Where Do (Eel) Babies Come From?

    Animal migrations are nothing sort of fascinating: the Arctic tern flies from the Arctic circle to the Antarctic, monarch butterflies travel the entire East Coast to end up in Mexico. Eels take a long journey, too: from the shores of Europe all the way to the Sargasso Sea, where they mate and give rise to their flat, clear-as-glass larvae. Or so we think. The truth is that no one has actually seen an eel mate in the wild.

    Even from ancient times the origins of eels have puzzled us; where and how they reproduce is still unknown. Aristotle thought that they wriggled up from the soil when it rained. In ancient Egypt it was believed they spawned from sunlight soaking the shores of the Nile. Freud dissected hundreds of eels trying to discover their sex organs; he was unsuccessful and refused to acknowledge the research. 

Maybe they're just a little shy.

    Today, we know that eels, specifically the European eel Anguilla anguilla, follow a predictable life pattern. They begin as flat, clear larvae, and mature into "glass eels", where they ride the Jet Stream to the coast of Europe. Once there, they migrate up the tiniest rivers and creeks, and then pull themselves onto land to colonize ponds and lakes. An eel on land can receive up to 50% of its oxygen from the air, making this journey less difficult than it sounds. Over the course of a decade, these young eels, or elvers, mature into the adult silver eels, and this is where their mating mystery begins.

Eel life cycle. Leptocephalus larvae were once thought to be a completely separate species.

    In the 20th century, Danish biologist Johannes Schmidt set out to track down the eel's origins. By capturing juvenile eels, he compared their sizes to see where the smallest and youngest eels were coming from, eventually pinpointing them to the Sargasso, that calm, shoreless sea near the Caribbean. But, though we have an idea where they spawn, no one has actually seen them do it. Eels have successfully reproduced in captivity, and, unlike Freud, we have located their sex organs. But the finer points are missing: no eggs or young larvae have ever been found.

    A new hypothesis, published in 2020, aims to narrow down the area that European eels may be spawning in. With the Sargasso stretching 2 million square miles, the chances of two hot single eels meeting in the same area is slim (barring some kind of aquatic Tinder). Using virtual eel simulations based on earlier observations of the migrations, researchers proposed a new spawning hotspot along the mid-Atlantic ridge; more precisely, where this oceanic mountain range hits a salinity front, an area near the Azores islands, outside of the Sargasso. The cues of both the change in terrain and salinity could make this location much easier to find for mating eels. The Japanese eel, a related species, is known reliably to spawn at the intersection of a seamount ridge and a salinity front, adding credence to this theory. Manganese, a trace element associated with underwater volcanoes, was also found in high concentration in the bodies of European eels, but not in Sargasso sea larvae. 

"Green and magenta circles indicate areas where American and European eels have been observed, respectively8, and the white dotted box indicates the examined area for the numerical experiment."

    While eels were plentiful throughout Europe in the past, being a cheap food enjoyed by common people, today their populations have declined up to 90%, largely due to overfishing, parasites, and habitat loss. While it also solves a long-standing scientific mystery, discovering the mating habits of eels could also be a step towards keeping them alive.

Eels were once so common in Britain they were cooked in stock and allowed to gelatinize, creating the dish of jellied eels, popular throughout London among the poor. Or, why British people should never be allowed around food.  

Ultrasound Could Save an Endangered Sea Snail

Ultrasound is one of the safest, easiest, and most useful methods of medical imaging available today. Ultrasound probes emit sound at a higher frequency than you can hear, which echoes off of tissues inside the body, returning to the probe to create an image. It's not unlike a bat's echolocation. Most people associate the technology with pregnancy. However, scientists at UC Davis are taking ultrasound out of the doctor's office and into the ocean to help recover one of our critically endangered sea creatures, the abalone.

Black Abalone, a critically endangered species on the IUCN Red List

Abalone make up several species of flat, spiral-shaped sea snails that live stuck to rocks in the shallow ocean, feeding on algae. They're prized for the iridescent mother-of-pearl that lines the insides of their shells, which is harvested to make jewelry or other decorations, and for their meat, which is a delicacy the world over. They're also major players in marine ecosystems, being food sources for marine mammals and helping to maintain kelp forests and reefs.

In recent years, though, their numbers have dwindled as a result of overfishing and ocean acidification, which erodes their shells with low pH. To aid in their survival and sustainability, scientists and farmers raise abalone in captivity, as in UC Davis's white abalone captive breeding program. However, telling when an abalone is about to spawn is difficult without being able to look inside at its gonads. And that's exactly what the researchers did.

To study an abalone requires prying it off of the rock it sticks to, a stressful experience that can harm the animal. Ultrasound technology, however, is far less invasive. To give an abalone an ultrasound, though, there's no jelly or gender reveal. The abalone is submerged in a tank, and the ultrasound wand is pressed to the outside of the tank, next to the abalone's foot. On a computer, the abalone's gonads show up as a thick dark band. The thicker the band, the more ready the abalone is to spawn. Identifying which snails are ready to spawn is useful for both abalone farmers and conservation experts to know which are going to be best for reproducing. For all the hardships abalone have faced, this is a promising step for reestablishing the creatures throughout the oceans.

Here's an interesting video of an ultrasound being given to an abalone.