The Revealing Nature of Eel Faces

This is one of about 200 species of moray eels that live in tropical and subtropical coral reefs.
This is one of about 200 species of moray eels that live in tropical and subtropical coral reefs. (Wolcott Henry)

Eels have an endearing smile. For Dr. Vinicius Espíndola their smile is the center of his world. Specifically, it is the fascinating muscle structure of an eel’s face that keeps him bent over a microscope for hours, meticulously outlining and noting every muscle fiber, bit of cartilage, and bone. While many scientists turn to the new, powerful, yet expensive DNA sequencing technology to determine species lineages and identify new species, Espíndola relies on an old method of species determination. He compares muscle size, location, and function to complete a similar task, a laborious process that few scientists are willing to use anymore. It may be hard work, but the study of facial structures has revealed not only new species, but provided invaluable information about eels that explains their importance within an ecosystem.

“I was always the different guy,” said Espíndola, postdoctoral fellow at the Smithsonian’s National Museum of Natural History. “I like the ugly things of nature, if you think eels are ugly. I prefer studying the complicated groups. The more complicated they get the more I’m into it.”

Eels are actually a type of fish. Together the 20 families of eels form the Anguilliform order. In total there are just over 1,000 species. All are slim with elongated bodies lacking pelvic fins, and some also lack pectoral fins. The largest is the European conger, a massive eel that can grow to over 10 feet long (3 m) and weigh over 240 pounds (110 kg). Most are also nocturnal, coming out at night to hunt under the cover of darkness.

But what makes eels truly unique is how dramatically different the face structures look and function from one species to the next. The moray eel is one of the more well-known species, but few know that it has not one jaw but a set of jaws that sit nestled within one another. Like in the movie “Alien,” the outer jaw opens to reveal a smaller, retractable set of jaws that shoot out to grab prey. The eel then uses those jaws to haul its catch into the esophagus, or stomach, where it digests the meal whole. 

Then there is the gulper eel. A deep-sea native, the gulper eel’s lower jaw is estimated to be a quarter of the length of its body, enabling the eel to expand its jaw to massive sizes. The size of its mouth is so large that it has also been called the pelican eel, in reference to the seabird. 

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It is the dramatic differences between these fish that led Espíndola to study eels in the first place. The moray and gulper eel are just two types of eels with unique physical features noticeable to the naked eye. They are also relatively well-known. Moray eels are favorite finds for scuba divers with their charismatic faces, and gulper eels even dazzle seasoned researchers on deep sea expeditions. 

In addition to the more charismatic eels, Espíndola studies eels that are extremely rare and less loved. For many of these species, information about their diet and role in an ecosystem is nearly nonexistent. Many are so rare they have been observed once or a handful of times. Some only have one collected specimen, which adds to the difficulty of their study. Because of this, Espíndola often is only allowed to dissect one side of the face, and gut content studies using clues from what the eels ate are out of the question since everything has to be preserved.

A recent project that led to the discovery of the new deep-sea eel species of Myroconger pietschi required a rescue mission to save the only existing specimen. It was collected at a depth between 650 and 985 feet (200 and 300 m) in 2001 as part of Brazil’s Revizee collection, a series of expeditions that ran from 1999 to 2003. Many ships donated their specimens to local collections, and this one was stored in a small collection in Ceara, a state in northeastern Brazil. But following the retirement of the curator-in-charge, the collection was abandoned in a basement. After it sat for 10 years in the musty space, Espíndola visited it and convinced them to donate the collection to Sao Paulo University.

“You could see scorpions underneath the jars, it was pretty scary. Mold all over the place,” said Espíndola.

a preserved eel specimen
The new deep-sea eel species of Myroconger pietschi. This specimen is the only collected specimen of the species. (Vinicius Espíndola)

Once in hand, Espíndola and colleagues compared the specimen’s facial anatomy to others in Japan, the United States, Ecuador, and France. Using dyes, Espíndola colored the facial structures to parse out the muscle from the bone and cartilage. Red colors cartilage, blue bone, and the muscles are left alone.  

“To someone who is not an expert the muscles look like a jumble,” said Espíndola. 

To the untrained eye M. pietschi looks very similar to other species in the genus. What Espíndola found was that the fish was unique when examined more closely. They only have one row of teeth, while most of the other species have two or three. Myroconger nigrodentatus is the only other species in the genus with one row of teeth, but upon comparison this species also has a significantly longer vomer plate, a pair of bones that form the roof of the mouth. It also had a much longer pectoral fin than M. pietschi.

From this information Espíndola suspects that M. pietschi is a poor swimmer that lurks between the rocks feeding on other fish, shrimp, or perhaps squid. Their sharp teeth indicate they’d be unable to grind any food with a protective shell.

Now that he has mastered the study of eel facial muscles, Espíndola is set on knowing every little detail about the entirety of their anatomy. Studying eels was Espíndola’s dream since he was a kid, but funding opportunities in his home country Brazil kept him from exploring the group until he was a PhD student. Now he is ready to dissect every part of their body.  

“I need to understand their anatomy,” said Espíndola. “I never stopped my dream of studying eels and now that I’m studying them, I want to know all there is to know about them.”

Once, the skills of dissection and comparison were perfected by every taxonomist. Now it is a methodology pursued by far fewer. As naturalists move in favor of DNA studies, Espíndola may very well become a rarity himself.

August 2021
Tags: Eels