Seagrass Aquarium Guide

Keeping Halodule, Thalassia and Halophila seagrasses: how they differ from macroalgae, why sediment depth and oxygen matter, light and nutrient needs, and the legal limits on sourcing.

What seagrasses are

Seagrasses are flowering plants, about 60 species in four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), and the only land-plant lineage that lives fully in the sea. They have roots, creeping rhizomes, vascular tissue, leaves without stomata and true flowers that are pollinated and set seed underwater. Seagrass meadows occur down to about 50 m where water is clear, and over 20% of the world's seagrass area has been lost since the late 19th century.

How seagrass differs from macroalgae

Macroalgae such as those grown in a refugium are not plants in this sense: they have no roots or vascular system and take nutrients directly from the water. Seagrasses can take nitrogen, as ammonium or nitrate, and phosphorus both from the water column and from sediment pore water through their roots, and their roots and rhizomes store nutrients and anchor the plant. Hobby writers also note that seagrasses grow slowly and steadily and do not overgrow rockwork or collapse suddenly in the way some fast macroalgae can. Algae can also work against seagrass: in a Malaysian estuary, thick mats of the green alga Ulva reticulata over seagrass beds shaded the plants and changed their leaf form and chemistry. Algae-based nutrient export is covered in the macroalgae-refugium-guide and the mangrove-refugium-guide.

The three genera kept in aquaria

• Halodule (shoal grass). Halodule wrightii, family Cymodoceaceae, ranges from Texas to Brazil, including Florida and the Caribbean, in shallow and intertidal water, with flat leaves up to 20 cm long. Florida's environmental agency describes it as an early coloniser of shallow water, and restoration projects have used it as a pioneer species ahead of turtle grass. Hobby sources describe it as the easiest seagrass to grow, spreading by dense runners.
• Thalassia (turtle grass). Thalassia testudinum, family Hydrocharitaceae, grows in the Caribbean and Gulf of Mexico from the low-tide mark to 30 m. Its rhizome lies 5–10 cm deep in the sediment, exceptionally down to 25 cm, and it spreads mainly by rhizome growth. It cannot grow in fresh water; growth is optimal at 25–38.5 ppt salinity and 20–30 °C. It is the largest of the three and grows slowly in aquaria.
• Halophila (paddle grass, star grass). A genus of about 18 accepted species with creeping stems and leafy nodes, widespread in tropical waters and extending into subtropical and temperate seas. Florida has three: Halophila engelmannii, Halophila decipiens and Johnson's seagrass, which Florida's agency now treats as Halophila ovalis, while WoRMS still lists Halophila johnsonii as accepted. Hobby sources describe Halophila as more delicate than Halodule and prone to shedding leaves after transplanting.

The sand bed: depth and oxygen

Seagrasses root in anoxic sediment. They survive because oxygen from photosynthesis travels down through internal air channels to the roots, and some leaks out into the root zone. This radial oxygen loss protects the roots from sulfide, the main plant toxin in seagrass sediments. Species differ: in a comparison of three tropical seagrasses, Halophila ovalis depended most on photosynthesis and lost oxygen fastest when its roots were in low-oxygen sediment, making it the most sensitive of the three.

Grain size and organic load decide how hostile the sediment becomes. Fine, silty sediment rich in organic matter turns anoxic and accumulates sulfide, which stunts roots and photosynthesis. In a tank experiment with the seagrass Thalassia hemprichii, mixing coarse beach sand into fine sediment from a eutrophic bay lowered organic matter, nitrogen and sulfur, made the sediment more permeable to oxygen and reduced sulfide uptake by the plants. For an aquarium this argues for a sand bed deep enough for the rhizomes of the chosen genus, with Thalassia needing the most depth, made of sand rather than fine mud, and not overloaded with uneaten food or detritus. Hobby sources add that sand from an established system, with its microbes, helps new plants settle.

Light

Seagrasses have some of the highest light requirements of any flowering plants, and in the wild their depth limit is set by water clarity. Field-based modelling of the Mediterranean seagrass Cymodocea nodosa put its long-term minimum at about 5–6% of surface light, falling to about 3% in plants that had acclimated to low light. Anything that shades the leaves, including epiphytic algae, diatom and bacterial films, or a canopy of macroalgae, reduces the light reaching them. One long-running hobby tank dominated by Halophila was lit by window daylight alone.

Nutrients

Because seagrasses feed through their roots as well as their leaves, the sediment is part of the nutrient supply. In Florida restoration work, fertilising Halodule wrightii transplants greatly increased their growth, which shows that new plantings can be nutrient-limited. Too much nitrogen in the water has the opposite effect: it feeds algae that cut the light available to seagrass. The goal in an aquarium is a sediment that can supply the roots while the water stays lean enough to keep epiphytes in check. Hobby sources also advise against large numbers of strongly herbivorous fish such as tangs or rabbitfishes in a seagrass tank.

Sourcing and legal status

Wild seagrass beds are protected habitat in many places. In Florida, Statute 253.04 names the state's seagrass species, among them turtle grass, shoal grass, star grass, paddle grass and Johnson's seagrass, and makes careless seagrass scarring by a vessel inside aquatic preserves an infraction with liability for the damage. Johnson's seagrass was the first marine plant listed under the US Endangered Species Act; according to Wikipedia, the National Marine Fisheries Service removed it from the list in April 2022 on taxonomic grounds, but Florida's agency page still describes it as federally listed. Other regions set their own rules through environmental, fisheries and nature-conservation authorities. Verify current status for the place and species before acquiring any plant.