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.  

The Perfect Witness Can't Lie. But Can They Buzz?

Insects are everywhere in our lives: visiting our flowers, poking through our trash, or hiding out in our cupboards. Most would consider them disgusting, especially the flies and maggots that hang around rotten food. Rotten food, though, has a lot in common with rotting bodies, and flies aren't picky. Investigators of crime scenes can use these scavenging bugs, then, to tell the story that a victim can't. In the field of forensic entomology, insect evidence gives valuable clues about how and where a killing happened.

Forensic entomology as a field dates back as far as forensics itself. Collected Cases of Injustice Rectified, a book from 13th century China written by judge and scholar Song Ci, is the first ever book of forensic science, detailing how to identify causes of death and perform autopsies. It is also the first known application of forensic entomology. 

In 1235, Song Ci writes, a stabbing occurred in a Chinese village. The weapon of attack was determined to be a sickle after various sharp objects were tested against an animal corpse. All the villagers were ordered to place their sickles on the ground, but only one of them attracted blow flies, which were drawn to the remnants of blood, tissue, and hair left on it. The owner of the sickle confessed to the murder and was led away ashamed.

Blowflies on some dead stuff. It's hard to find pictures when you have a massive phobia of maggots. Ew ew ew.

After nearly 8 centuries, the field has become considerably more sophisticated. Insects collected from corpses can give valuable information about both the time and location of death, and even if drugs are present in the deceased's system. 

Time of death is the most easily approximated, using what we know about insect life cycles. First on the scene, and most valuable to investigators, are flies, usually blowflies or fleshflies. Attracted by the chemicals created by decomposition, these scavengers feed near orifices such as the mouth, or open wounds, blowflies colonizing the body within a few minutes of death. 

Later on, flies lay their eggs on the body, which hatch into maggots. By identifying the species and age of the maggots, investigators can find out how long ago they hatched, and approximate a time of death that way. Other insects come along during later stages of decomposition: beetles, mites, and moths. Entomological data can be incredibly accurate: before 24 hours, it is even more accurate than soft tissue analysis in determining time of death, and after 72 hours, insect evidence is sometimes the only method of determination.

One of the major concerns of forensic entomology is finding bibles small enough to swear in the witnesses.

The insects present can also tell where the death took place with surprising specificity. For example, if flies are on a corpse that aren't native to where it was found, this is good evidence that the body was moved after death. Insects in general also like warm, moist environments. If a body is left out in the sun, then, larvae will exhibit faster development, and be more numerous. The same is true if in a humid or rainy place, allowing investigators to pinpoint which areas are likely for a murder.

Even still, insects can be drug-tested for compounds present in the corpse when a toxicology report is impossible or needs more evidence, such as when soft tissues or bodily fluids have decayed. In the emerging field of entomotoxicology, investigators can test insects to find if substances are present in their tissue. Some substances also alter the time of development: generally, cocaine and methamphetamine cause faster growth, while some poisons, such as barbiturates, slow down development.

In an age where many forensic techniques are being proved unreliable or even pseudoscientific, new approaches to convicting killers must be based on hard evidence. Bite mark analysis, bullet lead comparison, and fingerprinting, all things we think are damning evidence, have actually been shown to be highly unreliable. Forensic entomology fortunately seems less "buggy", and as the field expands, we may see more flies and beetles convicting bad guys.