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Johnson's seagrass (Halophila johnsonii)

Status | Species Description | Habitat | Distribution | Population Trends | Threats | Conservation Efforts | Regulatory Overview | Taxonomy | Key Documents | More Info

Status

ESA Threatened - throughout its range

Species Description

Weight:
n/a
Length:
leaves are 0.2-1.0 inches (0.5-2.5 cm)
Appearance:
green with pairs of linearly shaped leaves
Lifespan:
unknown
Diet:
unknown
Behavior:
reproduces only asexually, may be limited in distribution because of this characteristic

Johnson's seagrass can be identified by its smooth margins, spatulate leaves in pairs, a creeping rhizome (that is, a horizontal subterranean plant stem, like the runners on a strawberry plant) with petioles, sessile (that is, attached to their bases) female flowers, and long-necked fruits. The male flowers are unknown.

Outstanding differences between Johnson's seagrass and other similar species are its distinct asexual reproduction and leaf shape and form. The species is known to reproduce only asexually and may be limited in distribution because of this characteristic.

Johnson's seagrass plays a major role in the health of "benthic" resources as a shelter and nursery habitat. It has been documented as a food source for endangered West Indian manatees and threatened green sea turtles. We continue to conduct ecological research on Johnson's seagrass to better understand its life history and inform conservation decisions affecting the seagrass ecosystems.

Habitat

Johnson's seagrass prefers to grow in coastal lagoons in the intertidal zone, or deeper than many other seagrasses. It does worse in the intermediate areas where other seagrasses thrive.

The species has been found in coarse sand and muddy substrates and in areas of turbid waters and high tidal currents. Johnson's seagrass is more tolerant of salinity, temperature, and desiccation variation than other seagrasses in the area.

Critical Habitat
We designated critical habitat on April 5, 2000 in areas of Florida.

Distribution

Johnson's seagrass has a very limited distribution; it is the least abundant seagrass within its range. It has a disjunct and patchy distribution along the east coast of Florida from central Biscayne Bay to Sebastian Inlet. The largest patches have been documented inside Lake Worth Inlet. The southernmost distribution is reported to be in the vicinity of Virginia Key in Biscayne Bay, near Miami.

Population Trends

Johnson's seagrass is the rarest species of its genus. It has a limited distribution, limited ability to disperse and colonize habitats because of its asexual reproduction, and dependence on substrate stability.

Data on the species are rare, though one study found that abundance of all seagrass species is 16% less than in 1986 for the entire Indian River Lagoon complex (Ponce to Jupiter Inlet). Longer-term losses are thought to be approximately 50% for all seagrasses since the 1970s (Woodward-Clyde 1994).

Threats

Conservation Efforts

Research is ongoing on ways to successfully grow the species in captivity and transplant it to suitable locations. The Recovery Plan [pdf] calls for research on basic reproductive biology and life history of the species as well as general management and coordination among responsible local, state, and Federal agencies.

Regulatory Overview

On September 15, 1993, we published a proposed rule to list Johnson's seagrass as a Threatened species under the Endangered Species Act (ESA). Designation of critical habitat was subsequently proposed on August 4, 1994. A public hearing on both the proposed listing and critical habitat designation was held in Vero Beach, FL, on September 20, 1994. The public comment period was reopened until October 13, 1994, to include comments on both of the proposed actions.

Johnson's seagrass was listed as threatened [pdf] on September 14, 1998, and we designated critical habitat on April 5, 2000.

We published a recovery plan [pdf] on October 4, 2002.

Taxonomy

Kingdom: Plantae
Class: Liliopsida
Order: Hydrocharitales
Family: Hydrocharitaceae
Genus: Halophila
Species: johnsonii

Key Documents

(All documents are in PDF format.)
Title Federal Register Date
5-Year Review 72 FR 68129 12/04/2007
Recovery Plan 67 FR 62230 10/04/2002
Critical Habitat Designation 65 FR 17786 04/05/2000
ESA Listing Rule 63 FR 49035 09/14/1998
Status Review n/a 10/15/1997

More Information

References

Updated: January 15, 2015