Seagrass

Seagrasses are the only land plants—technically known as embryophytes—that live fully in the sea. Around 60 species make up this group, divided among four families: Posidoniaceae, Zosteraceae, Hydrocharitaceae, and Cymodoceaceae. All belong to the order Alismatales, within the monocotyledon clade. These plants are descendants of terrestrial ancestors that returned to the ocean between 70 and 100 million years ago.

The name “seagrass” comes from the long, narrow leaves of many species, which spread via rhizomes to form vast underwater meadows that look like grasslands. In fact, they often resemble true grasses of the family Poaceae. Like all autotrophic plants, seagrasses photosynthesize in the sunlit zone, and most grow in shallow, sheltered coastal waters anchored in sand or mud. Most species complete their entire life cycle underwater, including pollination. For a long time, scientists thought this pollination relied solely on water currents, but at least one species—*Thalassia testudinum*—uses a mixed strategy: it produces nutritious, sticky clumps of pollen that attract and cling to crustaceans (such as crabs and certain larvae) and syllid polychaete worm larvae, which then carry the pollen, much like insects do on land.

Seagrass meadows are among the most productive ecosystems on Earth. They act as major carbon sinks and provide habitat and food for a wide array of marine life, rivaling coral reefs in biodiversity.

**Overview**

Seagrasses are a paraphyletic group of marine flowering plants that evolved from land plants back to the sea in three or four separate events. A species is considered a seagrass if it meets these criteria: it lives only in estuarine or marine environments; its pollination occurs underwater with specialized pollen; its seeds are produced and dispersed underwater (by both living and non-living agents); its leaves have a reduced cuticle and lack stomata, with the epidermis serving as the main photosynthetic tissue; it anchors via rhizomes or underground stems; and its roots can survive in oxygen-poor sediment, relying on oxygen transported from leaves and rhizomes, while also playing a key role in nutrient uptake.

Seagrasses strongly shape the physical, chemical, and biological conditions of coastal waters. They provide invaluable ecosystem services, acting as breeding and nursery grounds for many organisms and supporting commercial fisheries. Yet much about their physiology remains poorly understood. In North American coastal waters, 26 seagrass species are found. Studies show that seagrass habitat is declining worldwide. Ten species—14% of all seagrasses—face an elevated risk of extinction, with three classified as endangered. This loss threatens marine biodiversity and the human communities that depend on the resources and services seagrasses provide. Protecting these underwater meadows is increasingly urgent.

**Evolution**

Seagrasses first evolved around 140 million years ago from early monocots—grass-like flowering plants whose seeds contain a single embryonic leaf. Terrestrial plants themselves arose from green algae perhaps 450 million years ago. Seagrasses then represent a return to the ocean by land plants. Between 70 and 100 million years ago, three independent seagrass lineages (Hydrocharitaceae, the Cymodoceaceae complex, and Zosteraceae) branched off from a single lineage of monocot flowering plants. Other marine colonizers—such as salt marsh plants, mangroves, and marine algae—have more diverse evolutionary histories. Despite their low species count, seagrasses have spread across the continental shelves of every continent except Antarctica.

Recent genome sequencing of *Zostera marina* and *Zostera muelleri* has shed light on how flowering plants adapt to the sea. During the evolutionary move back into the ocean, some genes were lost (for example, those for stomata) or reduced (such as those for terpenoid synthesis), while others, like those involved in sulfation, were regained. The genome also reveals that adapting to marine life required radical changes in cell wall composition, though seagrass cell walls are still not well understood. These walls appear to be complex blends of features from both land plants and marine macroalgae, incorporating new structural elements shaped by the salty environment and by different grazers and bacteria.

**Taxonomy**

Seagrasses are now considered a polyphyletic group of marine flowering plants, with about 60 species in five families (Zosteraceae, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae), all placed in the order Alismatales under the Angiosperm Phylogeny Group IV system. The genus *Ruppia*, which grows in brackish water, is not universally accepted as a true seagrass; some authors move it to Cymodoceaceae. The APG IV system and The Plant List offer further classification details.