Seagrass beds are protected sites

Seagrasses (Division Magnoliophyta, Class Magnoliopsida, Order Helobiae) differ from seaweeds (algae) in that the seagrasses produce flowers, fruits and seeds. Nutrients, such as phosphorus, can be absorbed by the root system and by the upright leaves. The term “seagrass” refers to the grass-like appearance of the strap-like leaves with horizontal rhizomes (runners) and its submerged habitat in seawater during high tides.

Seagrass beds are protected by the US federal government as one of the “special aquatic sites.” Only three species of seagrasses occur in Guam waters:
– Enhalus acoroides (Linnaeus f.) Royle.
– Halodule uninervis (Forsskal) Ascherson in the Family Potamogetonaceae.
– Halophila gaudichaudii J. Kuo.

The largest of Guam’s seagrasses, Enhalus acoroides, grows up to 1.5 meters in height and usually forms circular patches. It inhabits the sandy-silt areas near the mouths of rivers in southern Guam.

Halodule uninervis, up to 15 cm high, is abundant in Cocos Lagoon; a few patches can also be found on the shallow sandy reef flats near shore in the southern bays.

The third seagrass, Halophila gaudichaudii J. Kuo, is the only seagrass that is not grass-like but consists of small, stalked, elongated, veined leaves (up to 1.5 cm high) arising from horizontal runners. This species can be found in shallow sandy reef flats and deeper lagoons.

Seagrasses photosynthesize by converting inorganic carbon (carbon dioxide or bicarbonate ions) and water to sugar and oxygen, and serve as shelter, food and substrata for many marine animals and epiphytic algae. Seagrass beds with their extensive horizontal rhizomes and vertical root systems stabilize the sand during rough sea conditions.

Sea turtles are known to feed on seagrass and have been observed feeding on the flowers of Enhalus acoroides. Invertebrates and fish feed on the epiphytic algae and smaller animals living on the seagrass blades. The detritus formed by the dead and decaying seagrass blades also serve as sources of food and minerals in the marine environment. The seagrass beds also serve as shelter from predators for juvenile fish, e.g., rabbitfish and wrasses, and invertebrates.

By Roy T. Tsuda, PhD

For further reading

Amesbury, Steven S., and JH Francis. “The Role of Seagrass Communities in the Biology of Coral Reef Fishes: Experiments with Artificial Seagrass Beds.” Sea Grant Quarterly 10, no. 1 (1988): 1-6.

den Hartog, C. “The Sea-Grasses of the World.” Verhandelingen der Koninklijke Nederlandse, Akademie van Wetenschappen, AFD Natuurkude 59, no. 1 (1970): 1-275.

Gates, PD. “Browsing Patterns of Herbivorous Fishes in a Halodule uninervis Seagrass Bed of a Pacific Island Coral Reef (Guam, Micronesia).” MS thesis, University of Guam, 1986.

Phillips, Ronald C., and Ernani G. Menez. “Seagrasses.” Smithsonian Contributions to the Marine Sciences, no. 34 (1988): 1-104.

Tsuda, Roy T., and Nadiera Sukhraj. “Reassessment of Seagrass Species in the Marshall Islands.” Micronesica 4, (2016): 1-10.

Tsuda, Roy T., and S. Kamura. “Comparative Review on the Floristics, Phytogeography, Seasonal Aspects and Assemblage Patterns of the Seagrass Flora in Micronesia and the Ryukyu Islands.” Galaxea 9, (1990): 77–93.

Tsuda, Roy T., FR Fosberg, and MH Sachet. “Distribution of Seagrasses in Micronesia.” Micronesica 13, no. 2 (1977): 191–198.