Seagrasses are a diverse group of true marine vascular angiosperms (or flowering
plants) that carry out their entire life cycle – from seed to shoot to flower –
submerged in the temperate and tropical oceans of the world. Unlike the
macroalgae that are widely distributed within the marine aquarium hobby,
seagrasses have true roots and represent a distinct habitat that is intricately
tied to the health of the overall coastal system that includes reefs. Many
species of ornamental, sport and food fish rely on seagrass ecosystems as
nursery grounds for larval and juvenile stage young and the diversity of life
runs very high in these areas. Most seagrasses are characterized by a general
appearance resembling terrestrial grass with long strap or ribbon like leaves.
The genus Halophila, however, is a marked exception.
From left to right, relative sizes and
of H. ovalis, H. engelmannii and H. decipiens. Drawing
by Sara Lardizabal
(hal – oh – FY – luh), meaning "salt loving", is one of the largest genus of
seagrass extant today with fourteen existing members currently recognized by
marine botanists. Their phylogeny and evolution is currently under
research by several scientists as is their importance in seagrass ecosystems.
members are found in most of the tropical and subtropical oceans of the world
with both pandemic global distribution of some species and highly endemic
restricted ranges of others. H. decipiens boasts the widest
distribution including both of the American coasts, and every major ocean and
sea while H. johnsonii in Florida and H. hawaiiensis in Hawaii are
both highly restricted. H. johnsonii also has a threatened and
protected status, while its cousin, H. stipulacea, is actually an
introduced exotic species in the Mediterranean from its native Red Sea.
H. decipiens has also been cited as a potential invasive species of
Halophila in Hawaii. Clearly the Halophilas are a diverse group
of just fourteen species.
seagrasses are small in size, from one to four inches in height on average and
are distinguished by species on the morphological shape and arrangement of their
small, typically ovoid, leaves. Until recently access to them was
extremely limited and information was only available through the scientific
literature. However, three species, H. engelmannii (Star Grass),
(Oar Grass) and H. decipiens (Paddle Grass), have been newly introduced
to the hobby and their ease of culture and beauty should cement a certain place
in marine aquaria.
members are still rare in the marine aquarium trade, the most common of the
three species is Star Grass. These plants resemble miniature palm trees,
exhibiting leaves in beautiful star-like arrangements on upright stems. I
collected fragments of these plants in early 2005 while in Florida.
Stargrass has a distribution limited to the western Atlantic and Caribbean from
Florida, Puerto Rico, Cuba, the Bahamas, Bermuda and reportedly from Belize and
parts of the Yucatan Peninsula. The original plants were found after
spring storms amidst the beach wrack or floating at the surface and were coaxed
back to life quite easily over the course of a few months in seagrass dedicated
Oar grass shown at bottom with
Caulerpa prolifera and Halodule wrightii for size reference.
Photo by Sara Lardizabal
Oar Grass (H.
ovalis) and Paddle Grass (H. decipiens) are fairly similar species
and are somewhat difficult to tell apart by the shape of the leaves. Both
have oval shaped, paired leaves rising up from small rhizomes and are nearly
identical to Star Grass in root shape, growth rate and tank requirements.
Oar Grass is fairly widespread in the world’s oceans and was recently imported
through west coast marine wholesalers into the trade. It can be found
naturally from Australia’s tropical coasts, the Red Sea, Persian Gulf and
Africa’s eastern coasts, within the Indian Ocean and from the Philippines and
Japan and is, overall, a Pacific and Indian Ocean species. Paddle Grass
can be found coexisting with Oar Grass in nearly all of the above areas and its
range additionally includes Atlantic Ocean locales such as western Africa and
the Caribbean. These three species can be found mixed in with larger
seagrass species in extensive meadows, or can be found as small patchy beds of
Halophila in deep coastal environments.
seagrasses, Paddle, Star and Oar Grass require well-aged or mud- enriched sand
beds of at least two inches in height, which is much shallower requirements than
other available plants. Similarly, while high light levels provide
explosive growth, this species does well on far less light than usually
recommended for seagrasses. As little as 90 PPFD (about 4wpg in shallow
tanks less than 14” in height) of photosynthetically active light provided from
daylight flavored (5000 – 10,000K) fluorescent bulbs is sufficient. One
person has in fact reported that plants under metal halide lighting actually
seem to have slower growth rates than those under fluorescent lighting.
