In November 2008, a female manta ray got stuck in a fishing net off the coast of Okinawa, Japan. The fishermen called up the local aquarium, where scientists were studying how the creatures reproduce. The researchers lucked out: The ray was pregnant. Now, by using an ultrasound machine to take a close look at her unborn offspring, the researchers have figured out how manta ray embryos get oxygen without a mammal's life-support equipment.
Like many sharks and rays, manta rays give birth to live young, but they don't have an umbilical cord or a placenta to deliver oxygen. The uterus is closed off from the outside seawater, so the embryo has to be getting oxygen somehow, but nobody knew how.
When the researchers from Okinawa Churaumi Aquarium retrieved the manta ray, they corralled her in a portable pool that was 3.7 meters across—slightly smaller than the enormous ray, who had to fold up her long fins. Then they checked to see if she was pregnant using an ultrasound machine that had been modified to keep water away from the electronics. Holding the probe on her back, aquarium veterinarian Keiichi Ueda saw what came up on the screen: an embryo in the mother's uterus. "I was very excited because we could see the baby's mouth opening and closing," he writes in an e-mail. Eight months later, the manta ray gave birth to a healthy female, 2 meters from wingtip to wingtip and weighing 50 kilograms.
Later, Taketeru Tomita, a fish biologist at Hokkaido University Museum, examined the video from that ultrasound and several preserved manta ray embryos to figure out just how the baby was getting oxygen into its mouth. He worked out that the baby ray was raising and lowering its jaw, pumping uterine fluid in through its mouth and spiracle, an opening on the back of the head. Valves keep the flow going, presumably directing it over the gills so the embryo could extract oxygen from the fluid. Tomita's ideas on manta ray breathing were based on adults, which generally run water over their gills by swimming with their mouths agape.
"I was very surprised that the manta ray baby actively pumped the liquid," he says. Other studies have suggested that embryos also drink the uterine fluid for nutrition, making the mother's watery secretions a live-in oxygenated milkshake for the growing baby. Tomita says the research will help explain how sharks and rays evolved to produce live young. Sharks and rays that develop in eggs also breathe by pumping seawater with their mouths, but as far as he knows, this is the first time that researchers have seen the behavior in a vertebrate that gives birth to live young. He plans to publish results from similar ultrasounds on two other species of sharks and rays. The study appears online today in Biology Letters.
The work is the first to show how manta ray embryos get oxygen, says John Musick, a vertebrate ecologist at the Virginia Institute of Marine Science in Gloucester Point. "To actually see these respiratory movements in these rays is really cool."