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NATURAL HISTORY

JOHNSON’S SEAGRASS } Halophila johnsonii
FAMILY: Hydrocharitaceae


DESCRIPTION: Johnson’s seagrass is a small seagrass with short, hairless, elliptical leaves that have smooth edges. These leaves grow up to an inch long, occur in pairs, and have pointed tips. The plant spreads via its unbranched roots and horizontal, subterranean plant stem. Johnson’s seagrass can be identified by its female flowers and long-necked fruits. Male flowers are unknown.

HABITAT: Johnson’s seagrass grows in shallow waters of coastal lagoons in the intertidal zone. Found to depths of approximately six feet, the species occurs deeper than many other seagrasses. Johnson’s seagrass is also more tolerant of varied salinity and temperature ranges. This plant prefers coarse sand and muddy substrates in areas of turbid waters and high tidal currents.

RANGE: Johnson’s seagrass has very limited distribution. It has fragmented distribution along 125 miles of coastline in southeastern Florida from Sebastian Inlet to Biscayne Bay. The largest populations have been documented in Lake Worth Lagoon.

BREEDING: Reproduction of Johnson’s seagrass is by asexual branching and clonal reproduction. Female flowers have been observed, but even with decade-long studies, neither male flowers nor seeds have ever been recorded.

LIFE CYCLE: A perennial plant without a strong seasonal pattern, Johnson’s seagrass generally exhibits some winter decline.

THREATS: Johnson’s seagrass is imperiled by degraded water quality from agricultural and urban runoff, the dredging and filling of waterways, destruction of lagoon substrate by boating activity, trampling, and increased severity of hurricanes and storms driven by global warming.

POPULATION TREND: Because it seems to rely entirely on asexual reproduction and is dependent on substrate stability, Johnson’s seagrass is extremely vulnerable to human-caused disturbances. Information on this seagrass is difficult to come by, though one study found that all of the seagrass species in the Florida region have declined by 16 percent since 1986. Longer-term regional losses are thought to be nearly 50 percent since the 1970s. Johnson’s seagrass is known to be the least abundant seagrass within its range. Research is ongoing on ways to grow the species in captivity and successfully transplant it to suitable locations.

Photo by Lori Morris, NOAA