ANIMALS
An unusual, all-female assembly of great hammerhead sharks has been gathering in the tropical waters of French Polynesia every summer for over a decade, with numbers peaking around the full Moon. These critically endangered sharks convene between December and March around openings in two atolls, Rangiroa and Tikehau, in the Tuamotu Archipelago.
In the summers of 2020 and 2021, researchers recorded 54 female great hammerhead sharks (Sphyrna mokarran) and one whose sex could not be determined at the two atolls, which are nine miles apart. They noted that more than half of the sharks were seasonal residents, meaning they spent up to six days a month there for up to five months. Female sharks in the vicinity of Rangiroa mostly gathered in a spot called ‘hammerhead plateau’, an area 45 to 60 metres deep. “They were mainly sighted roving around the bottom of the plateau, independently from one another,” researchers wrote in the new study.
Great hammerhead sharks are a solitary species, so the high number of females sighted at the same time indicates the area is an aggregation site. The sharks likely have no relationship to each other, but are drawn there by external factors that appear to be linked to the lunar cycle and the presence of ocellated eagle rays (Aetobatus ocellatus). The number of sharks peaked in the days shortly before and after a full Moon during both summers, perhaps because the increased moonlight enhanced their ability to hunt around the atolls at night. The sharks may also have responded to changes in Earth’s geomagnetic field as the Moon waxes and wanes.
Large gatherings of grea
