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Halophila johnsonii: note elongate, linear leaf pairs. Illustration courtesy of: N. Eiseman, A.G.U.

Halophila johnsonii: detail of leaf. Photo courtesy of D. and M. Littler, National Museum of Natural History. Used with permission.

Species Name: Halophila johnsonii Eiseman
Common Name: Johnson's seagrass
Synonymy: None

    Kingdom Phylum/Division Class: Order: Family: Genus:
    Plantae Tracheophyta Angiosperm Najadales Hydrocharitaceae Halophila

    Voucher Specimen

    Holotype was sampled at Bessie Cove, Hutchinson Island, Martin County, FL. Lat.: 27° 15'; Long.: 80° 12' Number 00341 (HBFH) (Eiseman & McMillan 1980).

    Species Description

    Seven species of seagrass - Thalassia testudinum, Halodule wrightii, Syringodium filiforme, Ruppia maritima, Halophila engelmannii, Halophila decipiens and Halophila johnsonii - occur in the Indian River Lagoon. An illustrated key and guide to their morphology and distribution is presented by Eiseman (1980).

    Potentially Misidentified Species

    Halophila ovalis (R. Br.) Hook.fil


    Regional Occurrence

    Halophila johnsonii occurs only in the coastal lagoons of east Florida, from Sebastian Inlet to Biscayne Bay (Eiseman 1980).

    IRL Distribution

    Halophila johnsonii occurs in the southern half of the Indian River Lagoon, often in dense patches, in deep water and on shoals (Virnstein 1995). Eiseman reports that H. johnsonii occurs primarily in the intertidal zone, between stands of Halodule wrightii and the high tide line, but may also occur in mixed patches with H. wrightii. When H. wrightii is not present, H. johnsonii can occur below the low tide level.

    Halophila johnsonii is probably restricted to coastal lagoons in eastern Florida from Sebastian Inlet south to Virginia Key, Biscayne Bay. H. johnsonii was also documented in Sebastian Inlet, Pepper Park (north of Fort Pierce Inlet) and Big Mud Creek (Eiseman & McMillan 1980).

    Distribution, biodiversity, productivity and ecological significance of seagrasses in the Indian River Lagoon (IRL), FL, are summarized by Dawes et al (1995). Seven species of seagrass, including all 6 species occurring throughout the tropical western hemisphere, as well as Halophila johnsonii, occur in the IRL. Halodule wrightii is the most common. Ruppia maritima is the least common and is found in the most shallow areas of the lagoon. Syringodium filiforme can be locally more abundant than H. wrightii. Thalassia testudinum occurs in the southern portion of the IRL (Sebastian Inlet and south). Halophila decipiens, Halophila engelmannii and Halophila johnsonii can form mixed or monotypic beds with other species. Because of their abundance in deeper water and high productivity, the distribution and ecological significance of the 3 Halophila species may have previously been underestimated

    The northern area of the Indian River Lagoon supports the most developed seagrass beds, presumably because of relatively low levels of urbanization and fresh water inputs. Four species of seagrass - Halodule wrightii, Syringodium filiforme, Halophila engelmannii and Ruppia maritima - can be found north of Sebastian Inlet, while all 7 species occur to the south (Dawes et al 1995). Seagrasses were ranked in order of decreasing percent cover by Virnstein and Cairns (1986) as follows: Syringodium filiforme, Halodule wrightii, Halophila johnsonii, Thalassia testudinum, Halophila decipiens, Halophila engelmannii and Ruppia maritima.


    In the Indian River Lagoon, Phillips (1970) noted water depth for Halophila ranged from 6 inches (slack low tide) to 6 feet (at flood tide).

    Distributional Changes

    Changes in seagrass distribution and diversity pattern in the Indian River Lagoon (1940 - 1992) are discussed by Fletcher and Fletcher (1995). It was estimated that seagrass abundance is 11 % less in 1992 than in the 1970's and 16 % less than in 1986 for the entire Indian River Lagoon complex (Ponce to Jupiter Inlet). Decreases in abundance occurred particularly north of Vero Beach. In this area of the lagoon, it was also estimated that maximum depth of seagrass distribution has decreased by as much as 50 % from 1943 to 1992. Alteration of such factors as water clarity, salinity and temperature could affect the diversity and balance of seagrasses in the Indian River Lagoon system and should be considered when developing management strategies for this resource (Fletcher & Fletcher 1995).


