The "twilight zone" refers to a slice of the ocean between 200 and 1,000 meters below the surface, where sunlight is nearly absent and photosynthesis does not occur. Also known as the mesopelagic zone, it is home to a massive daily migration of planktonic organisms: They ascend the water column at night to feed closer to the surface, and descend at dawn to escape predators.

Elephant seals depend on this daily dance for sustenance: The females get their much-loved lanternfish and squid from this mobile buffet. A study published on February 14 in Science suggests that they could serve as valuable scientific allies in assessing the health of this little-known ecosystem. Moreover, they might help anticipate its fluctuations in response to larger-scale oceanic oscillations – such as the El Niño-La Niña cycle in the Pacific – or to changes driven by global warming.

To establish these connections, Roxanne Beltran and her colleagues analyzed demographic data from 1982 to 2015, covering 8,485 tagged and weighed elephant seals in the Año Nuevo Reserve, California. They identified correlations between maternal weight gain, pup growth, survival rates and reproductive success throughout their lifetimes. They were able to show retrospectively that these fluctuations were linked to oceanographic indicators such as temperature and surface current oscillations: The hidden link between the two was the abundance of prey in the mesopelagic zone.

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