs only in lagoons along roughly 200 km of the Florida coastline between

Sebastian Inlet Sebastian Inlet, located in Sebastian Inlet State Park in Brevard County, Florida and Indian River County, Florida, offers surfing and fishing opportunities. It is off State Road A1A just 12 miles north of Vero Beach. There are annual surf tour ...

and the northern part of

Biscayne Bay Biscayne Bay () is a lagoon with characteristics of an estuary located on the Atlantic coast of South Florida. The northern end of the lagoon is surrounded by the densely developed heart of the Miami metropolitan area while the southern end is la ...

, where it grows in small patches of a few centimeters to a few meters in diameter at depths ranging from the intertidal zone down to 3 meters. Prior to 1980, the species designation was uncertain. It was sometimes referred to as either ''Halophila decipiens'' or ''H. baillonis'' Ascherson, despite most closely resembling ''H. ovalis''.

Morphologically, Johnson's seagrass is recognized by the presence of pairs of linearly shaped foliage leaves, each with a petiole formed on the node of a horizontally creeping rhizome. The rhizome is located at or just below the sediment surface and is anchored to unconsolidated substrate by unbranched roots. The leaves are generally 2-5 cm long (including the petioles), and the rhizome internodes rarely exceed 3-5 cm in length, making this species appear diminutive relative to the larger seagrasses.

Both the Green sea turtle and the

West Indian manatee The West Indian manatee (''Trichechus manatus''), also known as the North American manatee, is a large, aquatic mammal native to warm coastal areas of the Caribbean, from the eastern US to northern Brazil. Living alone or in herds, it feeds on un ...

are known to feed upon the plant, as well as some

herbivorous A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...

fish. However, the main threat to species survival is probably human activity. Processes that threaten the plant include

eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...

, dredging,

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...

, and

thermal pollution Thermal pollution, sometimes called "thermal enrichment", is the degradation of water quality by any process that changes ambient water temperature. Thermal pollution is the rise or fall in the temperature of a natural body of water caused by hum ...

.''Halophila johnsonii''.

The Nature Conservancy The Nature Conservancy (TNC) is a global environmental organization headquartered in Arlington, Virginia. it works via affiliates or branches in 79 countries and territories, as well as across every state in the US. Founded in 1951, The Nat ...

References

External links

Johnson's Seagrass
page hosted by NOAA Fisheries Office of Protected Resources {{Taxonbar, from=Q5643434 johnsonii Endemic flora of Florida