These transparent eggs, known colloquially as mermaids’ purses, hold embryonic small-spotted catsharks, or Scyliorhinus canicula.
The eggs, about 2 to 3 inches long, rest in a tank at the Aquário Vasco da Gama near Lisbon, Portugal. When the fish hatch, they’ll be 3 inches long, and they will eventually grow up to about 3 feet. According to the Florida Museum of Natural History, scientists use this common species as a marine version of a lab rat to research hormones, blood and organs.