Temperature does not seem to have a major impact on this species. They do well
in temperate tanks as low as 66F and in tropical tanks upwards of 85F.
Their ideal temperature seems to be within 70-75F.
does far more to impact growth of these grasses than any of the other abiotic
qualities of their aquaria. They can consume large amounts of nutrients
(carbon, nitrogen, phosphorus) and can act as nutrient sinks in aquariums.
While this does mean growing seagrass will have a positive impact on phosphate
and nitrate levels, the typical marine aquarium may reach a level of nitrogen
limitation, especially in tanks with large colonies of seagrass, macroalgae,
live rock, live sand and a suitably-sized skimmer. In Star, Paddle and Oar
Grass, nitrogen limitation is marked by the slow of growth rate and new
plantlets emerge with red to purple colored leaves, which lack proper
Wild collected specimens of H.
engelmannii found floating in south east Florida. Photo: Sara
nitrogen, phosphorus and potassium, marine plants use micronutrients in small
amounts and very large quantities of carbon. Micronutrients include
calcium, magnesium, iron, manganese, biotin, vitamin B12, zinc, selenium,
cobalt, boron, molybdenum, nickel and even copper. The more useful
micronutrient needs can typically be met with regular partial water changes
(10-20% weekly), as most synthetic sea salts provide them suitably. Carbon
needs are typically met through the dissolved free carbon dioxide in solution
and through the alkalinity of the system. Aquarists with large stands of
seagrasses should monitor pH and alkalinity through several photoperiods to be
sure that there is not too much movement in these values. Alkalinity can
be supplemented through calcium reactors, kalkwasser dosing, or simple additions
of sodium bicarbonate (or baking soda) and commercial carbonate products, which
provide the carbonate molecules being targeted by the plants. Free CO2
levels can be brought back to equilibrium (and also help to lower artificially
high pH) by simply aerating the water with air lines or skimmers.
The best method
for transplanting these small plants is to harvest fragments with some soil
intact on the roots. It is especially important to keep several plants
together on a shared rhizome. While plugs of seagrasses in sand are
preferable, care must be taken when collecting the plug to not snap the
single-root structure of the plants. Halophila roots are
comparatively superficial considering they reach an average 2.5” into the
substrate compared to larger species like
Thalassia, whose roots can extend well beyond six inches. A rhizome
fragment needs to have at least five plantlets and a growing tip on the rhizome
to survive transplant.
Closeup of newly transplanted H.
ovalis and growing tip
of the rhizome, at left beneath the stunted leaf pair. Photo by Sara
Planting itself must be done with care. Preferably the aquarist would
first provide a well aged DSB or a substrate with mud layered with coarse to
fine-grained aragonite. Simply dig a suitably sized hole in the substrate
to lay the fragment within, orient the plants properly, and cover back with sand
to fill in the hole. It is not advisable to push the plants and roots into
the substrate as this can damage the roots. Damage to the root systems of
all seagrasses usually proves fatal.
Generally, if you
have a well-run reef or lagoon style tank, these small grasses will find a niche
and survive off even lean nutrient conditions. They may, however, not
reproduce in great numbers. Care must also be taken not to add these
species to tanks with large herbivores as they are highly grazable.
Currently there are only a few known grazers of Star Grass including Rabbitfish
(Siganus sp.), Lawnmower Blennies, some snails, Yellow Tangs and
Diadema urchins. Some Tangs, however, especially the comb-tooth
species like the Kole and Chevron (Ctenochaetus strigosus and C.
hawaiiensis respectively), do a nice job of cleaning the grass of epiphytic
alga growth that can sometimes occur without destroying its tender leaves.
In the wild, Manatees and Green Turtles have been observed to munch this
seagrass as well. As more aquarists attempt Star Grass in their tanks,
interactions with more herbivores is anticipated and we will certainly add to
the list of predators in the future.
