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Eel Migration Comes to Light

Evolution News


In his new book Evolution: Still a Theory in Crisis, Discovery Institute biologist Michael Denton gives example after example of living things — microbes, animals, and plants — whose traits could not have arisen by a Darwinian process. He describes one of the most bizarre of all, a fish whose life cycle and migration has challenged biologists for over a century: the European freshwater eel. Inhabiting most of the rivers of Europe and the British Isles, the eel undergoes radical metamorphoses, transforming from transparent, flat larvae to cylindrical “glass eels” and then again to yellow eels and, finally, silver eels that migrate to the Sargasso Sea to mate.

During these transitions, digestive and sex organs move about the anatomy. Early scientists were not even sure they were the same species. Individuals can apparently switch sexes depending on the environment, spending some time as hermaphrodites. Denton tells how young Sigmund Freud became baffled trying to find the gonads on an eel. They must develop later in the life cycle, Freud concluded before he abandoned biology for his more famous work in psychology.

“Their life cycle is no less baroque than their sexual development,” Denton continues. These vulnerable prey fish manage somehow to swim thousands of kilometers from their freshwater habitats out into the salty ocean to the Sargasso Sea, where they breed. Yet to this day, “no one has observed their mating and spawning of freshwater eels in the wild.”

Denton’s readers may be gratified to hear that scientists have, for the first time, tracked the migration routes of some of the eels using geolocators. That’s bound to be interesting. You may remember the geolocators attached to the feet of Arctic terns in Illustra’s film Flight: The Genius of Birds, but how does one fasten a tracking device to an eel, the veritable icon of slipperiness? The authors of an open-access paper in Science Advances used some clever intelligent design for this challenge.

Before tagging, eels were anesthetized using metomidate [d1-1-(1-phenylethyl)-5-(metoxycarbonyl) imidazole hydrochloride] at the concentration of 40 mg/liter. Eels tagged in the Mediterranean were anesthetized with Aqui-S (Aqua-S) at a concentration of 600 mg/liter. Fish were measured and weighed, and, where possible, their fat content was measured using a Distell Fatmeter …. Surgical implantation of i-DSTs was achieved by pushing the tags through a small (~1 cm) incision into the body cavity from the ventral surface. After tag insertion, the incision was closed with two independent single sutures and dusted with Cicatrin antibiotic. [Emphasis added.]

One cannot be sure that any animal will behave normally after radical surgery like this, but the multi-national European team is pretty confident it didn’t affect the eels too much. The tags consisted of external PSATs (pop-up satellite archival tags) and DSTs (data storage tags). Though not capable of continuous data recording, they monitored position, temperature and pressure every 2 minutes, and transmitted the data to low-earth orbit Argos satellites at set intervals or if the tag became detached. Considering that many of the 707 eels tagged succumbed to predators, it’s remarkable that over 80 of them collected enough data to reconstruct their migratory paths. Here were some of the key findings:

  • The eels did not make a beeline for the Sargasso Sea. Instead, they tended to congregate near the Azores islands, and then head west. Even so, there was variability depending on the release location.

  • Not all the eels swam at the same speed. Even though tests in artificial tanks show they can swim at 0.7 body lengths per second for 173 days, individual speeds varied from 3 to 47 kilometers per day, probably because of factors like currents or predator abundances.

  • Remarkably, the eels dive deep (as deep as 1000 m) during the day, and rise to near the surface at night. This probably helps them avoid predators, but it also exposes them to radically different lighting conditions, pressures and temperatures — sometimes as low as the freezing point of water. It also adds considerable distance to their overall travel.

The authors avoided any mention of evolution. We can infer, however, that these creatures are even better designed than previously known. They don’t just drift with prevailing currents. Like sea turtles and salmon, they know where they need to go and will battle currents to get there, even if it requires taking a roundabout path.

Historically, migration routes to the spawning area have been assumed to be “as the crow flies” from escapement to the Sargasso Sea…; however, as we have shown, migration routes were not simply error-free great-circle routes (that is, the shortest possible route to the Sargasso Sea from the departure points). Instead, many of the routes were in the reverse direction of the northern part of the subtropical gyre in the North Atlantic Ocean, which is consistent with the hypothesis that eels follow olfactory cues originating in the spawning area or that eels navigate using oceanic cues imprinted or learned during the leptocephalus [juvenile] phase. However, our reconstructions showed that the routes taken by eels contain meanders or deviations from a simple point-to-point migration along the shortest possible route. These meanderings are possibly related to entrapment within eddies or navigational responses to other hydrographic or bathymetric features [exemplified by leatherback turtle behavior in contrast to the “perfect” migration assumed in modeling studies].

Surely “olfactory cues” would be diluted to nothingness so far from the destination. How do they do it? Are eels equipped with magnetosensing, like salmon, sea turtles and Monarch butterflies? The authors don’t elaborate, but eels must engage multiple sensory modalities to navigate, especially when swimming in the dark at night. And like sea turtles, their “map” is inherited at birth, so that tiny juveniles 5mm long born in the Sargasso Sea know where their native stream is in Europe, and the adults that develop from them will be able to find the Sargasso Sea years later.

These new findings pile on problems for functionalist explanations. Neo-Darwinism would predict bare-minimum adaptation sufficient for survival. Travelling 5,000 to 10,000 km in open sea, rife with predators, makes no sense in evolutionary theory. Undergoing major morphological changes makes no sense, either. Evolution is indeed still a theory in crisis.

More Crises in Darwinian Fish Stories

Elizabeth Pennisi has more bad news for evolutionists. Writing for Science, she says, “Fossil fishes challenge ‘urban legend’ of evolution.” What urban legend does she speak of? It’s a favorite myth in Darwinian theory: the idea that gene duplication frees up a spare copy to evolve new innovations. Her fish story involves the most diverse group of vertebrates in the world.

Imagine a half-ton tuna laid out on a dock next to a seahorse, a minnow, and a moray eel. That’s just a snapshot of the astonishing diversity found in the group of fishes called teleosts, or ray-finned fish, which today have 30,000 species — more than all living mammals, birds, reptiles, and amphibians combined. For more than a decade, many researchers have assumed that teleosts’ dizzying array of body types evolved because their immediate ancestor somehow duplicated its entire genome, leaving whole sets of genes free to take on other functions.

Now, an examination of the fish fossil record challenges that view. Despite duplicating their genome about 160 million years ago, teleost fish hewed to a few conventional body types for their first 150 million years. Meanwhile, the holostean fishes, a related group with genomes that never underwent a doubling, evolved a stunning diversity of body plans. The work “demonstrates beautifully how necessary it is to look at the fossil record when testing hypotheses about … largescale evolutionary changes,” says Robert Sansom, a paleontologist at the University of Manchester in the United Kingdom.

Pennisi’s lesson is clear. How many times have lazily accepted assumptions blinded scientists from the truth? You have to look at the data. You have to test theories with observations.

One can get an idea of the trajectory of a scientific paradigm by watching how rapidly anomalies accumulate. Evolution is more a theory in crisis than Denton’s first book was published in 1985. These entries show it has gained more crises since his second book was published earlier this year. Things do not look good for Darwinism. There’s an alternative position that doesn’t have these problems. Its evidence is growing year by year. It’s called intelligent design.

Photo: European eel, via Wikipedia.

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