Dugongs and manatees, said to be the basis for the mermaid myths, are enormous aquatic mammals that can weigh over a tonne. Over time, their bodies have evolved to become streamlined and they have lost their hind limbs as part of this process. They have unusually dense bones that probably help them stay submerged.
Scientific name: Sirenia
The shading illustrates the diversity of this group - the darker the colour the greater the number of species. Data provided by WWF's Wildfinder.
The Sirenia (commonly referred to as sea cows) are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. Four species are living, in two families and genera. These are the dugong (one species) and manatees (three species). Sirenia also include Steller's sea cow, extinct since the 18th century, and a number of taxa known only from fossils. The order evolved during the Eocene, more than 50 million years ago.
Sirenia, commonly sirenians, are also referred to by the common name sirens, deriving from the sirens of Greek mythology. This comes from a legend about their discovery, involving lonely sailors mistaking them for mermaids.
"Sea cow" (seekoei) is also the name for a hippopotamus in Afrikaans. In Germanic languages, the word See can mean either a body of fresh or salt water, so this follows from the species inhabiting lakes in southern Africa rather than the sea itself.
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