Female flowers, as green spikes at
center, of H. engelmanni. Photo by Sara Lardizabal.
Male flower from H. engelmannii
formed during a stressful dinoflagellate outbreak. Photo by Sara
One of the most
interesting, and still perplexing, observations of H. engelmannii in my
care has been the development of female flowers on several rhizomes in the tank.
These appeared first as odd spike like structures in the center of several
plantlets. They are very different flowers from those we are familiar with
here on land, but the overall effect with Star Grass is of a single large green
flower. Each spike eventually produced over the course of five days three
small strands that became the pollen receiving structures of the flowers.
A male flower was produced under stressful growing conditions and was a small
red capsule formed at the center of a plantlet. This later burst open,
releasing pollen into the aquarium though there were unfortunately no female
flowers to fertilize. I am still awaiting a reproduction event in my tank
that will produce viable seeds and fruits from Star Grass.
With good nutrients and light, star
grass is capable of growing rapidly, seen here growing mixed in with
Halodule wrightii, shoal grass. Photo by Sara Lardizabal
However, vegetative propagation – wherein the rhizomes extend and new plantlets
are produced - has been extremely quick. Inside of three months my
original three colonies of twelve total plantlets became a starry field of
grasses covering the footprint of a ten gallon tank (20” x 10”). Over the
past year of culture I have harvested over fifty starter colonies, each at least
eight plantlets. Halophila species are amazing colonizing plants,
relatively unbothered by transplant into new systems, and the easiest, most
resilient seagrasses this author has attempted so far.
While the Halophilas have some potential as refugium export species (in
extremely brightly lit refugia only) the best application will likely fall in
systems setup as lagoonal reefs, seagrass aquaria, and in Syngnathid species
specific tanks. I expect them to be found most frequently in the tanks of
marine aquarists who are tending to marine planted aquarium gardens. Due
Halophila’s small size and shallow bed requirements it may also find a ready
home as an accent in nano reef tanks where aquarists are hoping for better color
balance in the aquascape. Ultimately, these three seagrasses, while newly
introduced to the hobby this year, seem very likely to become common showpieces
and a great addition to the biodiversity of marine aquaria.
References and Further
Fish and Wildlife
Service. Multi Species Recovery Program for South Florida:Seagrasses. Pg. 3-597.
Green, E.P. and
Short, F.T. 2003. World Atlas of Seagrasses. Prepared by UNEP World Conservation
Monitoring Centre. Univ. California Press, Berkeley, USA.
Kenworthy, W.J., Fonseca, M.S., Whitfield, P.E. 2006. Seed bank, biomass and
productivity of Halophila decipiens, a deep water seagrass on the west Florida
continental shelf. Aquatic Botany 84: 110 - 120.
Hammill and Sumby.
2002. In vitro culture of Heterozostera tasmanica and Zostera muelleri.
Presented at Western Port Sea Grass Seminar in Hastings, Victoria, AUS.
Union for Conservation of Nature and Natural Resources (IUCN). 2004. IUCN Red
List of Threatened Species. IUCN, Gland Switzerland.
and McMillan, C. 1990. Germination and seedling development of
Halophila engelmannii Aschers. (Hydrocharitaceae) under axenic
conditions. Aquatic Botany, 36: 167-177.
1990. Germination and seedling development of H. engelmanni. Aquatic
Botany. 36: 167 - 177.
1994. Development of a medium and culture system for in vitro propagation of
H. engelmanni. Canadian Journal of Botany. 72: 1503 - 1510.
2006. Beyond the Refugium: Seagrass Aquaria. Reefkeeping. Vol
5 issue 3 (April).
2005. The ‘Grass Menagerie. Online weblog at
Pulich, WM. 1983.
Growth response of H. engelmanni to sulfide, copper and organic nitrogen
in marine sediment. Plant Physiology. 71 (4): 975 - 978.
Short, FT and
Coles RG. 2001. Global Seagrass Research Methods. Elsevier
Sprung, J. 2006.
Seagrass Aquariums. Coral. 2 (6): 70 – 77.