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Beautiful Seagrasses – Keeping True Flowering Plants in Your Marine Aquarium

Beautiful Seagrasses – Keeping True Flowering Plants in Your Marine Aquarium
By: Anthony Calfo

Sought after by aquarists around the world, the Hawaiian bristle-tooth, Ctenochaetus hawaiiensis, is a member of the best Tang genus for algae control on soft substrates like in seagrass displays. Picture and Caption by Anthony Calfo

For some years now, private aquarists have begun to realize success with the culture of true, vascular, marine plants. While the number of flowering species available in the hobby is rather small, their natural distribution on or near coral reefs is quite great! Some can be found approaching the deepest range of photosynthetic life in the sea, while others occur within the first few meters of the surface.

These plants live in a wide range of conditions too, from cold temperate seas through to warm tropical waters. Yet for all the geographical differences between them, they have many common traits in husbandry and handling that make their care in aquaria a very straightforward endeavor. For the purpose of this article, the focus will be on genera that we commonly call “seagrass”, although other marine plants such as Rhizophora (Red Mangrove) and Halophila (Paddleweed) share similar needs, benefits and purpose in aquaria.

There are even some interesting brackish species like Ruppia maritima (Beaked Tasselweed) that are both interesting and hardy, ranging from fresh to full seawater. A closer look at coastal waters and salt marshes will reveal quite a number of other candidates to you for aquarium use. And with the increased application of refugiums and natural filtration strategies, our appreciation of marine plants will grow stronger in time!

Best Bets:

For American aquarists, the most familiar seagrass is Thalassia testudinum, collected from Atlantic waters, although imports (seeds) from islands in the Tropical Pacific (such as Fiji) do make seasonal appearances. This species, Thalassia hemprichii, is widely distributed – including the Philippines. Either species is suitable for aquarium use, largely do to the sturdy form (reasonably hardy to handle and prune) as well as modest maximum size; they can be cropped nicely at 12″/30 cm. Blades are flat and wide (1/2″/12 mm.) and can reach one meter in height, although less than half of that is more common. Thalassia produces the largest flowers (pale white/pink) and seeds of all the currently kept seagrasses.

“Manatee grass”, Syringodium filiforme, can be found in many of the same regions of the Atlantic/Caribbean as Thalassia. They also make their way into the US hobby. A Pacific species, S. isoetifolium, occurs in Australia and New Zealand waters. Although hardy enough to establish and culture, this genus is slightly more challenging to keep by private aquarists if only for their need for very large, tall tanks (towards 1 m in height) and the significant hardware (large, expensive pumps) needed to produce adequate water flow in such vessels. Like Turtle grass, Manatee grass requires regular cropping of the blade tops to prevent overgrowth, disease or infection. “Blades” of manatee grass are tubular or cylindrical in nature. The flowers are very small, inconspicuous and slow to appear in home grown colonies.

Zostera marina, “Eelgrass”, looks very similar to Syringodium, but is impractical for casual reef aquarists due to its cool water needs. In point of fact, however, it is a good candidate for cultivation in temperate displays for dedicated hobbyists keeping biotopes, for example. Some public aquariums have used this relatively hardy species for unique Eastern Pacific exhibits. This seagrass naturally occurs in deeper, calmer waters, which translates well in typical home aquaria that commonly lack adequate light or water movement. Zostera is usually found just below the intertidal zone. Sexual reproduction (and flowering) in this plant seems to be influenced by warmer water temperatures and, as such, may limit propagation strategies and dispersal in cooler climes. As an interesting aside, this plant is quite edible and was consumed by Native American (Indian) coastal peoples.

Although hardly seen in the hobby, Shoal (Sea)grass, Halodule, is a very appealing genus for aquarium use. Its benefits include small size (more narrow blades than Thalassia and most are short at 4″/10 cm tall or less), and rather weak root system. What this means for aquarists is that large, deep sand beds (> 6″/15 cm.) are not so critical for success as it is with the other seagrasses. Halodule has much appeal for smaller refugiums and home-sized lagoonal displays. Specimens collected in the Atlantic (H. beaudettei) occur in very shallow water and tolerate a wide range of salinity. I do hope we see more of this and like species in the hobby soon.

For aquarists searching for these plants and other uncommon creatures, it has been a bit of a challenge to date. As the popularity of keeping such organisms grows, so too will support from merchants. For some of these and more, there’s a good chap at who can help you with his experience as a lifetime marine biologist as well as dedicated hobby mentor and volunteer.

The root systems of seagrasses are very delicate! Never push specimens into the sand to plant, but dig a hole and bury them gently. Picture and Caption by Anthony Calfo


One of the biggest challenges to keeping seagrasses is getting healthy starters! The rhizomes/”roots” of these plants are somewhat to very sensitive to being disturbed. The very best way to transplant seagrass is to dig deep and around a patch, to be taken whole as a “plug” with undisturbed roots and substrate together. For shipping, however, this is too difficult and too expensive (the heavy weight of muddy substrates and the postage to deliver it). It is inevitable that we must accept starters as “bare root” specimens in most cases. If shopping on sight, select pieces with the longest, unbroken rhizomes.

At all stages of transit, be very gentle with the runners/roots. You will notice that the roots may have an offensive (sulfur) odor from the muddy, anoxic substrates they are harvested from. This is no cause for concern. But it should remind you about the need for like (mature, nutrient rich) substrates in aquaria. More about this below.

Planting seagrass is a sensitive matter just the same. They generally must be rooted at great depth. Arguably, anything less than 6″/15 cm. of substrate is not enough for long term success. By comparison, consider the potting needs of a 1-3 feet tall houseplant! Yes… seagrasses need tall tanks and deep fine substrates. Oolitic sand can be used alone if it is very mature (over one year established). But a mix of mud and fine sand (sugar-fine aragonite) may be best overall. I recommend a bed depth of at least 6″/15 cm., and preferably substrates approaching 12″/30 cm. for long term success.

