New York Times
"Green spoon worms.
This animal, which lives on the sea floor, has one of the largest known size differences between male and female: the male is 200,000 times smaller. He spends his whole life in her reproductive tract, fertilizing eggs by regurgitating sperm through his mouth. He's so different from his mate that when he was first discovered by science, he was not recognized as being a green spoon worm; instead, he was thought to be a parasite.
...
The magnificent difference between male and female green spoon worms has nothing to do with their having different genes: each green spoon worm could go either way. Which sex it becomes depends on whether it meets a female during its first three weeks of life. If it meets a female, it becomes male and prepares to regurgitate; if it doesn't it becomes female and settles into a crack on the sea floor."
This animal, which lives on the sea floor, has one of the largest known size differences between male and female: the male is 200,000 times smaller. He spends his whole life in her reproductive tract, fertilizing eggs by regurgitating sperm through his mouth. He's so different from his mate that when he was first discovered by science, he was not recognized as being a green spoon worm; instead, he was thought to be a parasite.
...
The magnificent difference between male and female green spoon worms has nothing to do with their having different genes: each green spoon worm could go either way. Which sex it becomes depends on whether it meets a female during its first three weeks of life. If it meets a female, it becomes male and prepares to regurgitate; if it doesn't it becomes female and settles into a crack on the sea floor."