There are 12 genera of seagrasses, belonging to four monocotyledonous families.
In external design, the halophytic seagrasses from different families can appear very similar, but evolution of these groups of genera has taken place several different times, quite likely early in the differentiation of monocotyledons, and likely from ancestors that inhabited brackish water of coastal salt marsh and estuary.
Some authors also list Ruppia maritima (Family Ruppiaceae) as a seagrass, whereas this species is generally not found beyond the margin of salt marsh.
Marine phanerogams have been observed along all continents except Antarctica (even though ironically one species is named Amphibolis antarctica!). Thirty of the 58 species are found around Australia, where much of the experimental work on seagrasses has been done. The largest seagrass meadow in the world occurs in the hypersaline Shark Bay of western Australia, where 12 species occur, but 85% of that coverage is by A. antarctica, which forms expansive monospecific stands. Shark Bay has very few rocky substrates, and the entire inlet is dominated by seagrasses. There are as many as 17 species in the general region of Perth, Australia in nearshore environments.
The rich assemblage of seagrasses around the continent of Australia produces a great amount of variation between localities. This differs greatly from the Neotropics, where there are only four major species, Halophila decipiens, Halodule wrightii, Thalassia testudinum, and Syringodium filiforme. Remarkably, there is only one tiny population of a seagrass, Heterozostera tasmanica, on the entire western coastline of South America, at Coquimbo, Chile. Absence of seagrass is attributed in large part to the lack of a protected, shallow, flat site with clear water.
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