Halophila johnsonii is dioecious (contains flowers of only a single sex on one
plant). However, the
male flower has never been recorded in the field or in laboratory culture. The absence of male
flowers supports
the hypothesis that sexual reproduction is absent in this species. Scientists have not observed
new Halophila
plants growing from root or stem fragments. Rather, the plant extends by branching and by
growth of the
rhizomes. This limited reproductive capacity further threatens the ability of the rare plant to
survive
human-induced or natural disturbances. Because it is most abundant amidst the heavy boating
traffic of south
Florida coastal area inlets and channels, potential threats to the diminuitive seagrass include
dredging activities,
boat propellor and anchor damage, and storm events.
Identifying characteristics of H. johnsonii include smooth linear leaves with
entire margins. Leaves
are 10-20 mm long on long petioles and occur in pairs at each node. The plant has a creeping
rhizome and
sessile female flowers. The plant tolerates broad ranges of salinity, temperatures, and water
levels. It is found
on sandy intertidal shoals where it is exposed to drying, intense sunlight and extreme temperature
changes
during low tide. It also is found in deeper channels with swift, eroding currents.
Johnson’s seagrass is one of twelve species of the genus Halophila. Most
Halophila species are less
than four inches tall, shallow rooted, and have two to three orders of magnitude less biomass per
unit area
compared to all other seagrasses. In contrast to the restricted range of Johnson’s seagrass, other
members
of the genus have a pantropical range. In addition, Halophila seagrasses cover the
greatest range of water
depths for seagrasses. They have been found growing in water depths of more than 100 feet deep,
as well as
in shallow estuaries and intertidal shoals. These seagrasses are known to provide a food source to
green sea
turtles, West Indian manatees, and dugongs.
References:
1) Durako, M. and F. Wettstein. 1994. Johnson’s seagrass. The Palmetto, Winter 1994, pp.
3-5.
2) Eiseman, N.J. and C. McMillan. 1980. A new species of seagrass, Halophila
johnsonii, from the
Atlantic coast of Florida. Aquatic Botany 9:15-19.
3) U.S. Federal Register 63(177):49035. 1998.
NOTE: The APIRS database contains 27 references on Halophila johnsonii.
Aquaphyte Contents Aquaphyte page Home