Halophila decipiens

Halophila decipiens, called paddle grass, is a seagrass that grows in tropical waters across the Atlantic, Indo-Pacific, and beyond. Its oval leaves, 1 to 2.5 centimeters long, have serrated edges and are covered in tiny hairs. These leaves sprout from thin, white rhizomes—slightly thicker than the roots—that form branching networks on the seafloor. Unlike other seagrasses, Halophila species lack basal sheaths, the protective casings around the stem base.

This species is one of the few seagrasses with a truly global range, found in shallow waters worldwide. Genetic data show very little DNA variation between distant populations, suggesting its spread happened recently in geological time. How it achieved this global dispersal is unknown, though its small seeds may hitch a ride inside large drifts of algae or other vegetation. While present globally, it is most common in subtropical and tropical areas, forming meadows near reefs or on sandy bottoms. It grows in water deeper than 50 meters, except off southern Australia and southern Mozambique, where it appears near the shallow mouths of open estuaries. As a euryhaline species, it tolerates low salinity.

Halophila decipiens is monoecious, with male and female flowers on the same stem. The male flower emerges first, producing pollen without an exine. The female flower follows, sometimes developing into green fruits that hold up to 30 seeds. Flowering occurs in January, with fruiting and seed germination in late spring.

Seagrass beds provide habitat and breeding grounds for many organisms and are a key food source for sea urchins, sea turtles, parrotfish, and surgeonfish. This species is ephemeral and annual: its population dies off in late autumn and regenerates from seeds in spring. It often grows in mixed meadows with Halophila ovalis and Halophila spinulosa. In Hawaii, where it was discovered in 2000, it is classified as invasive and considered a threat to native Halophila hawaiiana. Elsewhere, its global distribution prevents an invasive label, but its easy dispersal and reproduction may allow it to outcompete other seagrasses in some areas. The dense root networks of Halophila decipiens have been shown to trap microplastics in the roots and surrounding sediment, raising the possibility of using it as a natural filter for ocean microplastic pollution.