The depth of the aquarium is somewhat more flexible, but a water depth of 24″/50 cm. above the sand is a fair minimum. Note: Halophila, Ruppia and Halodule are exceptions among marine plants; they tolerate shallower substrates and less deep water.

Never push a seagrass pod or cluster directly into the sand; driving it forcibly into the substrate can damage the crown or roots, which may be fatal for the specimen! Instead, always dig a pit and then gently lay the pod or cluster down inside before covering the roots gently. It is important to note that most seagrasses need to be planted rather deep into the new substrate. About 3″/7.5 cm. minimum below the surface of the substrate is recommended. Anything less reduces the likelihood of a successful transplant.

Seagrasses transplanted from the wild will generally lose their original (shipped) leaves in the ensuing weeks and months. This is very normal and is commonly observed in other aquatic plants after a change in light or water depth. In some cases, the plant appears to die back completely with no sign of life for many months. But after five months or even longer, the roots may sprout anew! Leave the rhizomes buried even after the blades have died back with hope for such recovery.

Deep, fine, soft substrates are best for seagrasses. Six inches (15 cm.) is the minimum recommended depth. Add mud for better results. Picture and Caption by Anthony Calfo

As mentioned above, a nutrient rich substrate is necessary for best success with seagrasses. If you ever get the chance to dig around in the substrates of a seagrass meadow, you will appreciate this intimately. A bit too intimately… bring nose plugs! Using mud in a fine sand mix is helpful, but patience and time is the best recipe for success here. Plumb the seagrass aquarium inline with the rest of the aquarium system and allow the substrate to mature for at least six months. Twelve months or more is better. Fertilization of the roots may be helpful, but must be done carefully (small, weak doses). Aquatic plants and algae are unique in that they do not depend on substrates like terrestrial plants do for their principal nutrient base. Aquatics can draw such elements through their stems and leaves too.

Be sure to provide very strong water flow. If possible, generate surge-style flow to help thrash and wash sediments and epiphytic matter off the seagrasses (this wash is very good matter for filter feeders!). You will likely need to employ some gastropods or fishes to rasp the leaves of seagrass for improved health and vigor. Trim dead or dying tips off actively. One of the theories revolving around seagrass diseases in places around the world is that larger grazing animals such as turtles and manatees have been overfished. Some such large herbivores are active grazers on seagrass, but dwindling numbers of these creatures in recent centuries has led to overgrowth of the plants. Overgrowth stifles vigor and water flow and increases the risk of disease. Prune your seagrasses actively (monthly). This is not only a means of nutrient export, but it stimulates health and vigor exactly as it does in terrestrial plants!

Lighting varies by species, but generally speaking… bright warm daylight is best for seagrass species. A minimum of 5 watts per gallon of 4000-7000K lamp color over aquaria less than 30″/75 cm in height is a good starting point. Excessively blue spectrums will handicap the growth of most common seagrass species. Look for lamps that have a high CRI rating as well (over 90 ideally). You can find inexpensive and useful plant-suitable lamps and fixtures at the local DIY home store (lumber and hardware store). A search of reef hobby message boards will often lead to specific brand and model recommendations for stores in your area. Aquarium-specific lighting, although more expensive, is generally best of all though. Daylight temperature metal halides will generally support the fastest growth of shallow water species.

If the seagrass display is to keep cnidarian animals, lamp temperatures closer to 10, 000K are acceptable and perhaps preferable, depending on the needs of the species kept. Seagrasses will still grow very well under such “10k” light. If nothing else, the bluer light is aesthetically more attractive to most people.
Why Keep Seagrasses:

There are many reasons for keeping seagrass displays and refugiums. Studying these unique plants helps us see the much broader picture of how expansive reef communities really are. Specific biotopes like seagrass meadows and mangrove swamps play crucial roles in the overall support and survival of coastal and reef ecosystems. They bring many of these benefits to aquarium culture as well. Just like desirable algae species, seagrasses trap and utilize nutrients like phosphate and nitrate. In fixing these compounds, they improve water quality and serve as a vehicle for nutrient recycling when eaten by creatures, or nutrient export when harvested out of the sea or aquarium. The very surface of grass blades is a living substrate for the cultivation of epiphytic matter that gets liberated with surging water flow and rasping herbivores. Some species of cnidarians, like free-living Goniopora, have been linked to seagrass meadows with an implicated need for the relationship and proximity of the plants for health/survival. Of perhaps worthy anecdotal mention: the author’s own best success, like that of some other aquarists, with Goniopora stokesi living for many years and reproducing by numerous daughter satellites have all been in the company of seagrasses.

Many, many years ago… one of the author’s 240 gallon seagrass refugiums with free-living, reproducing Goniopora stokesi in residence. Picture by Bob Fenner, Caption by Anthony Calfo

Seagrass exhibits also afford more natural and successful displays of lagoonal species of fishes and other reef creatures. Syngnathids (pipefish and seahorses) do remarkably well in seagrass systems for having behavioral enrichment as well as matrices and substrates for cultivating more natural prey such as copepods. Some anemones, jellyfishes and Fungiid corals, for example, are displayed more naturally in lagoonal displays.

Seagrass habitats in the wild are largely in need of protection. To do this successfully, we must study and understand the organisms. Aquarists can contribute to this understanding and conservation by learning to successfully culture seagrasses and the natural species they support, and sharing results with the aquatic science community of hobbyists and academics at large.

With kindest regards,
Anthony Calfo

Bibliography and Recommended Reading:

Littlers, Diane and Mark, 1989 Smithsonian Institution Press.