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The genus Halophila

Stars, Paddles and Oars
for Marine Planted Aquaria

By Sarah Lardizabal


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 leaf shapes
of H. ovalis, H. engelmannii and H. decipiens.  Drawing by Sara Lardizabal

Halophila (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.

Halophila 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. 

Typically these 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), H. ovalis (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. 

While Halophila 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 aquaria. 

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 only Halophila in deep coastal environments. 


Husbandry Requirements

Like other 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.

Nutrient supply 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 chlorophyll levels.

Wild collected specimens of H. engelmannii found floating in south east Florida.  Photo: Sara Lardizabal

Along with 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 Lardizabal

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 Lardizabal

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 to 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 Reading:

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.

Hammerstrom, K.K., 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. 

International Union for Conservation of Nature and Natural Resources (IUCN). 2004. IUCN Red List of Threatened Species. IUCN, Gland Switzerland.

Jewett-Smith, J. and McMillan, C. 1990.  Germination and seedling development of Halophila engelmannii  Aschers. (Hydrocharitaceae) under axenic conditions.   Aquatic Botany, 36: 167-177.

Jewett-Smith, J. 1990. Germination and seedling development of H. engelmanni. Aquatic Botany. 36: 167 - 177.

Jewett-Smith, J. 1994. Development of a medium and culture system for in vitro propagation of H. engelmanni.  Canadian Journal of Botany. 72: 1503 - 1510.

Lardizabal, S. 2006.  Beyond the Refugium: Seagrass Aquaria.  Reefkeeping.  Vol 5 issue 3 (April).

Lardizabal, S. 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 Amsterdam.

Sprung, J. 2006.  Seagrass Aquariums.  Coral.  2 (6): 70 – 77.

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