Johnson's seagrass

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Johnson's seagrass, scientifically known as Halophila ovalis, is a small, asexual seagrass in the tape-grass family. It was once called Halophila johnsonii, but scientists now treat it as a clone of Halophila ovalis. By 2022, U.S. authorities removed Halophila johnsonii from the Endangered Species Act because it isn’t a distinct species.

Where it lives
This seagrass occurs only along the southeastern coast of Florida, in lagoons spread across about 200 kilometers from Sebastian Inlet to the northern part of Biscayne Bay. It grows in small patches ranging from a few centimeters to a few meters across, in shallow water from the intertidal zone down to about 3 meters deep.

What it looks like
Johnson's seagrass has pairs of narrow, linear leaves about 2–5 cm long (including the leaf stalk). The leaves grow from a creeping rhizome near or just below the sediment surface and are held in place by roots. The rhizomes are close together, so the plant often looks small.

Reproduction and genetics
Female flowers have been observed, but male flowers and seeds have not been seen in long-term studies. Genetic research shows this plant is not a distinct gene pool; samples from Indian River Lagoon were genetically identical, indicating it is a clone of Halophila ovalis, related to populations in Africa and Antigua. The clone may have come from a recent introduction rather than representing a separate species.

Ecology and threats
Green sea turtles, West Indian manatees, and some herbivorous fish eat Johnson's seagrass. The main threats come from human activity, including nutrient pollution (eutrophication), dredging, increased water turbidity, and heat pollution.