    Sources of mapped distributions of Indian River Lagoon seagrasses include the following: 1) Seagrass maps of the Indian & Banana Rivers (White 1986); 2) Seagrass maps of the Indian River Lagoon (Virnstein and Cairns 1986); 3) Use of aerial imagery in determining submerged features in three east-coast Florida lagoons (Down 1983); and 4) Photomapping and species composition of the seagrass beds in Florida's Indian River estuary (Thompson 1976). Data from the first two sources (White 1986; Virnstein & Cairns 1986) is now available in GIS format (ARCINFO) (see Fletcher & Fletcher 1995).



    H. johnsonii can be locally abundant. It occurs in dense monospecific stands, or in mixed patches only within the confines of the Indian River Lagoon system.




    Pistillate flowers were observed in Halophila johnsonii from April to July (Eiseman 1980). Staminate flowers were not observed. This could imply that H. johnsonii is apomictic i.e., reproduces without fertilization. However, the possibility that male flowers in H. johnsonii could be rare, was mentioned by Eiseman and McMillan (1980). An Indian River Lagoon population of H. johnsonii near Jim Island in Fort Pierce, FL was observed flowering in May and June (Dawes et al 1989). Moffler and Durako (1987) report that H. johnsonii flowers April through July and fruits in August.

    Seasonality of both growth and biomass is exhibited by all species of seagrass in the IRL, being maximum during April - May and June - July respectively. The significance of seagrass beds as habitat, nursery and food source for ecologically and economically important fauna and flora, as well as various management strategies for seagrass beds of the IRL are discussed in Dawes et al. ( 1995).

    Water temperature, moreso than photoperiod, appears to be more influential in controlling floral development as well as subsequent flower density and seed production in seagrasses. Laboratory experiments showing flowering induction under continuous light suggests that photoperiod probably plays a limited role in sexual reproduction (Moffler & Durako 1987).



    Water temperatures in the IRL ranged from a low of 21.0 to a high of 36.0°C, and cover the period from March through September (Phillips 1970). When Halophila johnsonii, an intertidal to shallow subtidal species, was compared with deeper water populations of H. decipiens , H. johnsonii showed greater tolerance to variations in salinity and temperature as well as higher irradiances (Dawes et al 1989).


    Salinity for Halophila johnsonii occurring in the IRL ranges from 24.3 to 38.0 ppt.  The occurrence of Halophila johnsonii throughout the year at salinities as high as 43 ppt was documented (Eiseman & McMillan 1970, Eiseman 1980). H. johnsonii showed greater tolerance to variation in salinity than did a related species, H. decipiens.


    Trophic Mode



    In the Indian River Lagoon, FL, Halophila johnsonii occurs both in firm substrata composed of fine sediment (Eiseman & McMillan 1980), as well as in sandy, mud substratum (salinity - 38 ppt, temperature - 33 - 36°C) (Dawes et al 1989). Eiseman (1980) reported that H. johnsonii prefers firm substratum.

    Virnstein (1995) suggested the "overlap vs. gap hypothesis" to explain the unexpectedly high (e.g., fish) or low (e.g., amphipods) diversity of certain taxa associated with seagrass beds. In a highly variable environment such as the Indian River Lagoon, diversity of a particular taxa is related to its dispersal capabilities. For example, amphipods, lacking a planktonic phase, have limited recruitment and dispersal capabilities, whereas highly mobile taxa such as fish (which also have a planktonic phase) would tend to have overlapping species ranges and hence higher diversity (Virnstein 1995). For an extensive treatment of seagrass community components and structure including associated flora and fauna, see Zieman (1982).

    Associated Species

    When not occurring in monospecific beds, Halophila johnsonii co-occurs with Halodule wrightii.

    For an extensive treatment of seagrass community components and structure, including associated flora and fauna, see Zieman (1982). The significance of seagrass beds as habitat, nursery and food source for ecologically and economically important fauna and flora as well as various management strategies for IRL seagrass beds are discussed by Dawes et al (1995).

