Hammerhead Sharks Unite Under the Full Moon: In a captivating phenomenon, female great hammerhead sharks have been discovered congregating in large numbers under the enchanting glow of the full moon.
These awe-inspiring sharks were observed forming clusters in the tropical waters of French Polynesia during the Southern Hemisphere summer months, spanning from December to March. Notably, their numbers reached a crescendo during the full moon phase.
Curiously, the gatherings appeared to exclude male counterparts, as indicated by a recent study published in the journal Frontiers in Marine Science. The research team documented an unprecedented gathering of 55 critically endangered great hammerheads at two Pacific atolls in 2020 and 2021. Among these magnificent creatures, 54 were confirmed as female, while the gender of one remained undetermined. This social behavior is remarkable, considering that great hammerheads are typically solitary beings, seldom interacting with their own kind.
The study, employing non-invasive techniques in an untouched ecosystem, offers an invaluable opportunity to investigate the life history of great hammerhead shark populations in the Central Pacific region, as affirmed by the authors.
Great hammerheads, the largest among the nine hammerhead shark species, can reach an average length of 13 feet. These sharks predominantly lead solitary lives, embarking on extensive migrations through tropical waters. Classified as critically endangered on the IUCN Red List, they face significant threats, primarily stemming from overfishing.
The recorded gatherings took place at two sites: Rangiroa atoll and Tikehau atoll, both part of the Tuamotu archipelago in French Polynesia. Many of these sharks spent up to six days each month at these locations during the summer season, reaching their zenith before, during, and after the full moon. Researchers propose that the enhanced moonlight enables the sharks to optimize their hunting activities around the atolls, which consist of ring-shaped islands or coral reefs.
Furthermore, the scientists suggest that these gatherings may be influenced by changes in the Earth’s geomagnetic field during different lunar phases, or they may align with higher populations of one of their prey species—the ocellate eagle ray.
“The gathering of A. ocellatus is thus a predictable event that sharks could try to intercept, indicating that predation is a likely driving factor of the seasonal presence of great hammerhead sharks,” the authors stated in their study.
While gender segregation during these gatherings has not been observed in great hammerheads before, it has been documented in scalloped hammerhead sharks. The absence of males during this period may hint at a connection between these gatherings and the sharks’ mating cycle.
“Lagoons and their protected warm-shallow-coastal waters are known to serve as nursery areas for various shark species,” the researchers emphasized.
Although the exact purpose of these gatherings remains unconfirmed, these findings shed light on a crucial habitat for these critically endangered creatures, emphasizing the need for conservation efforts to ensure their survival and prosperity. The study highlights the significance of these atolls within the Tuamotu archipelago, where great hammerhead shark sightings—inside or outside lagoons—have been reported in at least 18 out of the 80+ atolls that make up.