Halophila johnsonii 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Plantae Tracheophyta Liliopsida Alismatales Hydrocharitaceae

Scientific Name: Halophila johnsonii Eiseman
Common Name(s):
English Johnson's Seagrass, Species code: Hj
Taxonomic Notes: This species was first observed in the1950s by Phillips (1960) when he misidentified this species as H. baillonis. It was then collected in 1980 and described as a new species (Eiseman and McMillan 1980). There is some controversy with this species in that genetic analysis showed that H. johnsonii is genetically very similar to H. ovalis, which is an Indo-Pacific species (Waycott et al. 2006, Short et al. 2010), suggesting that H. johnsonii may be an introduced representative of H. ovalis, a concept strengthened by the fact that only female plants have been found.

Assessment Information [top]

Red List Category & Criteria: Least Concern ver 3.1
Year Published: 2010
Date Assessed: 2007-10-27
Needs updating
Assessor(s): Short, F.T., Carruthers, T.J.R., van Tussenbroek, B., Zieman, J. & Kenworthy, W.J.
Reviewer(s): Livingstone, S., Harwell, H. & Carpenter, K.E.
Halophila johnsonii is limited to the east coast of Florida. There are localized threats due to human activities, but none that cause an overall decline of population. The population is fluctuating and has been increasing in some parts of its range. There is some controversy concerning the taxonomy of this species, with evidence suggesting that it is an introduced representative of H. ovalis. However, it is currently a good species under published documentation. This species is listed as Least Concern.

Geographic Range [top]

Range Description:Halophila johnsonii is endemic to Florida, USA. It is found from north of Sebastian Inlet to Biscayne Bay.
Countries occurrence:
United States (Florida)
FAO Marine Fishing Areas:
Atlantic – western central
Additional data:
Lower depth limit (metres):4
Upper depth limit (metres):1
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:The population of Halophila johnsonii has been fluctuating in abundance from 1994-2007 in the portion of its range where surveys have been conducted. There has been a recent report of an 18.5 km extension north of its known geographic range (Virnstein and Hall 2009).
Current Population Trend:Increasing
Additional data:
Population severely fragmented:No

Habitat and Ecology [top]

Habitat and Ecology:All members of the known population are female and ovules but no seeds have been found. An estuarine species, it is found in intertidal to shallow subtidal areas, mostly on sandy intertidal flats. The distribution of this species is often discontinuous and patchy. This species is highly transient and plants are quick growing, reaching mature size in about two weeks with beds only persisting for a few months. (Zieman 1982, UNESCO 1998, Hemminga and Duarte 2000, Green and Short 2003, Larkum et al. 2006).

Research shows high fragment viability suggesting high dependence on fragmentation as a means of dispersal. Data show that fragment viability is dependent on seasons with fragments being viable for up to four days during the spring months and up to eight days during the fall months. As only female plants have been observed, reproduction is most likely through fragmentation (Hall et al. 2006).

Threats [top]

Major Threat(s): Halophila johnsonii has a skewed sex ratio since all known individuals of the population are female and there is no known seed production. This species may be locally threatened by coastal development and pollution, and also by mechanical damage from recreational and commercial boats.

Conservation Actions [top]

Conservation Actions: This species is listed as a threatened species in the US Federal Register Volume 63 no. 177, 1998: 49035-4904.

Removal and destruction of seagrass in the Florida state parks is forbidden, and some areas limit boats with engines in order to decrease propeller scaring of the grass beds. There are also plans to decrease negative human impacts through educational efforts aimed at sports fishers and boaters.

Citation: Short, F.T., Carruthers, T.J.R., van Tussenbroek, B., Zieman, J. & Kenworthy, W.J. 2010. Halophila johnsonii. The IUCN Red List of Threatened Species 2010: e.T173324A6990996. . Downloaded on 22 September 2018.
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