|Photographer: Ltshears (via Wikimedia Commons)|
So, how in the world did people mistake this 12-foot, 1,200 pound marine mammal for a creature that's half woman and half fish? For the most part, manatees remain underwater, and the sailors would usually only see a back and tail with no dorsal fin, which is how mermaids were often depicted. If a manatee head did surface so that a sailor could see the vaguely human-like eyes and face in the right light, it would add to the illusion. Female manatees also have two breasts, one under each armpit (in fact, the word "manatee" comes from the Carib word "manati," which means "with breasts"). One should also keep in mind that these sailors were malnourished, trapped in the poor conditions of the ship, and starving for female contact. They simply weren't thinking straight.
The manatee-mermaid association has made it into the scientific classification of these animals. They belong to the order Sirenia, which is named after the Sirens of Greek mythology (even though the Sirens were originally part woman and part bird, they later became associated with mermaids).