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Lancetfish are providing a unique glimpse into the ocean’s twilight zone

By Jessica Perelman, Guest Blogger When Assistant Professor at Scripps Institution of Oceanography, UCSD, Anela Choy first conducted doctoral research on the feeding habits of large midwater fish predators, it quickly became clear to her that prey items found in the stomachs of longnose lancetfish (Alepisaurus ferox) were hardly digested and fairly easy to identify....

By Jessica Perelman, Guest Blogger

When Assistant Professor at Scripps Institution of Oceanography, UCSD, Anela Choy first conducted doctoral research on the feeding habits of large midwater fish predators, it quickly became clear to her that prey items found in the stomachs of longnose lancetfish (Alepisaurus ferox) were hardly digested and fairly easy to identify. These observations inspired an innovative idea: if scientists could track the lancetfish diet over a long period of time, perhaps they could compile a time series that may help us better understand deep-living prey communities in the middle of the marine food web.

So began the lancetfish project.

The rarely sighted longnose lancetfish (Alepisaurus ferox) is distinct in appearance: an elongated body, sharp fangs, large eyes, and a tall sail fin. Photo: NOAA

Deep in the ocean, where the sun’s rays barely reach and visible light has nearly disappeared, swims a poorly known fish with a tall sail fin and sharp fangs. It can reach nearly six feet in length and is a top predator in the mesopelagic, or “twilight zone.” Most people are unlikely to ever encounter a lancetfish unless they stumble across one washed ashore. But over the past decade, lancetfish have been caught on longlines so frequently that they are now the most commonly caught species in the Hawaii longline fishery, which actually targets bigeye tuna and swordfish. Despite their abundance as incidental catch, scientists know very little about the biology and behavior of lancetfish. Nevertheless, some researchers have taken a great interest in them, and for good reason.

Adding to years of diet data already collected, the lancetfish project is now a collaboration between researchers at the University of Hawaiʻi, Scripps Institution of Oceanography at the University of California, San Diego, NOAA Pacific Islands Fisheries Science Center (PIFSC), and Stanford University. Lancetfish are collected by NOAA longline vessel observers from the Pacific Islands Regional Office (PIRO). While several large fish species appear as common by-catch in this fishery, including moonfish and escolar, the unusual digestion and high occurrence of lancetfish makes them the best candidates for our “biological samplers” of the twilight zone.

As a graduate student at the University of Hawaii, I have the chance to continue exploring the ecology of lancetfish and their prey in the central North Pacific Ocean.

A “prey platter” depicting the stomach contents of a 4.5-foot lancetfish. Clockwise from left, common prey items include: juvenile fangtooths and lancetfish, eel larvae, deep-sea hatchetfish, amphipod crustaceans, polychaete worms, heteropod molluscs, and a variety of squids and octopods. Photo: Elan Portner

What do lancetfish eat?
The small crustaceans, squid, and fish consumed by lancetfish–collectively called micronekton–comprise a poorly known community of animals in the middle of the food web that are inherently difficult to study. Micronekton are an important link between the base of the food web and top predators such as tuna and billfish, which may very well end up on your dinner plate. As they inhabit one of the largest ecosystems on the planet, many species in the twilight zone are difficult to collect with traditional sampling equipment and can easily avoid being caught by nets. Lancetfish act as scientific “nets” in that they continuously ingest and store deep-living micronekton which we can later extract from their stomachs and identify. The method also provides valuable insight into how this ocean ecosystem may be changing across time and space.

Left: Seventy-one hatchetfish were found in the stomach of a 2-foot lancetfish. “Aggregations” like this are found regularly in the diet and are often observed for other fish and crustacean prey as well. Photo: Jessica Perelman
This 5-inch cow fish is one of many perfect specimens that have been recovered from lancetfish stomachs. Although this is a coastal reef species, they likely remain in the open-ocean as juveniles before settling on reefs around the Hawaiian Islands. Photo: Jessica Perelman

Odd Meal Choices
Among the common prey items found in stomachs are deep-sea hatchetfishes, amphipods, and polychaete worms. But there are a few particularly interesting categories of prey that we frequently observe. One of these groups is other lancetfish. Yes, this species is highly cannibalistic, and stomachs are often found containing many small lancetfish simultaneously. In fact, those cannibalized lancetfish sometimes have lancetfish within their own stomachs as well. Though there does not seem to be a shortage of these predators in the central North Pacific Ocean, the consequences of their long-term removal by the fishery are uncertain.

Cannibalized juvenile lancetfish removed from the stomach of a 4.25-foot lancetfish, with even smaller juveniles emerging from the stomachs of these prey. Photo: Jessica Perelman

Another noteworthy prey item is one that is inorganic: plastic. Plastic is found in lancetfish stomachs in many different sizes, shapes, and forms. We see plastic bags, twine, fishing net, bottles, and miscellaneous debris. Objects that one would never expect to appear so far from land, such as a 3-cm LEGO® piece, are regularly recovered from stomachs with little to no degradation. Whether or not the presence of plastic in the digestive tract of lancetfish influences their health or nutritional state is not well understood, especially since the food in their stomachs is generally undigested regardless. Still, marine debris is somehow making it into deep waters and working its way into the marine food web.

Plastics and other debris are recovered from more than 25% of lancetfish stomachs, including (from left) this large white bottle, a 3-cm LEGO® piece, and various unidentifiable plastic fragments. Photos: Jessica Perelman

Thoughts from the Lab
Sifting through the contents of a fish’s last meal is a messy task, and probably not the most glamorous job. A high school student once asked during a lab demonstration, “Do you do this for a living?” To which I responded, “Yes I do! Isn’t it fascinating?” In fact, I feel pretty lucky to have this opportunity. Lancetfish and their prey are creatures that many people do not even know exist, from depths in the ocean far too deep for a human diver. The creative use of lancetfish as a sampling tool is providing unprecedented information about mesopelagic animals in an often-overlooked part of the food web. There is so much we can learn from this unique species, and every discovery is an exciting step towards understanding, appreciating, and protecting a precious ecosystem that is largely unexplored.

Keep reading here to learn more.

Live encounters with the cryptic lancetfish are very rare. This image was recorded in 2017 by a remotely operated vehicle (ROV) near the Musician Seamounts at a depth of 900 meters. Photo: NOAA Ship Okeanos Explorer

Author Bio: Jessica Perelman is pursuing her PhD in Biological Oceanography at the University of Hawai’i in collaboration with NOAA Pacific Island Fisheries Science Center (PIFSC). As a member of the university’s Deep-Sea Fish Ecology Lab, she aims to better understand the impacts of environmental change on the biology and behaviors of mesopelagic animals.

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Meet the Author

Carl Safina
Ecologist Carl Safina is author of seven books, including the best-selling “Beyond Words; What Animals Think and Feel,” and “Song for the Blue Ocean,” which was a New York Times Notable Book of the Year. His writing has won a MacArthur “genius” prize; Pew and Guggenheim Fellowships; book awards from Lannan, Orion, and the National Academies; and the John Burroughs, James Beard, and George Rabb medals. His work has been featured in The New York Times, The Los Angeles Times, The Washington Post, National Geographic, and elsewhere, and he hosted the 10-part “Saving the Ocean” on PBS. Safina is founding president of The Safina Center at Stony Brook University.