Since the unexpected discovery of a megamouth shark (Megachasma pelagios) entangled in a parachute sea anchor off the coast of Hawaii in 1976, the scientific community has been captivated by this rather odd species.
Despite its impressive size — growing over 23 feet (7 meters) long and weighing more than 2425 pounds (1100 kilograms) — less than 300 individuals have been documented since that first encounter. Unlike the more familiar whale (Rhincodon typus) and basking sharks (Cetorhinus maximus), the megamouth remains a shadowy figure in our history books. Rarely seen… and even more rarely studied. Unlike more commonly studied sharks, the megamouth shark has not been the subject of long-term tracking studies. As a result, scientists have been left to hypothesize about their migratory patterns, seasonal habitat use, and the driving factors behind these movements based on sparse data. One study tracked a single megamouth shark off southern California for just over two days, revealing a pattern of vertical migration—rising toward the surface at night and descending into deeper waters during the day. However, whether this behavior is typical of the species or just one of several movement strategies remains unclear.
What we do know about them is pieced together from chance captures, strandings, and a few sightings. For example, scientists know megamouths roam the neritic and oceanic waters of tropical and subtropical latitudes, spanning depths from the surface down to the bathypelagic zone over 3937 ft (1200 m) deep. Their broad depth range and diet of vertically migrating zooplankton suggest that the megamouth shark may rely more on deep-sea prey than previously thought. Unique to this species is a white band of denticles across its upper jaw, which researchers believe could reflect bioluminescent light from deep-sea plankton, potentially attracting prey in the dark waters where these sharks often roam. Understanding these deep-sea behaviors is crucial, especially since the megamouth shark inhabits some of the most polluted waters on Earth, where microplastics pose a threat to other filter-feeding species.
And while the International Union for Conservation of Nature lists the megamouth shark as Least Concern, with an unknown population trend, this status offers little comfort to conservationists, given the species’ vast and largely unexplored range. With their slow reproductive rates and potentially long lifespans, any increase in mortality could have a significant impact on their populations.
Thus, a recent study set out to investigate the horizontal and vertical movements of megamouth sharks in the northwest Pacific, focusing on waters off the coast of Taiwan. By tagging adult megamouth sharks with pop-up satellite archival transmitters, researchers were able to track their movements for periods ranging from 12 to 244 days. The data revealed that megamouth sharks exhibit a pattern of normal diel vertical migration, spending most of their daylight hours in the mesopelagic zone, which lies between 656 – 3280 ft (200 – 1,000 m) deep. At night, they ascend to the epipelagic zone, closer to the surface. This behavior is consistent with the movement of their prey—zooplankton that also migrate vertically. By following this food source, megamouth sharks likely optimize their feeding efficiency while minimizing the risk of predation.
The study’s findings also revealed that megamouth sharks may exhibit behavioral thermoregulation, adjusting their depth to remain within an optimal temperature range. This is a significant insight into megamouths, as it suggests that these sharks are not merely passive drifters in the ocean currents but active participants in managing their thermal environment. Such behavior would allow them to conserve energy and maintain their physiological functions in the varying thermal conditions they encounter during their vertical migrations.
SIZUOKA, JAPAN: Children peer into a stuffed specimen of a 4.2-meter-long megamouth shark at Tokai … [+]
Yet, the most intriguing finding was their apparent fidelity to specific regions, particularly the waters east of Taiwan. One of the tagged sharks remained in this area for an extended period, suggesting that megamouth sharks might have preferred habitats or seasonal residences. However, the data also showed broader-scale movements, with one shark traveling into the South China Sea, indicating that their range can extend well beyond these core areas. The authors believe this data challenges previous assumptions that megamouth sharks are primarily nomadic, suggesting instead that they may have more intricate spatial strategies that vary with life stage, season, and possibly sex.
Despite the valuable insights gained from this study, many questions about megamouth sharks remain unanswered. For example, it is still unclear whether the observed migratory and behavioral patterns are consistent across different populations or if they vary depending on environmental conditions. There is also a considerable lack of data on juvenile megamouth sharks that leaves a significant gap in our understanding of their early life stages and how these might influence their later behavior and survival. While this study is a significant step forward, it also underscores the need for further research to fully understand the species’ role in the marine ecosystem and the potential threats it faces. Future research will need to focus on longer-term tracking and broader sampling to fully uncover the secrets of the megamouth shark’s movements and behavior in the deep ocean.
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