    Direct grazing on Florida seagrasses is limited to a number of species, e.g., sea turtles, parrotfish, surgeonfish, sea urchins and perhaps pinfish. Other grazers e.g., the queen conch scrape the epiphytic algae on the seagrass leaves (Zieman 1982). One hundred and thirteen epiphytes and up to 120 macroalgal species have been identified from Florida's seagrass blades and communities respectively (Dawes1987).


    Special Status

    Habitat structure

    Notes on Special Status

    Virnstein (1995) stressed the importance of considering both geographic scale and pattern (landscape) in devising appropriate management strategies to maintain seagrass habitat diversity in the Indian River Lagoon. It was suggested that goals be established to maintain seagrass diversity and that these goals should consider not only the preservation of seagrass acreage but more importantly, the number of species of seagrass within an appropriate area. By maintaining seagrass habitat diversity, the maintenance of the diverse assemblage of amphipods, mollusks, isopods and fish, associated with seagrass beds, will be accomplished (Virnstein 1995).

    Benefit in the IRL

    Because of the vital role of seagrasses as habitat, the health of the Indian River Lagoon ecosystem is reflected in the health of its seagrass communities. Thus, the implementation of sound management strategies designed to protect and promote seagrass habitat helps insure protection for many of the commercially and recreationally important species resident in the Indian River Lagoon.


    Dawes CJ. 1987. The biology of commercially important tropical marine algae. In: Bird KT, Benson P (eds), Seaweed cultivation for renewable resources. Elsevier, Amsterdam: 155 - 190.

    Dawes CJ, Hanisak D, Kenworthy JW. 1995. Seagrass biodiversity in the Indian River Lagoon. Bull Mar Sci 57: 59-66.

    Dawes CJ, Lobban CS, Tomasko DA. 1989. A comparison of the physiological ecology of the seagrasses Halophila decipiens Ostenfeld and H. johnsonii Eiseman from Florida. Aquat Bot 33: 149-154.

    Down C. 1983. Use of aerial imagery in determining submerged features in three east-coast Florida lagoons. Fla Sci 46: 335 - 362.

    Eiseman NJ. 1980. Illustrated Guide to the Sea Grasses of the Indian River Region of Florida. Harbor Branch Foundation. Technical Rep 31. 27 pp.

    Eiseman NJ, McMillan C. 1980. A new species of seagrass, Halophila johnsonii, from the Atlantic coast of Florida. Aquat Bot 9: 15-19.

    Fletcher SW, Fletcher WW. 1995. Factors affecting changes in seagrass distribution and diversity patterns in the Indian River Lagoon complex between 1940 and 1992. Bull Mar Sci 57: 49-58.

    Moffler MD, Durako MJ. 1987. Reproductive biology of the tropical-subtropical seagrasses of the southeastern United States. In: Proc Sym Subtropical-Tropical Seagrass Southeast US. pp. 77-88.

    Phillips RC. 1960. Observations on the ecology and distribution of the Florida seagrasses. Professional Paper Series No. 2. Florida State Board Conserv Mar Lab, St. Petersburg, FL.

    Thompson MJ. 1976. Photomapping and species composition of the seagrass beds in Florida's Indian River estuary. Harbor Branch Foundation. Technical Rep 10: 49 pp.

    Virnstein RW. 1995. Seagrass landscape diversity in the Indian River Lagoon, Florida: The importance of geographic scale and pattern. Bull Mar Sci 57: 67-74.

    Virnstein RW, Cairns KD. 1986. Seagrass maps of the Indian River lagoon. Unpublished report.

    White CB. 1986. Seagrass maps of the Indian and Banana Rivers. Final Report to the Coastal Zone Management Program, Florida Department of Environmental Protection.

    Zieman JC. 1982. Ecology of the seagrasses of south Florida: a community profile. No. FWS/OBS-82/25. Virginia Univ: Charlottesville, VA (USA). Dept Environ Sci.

Report by: J. Dineen, Smithsonian Marine Station
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Page last updated: July 25, 2001

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