Our Changing World: The dance of the lanternfish

In World War II, naval sonar operators spotted something strange: a 'false seafloor' hundreds of metres below the surface of the water, that mysteriously moved closer to the surface at night. They named it the 'deep scattering layer'.

It turned out that the scattering was due to a massive concentration of marine life. Their movement is the largest daily animal migration on earth, involving trillions of critters. Now researchers are investigating the part this commute plays in controlling the oceanic food web, and in the carbon cycle.

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Lanternfish in a landscape of fear

It's a tough life in the ocean, says Victoria University of Wellington fish ecologist Professor Jeff Shima. "It's been described as kind of a landscape of fear. Everything's trying to eat everything else and you're trying to eat things but not be eaten yourself."

His work on reef fish life cycles has led him to investigate the lanternfish family of fishes. These anchovy-sized, deep-sea fish are so-named because of their light-producing organs on their belly and sides called photophores.

With about 250 species in the family, they are found in deep water across the globe, in huge abundances. They make up around 65 percent of the deep-sea fish biomass and are a main player in the deep scattering layer.

To navigate the 'landscape of fear' they make a daily vertical migration. Each night they rise from the twilight zone of the ocean (200-1000 metres below) to feed near the surface on smaller prey like phytoplankton, zooplankton and tiny fish larvae. When the sun rises, they retreat to the depths, avoiding predators by matching their belly glow to the ambient light above, effectively making themselves invisible.

Dancing in the moonlight

While working on reef fish life cycles, Jeff was puzzled to discover that growth rates of larval fish seemed to follow a lunar cycle. Exploring this further has led him to study lanternfish, whose migration is highly sensitive to moonlight.

Passing clouds or different moon phases can influence how far lanternfish travel upward. If it's too bright, they stay deeper to avoid being seen, potentially shifting the behaviour of other species - especially the larvae of reef fish trying to return to the reef under cover of darkness.

Reef fish hatch offshore and spend their early lives in the open ocean before returning to the reefs as juveniles. These early stages are important, says Jeff. "If we can understand what's driving variation in the survival of baby fish while they're out at sea, that translates into big effects down the road in terms of the size of a fish population or the biomass of a fishery that we can harvest."

He would like to figure out whether lanternfish movement and predation, determined by moonlight levels, is influencing when larvae make their return journey.

To investigate this, PhD candidates Alisha Gill and Niamh Smith are conducting fieldwork in Moorea, French Polynesia. They use nets to capture and count overnight larval fish arrivals across the lunar cycle, while also keeping track of predator movement using sonar, moonlight levels using a sensor, and ocean conditions such as tides and wind.

The carbon question

Because of their massive numbers, lanternfish might also play a crucial role in carbon sequestration and global climate regulation.

Carbon from the atmosphere enters the ocean at the surface - including when phytoplankton photosynthesise, converting sunlight and carbon dioxide into food. By consuming carbon-rich prey at the surface and then moving downwards - where they might release waste or become prey themselves - lanternfish could help transport carbon to the ocean depths much faster than if it simply drifted downwards. Though a lot of carbon is recycled to the surface by ocean currents, if carbon-containing material reaches the seabed floor it can get buried in sediments and locked away for a long time.

However, there's a lot of uncertainty about this process. Lanternfish are fragile and hard to keep alive in labs, making direct study difficult, says Jeff. Instead, the team is analysing the fish's ear bones (otoliths), which store daily chemical records, offering clues about their movements and metabolism.

From predator-prey dynamics to climate science, the researchers hope their work will lead to better understanding of how these tiny fish are shaping our oceans.

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