Dozens Of Female Hammerhead Sharks Have Been Gathering Under A Full Moon Every Year
For more than a decade, dozens of female great hammerhead sharks have been gathering in the tropical waters of French Polynesia every summer. Their numbers peak around the time the full moon appears.
These sharks are critically endangered, and they are usually a solitary species, which makes the phenomenon even more unusual.
Each year, they assemble between December and March, during the austral summer, around openings in the neighboring Rangiroa and Tikehau atolls in the Tuamotu archipelago. The atolls are nine miles apart.
An atoll is an island or coral reef shaped like a ring that encloses a lagoon. It forms when land erodes and sinks below the ocean’s surface.
Researchers recorded 54 female great hammerhead sharks (Sphyrna mokarran), and one shark whose gender could not be determined, at the two atolls in the summers of 2020 and 2021.
More than half of the sharks were seasonal residents, so they only spent up to six days a month there for no longer than a span of five months.
Female sharks near the Rangiroa atoll mostly meet up in a spot known as the “hammerhead plateau,” an area that is 150 to 200 feet deep. They were swimming independently from each other around the bottom of the plateau.
The high number of female great hammerhead sharks appearing at the same time around the atolls indicates that the area is an aggregation site.
The sharks probably do not have a relationship to each other but are drawn there by factors that seem to be linked to the lunar cycle and the presence of ocellated eagle rays (Aetobatus ocellatus).
Janos – stock.adobe.com – illustrative purposes only
In the days shortly before and after a full moon during both summers, the number of sharks reached a peak.
This may be due to the increased moonlight, which allows the sharks to hunt around the atolls at night.
Additionally, the sharks may be influenced by changes in Earth’s geomagnetic field as the moon transitions through its phases.
The large gatherings of great hammerheads also occurred simultaneously with high numbers of ocellated eagle rays in the lagoons, where they go to reproduce. Their mating season is pretty predictable, and sharks hunt these rays, so they may be trying to take advantage of the event.
Furthermore, warmer water temperatures following the winter months may attract sharks to the Tuamotu archipelago.
The research team compared their observations with previous data collected on the atolls. They discovered that some sharks had been returning to the atolls every summer for 12 years.
They were able to identify another 30 male and female sharks from the records and found that the males typically showed up from August to October rather than the summer.
In the past, a divide between males and females had been reported for scalloped hammerhead sharks but not for great hammerheads.
The great hammerhead males might stay away from the sites that females occupy during the austral summer because of something to do with their breeding period.
It is unclear whether the atolls serve as nursery grounds for the great hammerhead sharks, but further research efforts are underway to find out if that is the case.
The findings were published in Frontiers in Marine Science.
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