Mother Sharks Return 'Home' To Give Birth: DNA Evidence Proves Lemon Sharks 'Remember' Their Birthplace
An intriguing behavior found in mother sharks has been discovered as a result of a two-decade long study in Bimini, a chain of islands in the westernmost district of the Bahamas. The study that began in 1995 shows that female lemon sharks born 15 years ago at this location came back 15 years later to give birth to their own young ones. This is the behavior noticed for the first time in sharks.
Lemon sharks are viviparous — meaning they give birth to live young that have developed inside the bodies of the mother shark - and give birth to litters of 4-17 pups. Their gestation period is between 10 and 12 months, and there is evidence that the female may take a year off before mating again. For a long time, experts were pretty sure that female lemon sharks return to the same nursery areas each time they give birth. But now the behavior — which mimics that of salmon and, famously, sea turtles — has been proven. The study, published online in the journal Molecular Ecology, is a collaborative effort involving scientists from The Field Museum, the Bimini Biological Field Station Foundation, the University of Miami, and Stony Brook University.
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For years, researchers had speculated upon the homing back ability of mother sharks, but this is the first definitive study that proves this link. One of the reasons it took so long is that it's especially difficult to track sharks from birth to maturity. Hundreds of student volunteers participated in the 19-year ongoing research project in the Bahamas, where "the lagoon in Bimini is almost like a lake," said Dr. Samuel Gruber, project founder and President/Director Bimini Biological Field Station Foundation. The study team captured nearly every shark born into the lagoon each year for nearly two decades to observe if the females actually came back to deliver babies.
"We used each shark's individual DNA fingerprint to construct a large family tree," said Dr. Kevin Feldheim from The Field Museum and the lead author of the study. "We found that newborn sharks captured in the mid-1990s left the safety of the islands when they were between five and eight years old. Yet, despite leaving and visiting many other islands in their travels, these sharks 'remember' where they were born after a decade of roving, and are able to find the island again when they are pregnant and ready to give birth," he went on to explain further.
The recent study on breeding grounds has important implication for conservation efforts. Sharks live a relatively long life for sea creatures. "When we tagged the first baby sharks in Bimini, Bill Clinton was President of the United States," said Dr. Chapman assistant professor at the School of Marine and Atmospheric Sciences, who started out 17 years ago as a volunteer researcher at Bimini charged with catching sharks for tagging. "When they started to mature and return to give birth, Barack Obama was President," he added.
Because they take so long to mature, sharks are extremely vulnerable to overfishing. In addition, they are considered a delicacy in many Asian cuisines, and as a consequence their numbers have diminished in recent years. The study's authors argue that it is important to preserve local nursery habitats because sharks use the same nursery areas across generations. By designating inshore marine reserves, individual nations could protect sharks of future generations. While many nations are already alert to the threats posed to sharks by unregulated fishing, the Bahamas recently enacted a law to protect sharks recently to sustain their $80 million annual shark tourism industry. The study's authors urge individual countries (and groups of nations) to follow the Bahamas' lead.
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