Sleep Learning About Axolotls, Lakes Xochimilco, & Texcoco

In this episode of the I Can’t Sleep Podcast, drift off while learning about axolotls, Lake Xochimilco, and Lake Texcoco. Why three topics this time? Well, the first two weren’t quite long enough, so here we are. Thankfully, they’re all connected—axolotls and their habitats. These fascinating creatures are even more incredible than I expected—they can regrow limbs and parts of their brain! Amazing, right?

Anyway, I know I just hyped this one up, so send me a message if you made it all the way through without falling asleep. Happy sleeping!

Transcript

Welcome to the I Can't Sleep Podcast,

Where I read random articles from across the web to bore you to sleep with my soothing voice.

I'm your host,

Benjamin Boster,

And today's episode is a collection of three Wikipedia articles around the same topic.

The first one is about axolotls,

The second is Lake Xochimilco,

And the third is Lake Texcoco.

The axolotl is a pedomorphic salamander closely related to the tiger salamander.

It is unusual among amphibians in that it reaches adulthood without undergoing metamorphosis.

Instead of taking to the land,

Adults remain aquatic and gilled.

The species was originally found in several lakes underlying what is now Mexico City,

Such as Lake Xochimilco and Lake Chalco.

These lakes were drained by Spanish settlers after the conquest of the Aztec Empire,

Leading to the destruction of much of the axolotl's natural habitat.

Axolotls are used extensively in scientific research due to their ability to regenerate limbs,

Gills,

And parts of their eyes and brains.

Notably,

Their ability to regenerate declines with age,

But it does not disappear.

Axolotls keep modestly growing throughout their life,

And some consider this trait to be a direct contributor to their regenerative abilities.

Further research has been conducted to examine their heart as a model of human single ventricle and excessive trabeculation.

Axolotls were also sold as food in Mexican markets and were a staple in the Aztec diet.

Axolotls may be confused with the larval stage of the closely related tiger salamander,

Which are widespread in much of North America and occasionally become pedomorphic or,

With mud puppies,

Fully aquatic salamanders from a different family that are not closely related to axolotl,

But bear a superficial resemblance.

A mature adult axolotl at age 18 to 27 months ranges in length from 15 to 45 cm,

Although a size close to 23 cm is most common and greater than 30 cm is rare.

Axolotls possess features typical of salamander larvae,

Including external gills and a caudal fin extending from behind the head to the vent.

External gills are usually lost when salamander species mature into adulthood,

Although the axolotl maintains this feature.

This is due to their neoteny evolution,

Where axolotls are much more aquatic than other salamander species.

Their heads are wide and their eyes are lidless.

Their limbs are underdeveloped and possess long,

Thin digits.

Three pairs of external gill stalks originate behind their heads and are used to move oxygenated water.

The external gill rami are lined with filaments to increase surface area for gas exchange.

Four gill slits lined with gill rakers are hidden underneath the external gills,

Which prevent food from entering and allow particles to filter through.

Axolotls have barely visible vestigial teeth,

Which develop during metamorphosis.

The primary method of feeding is by suction,

During which their rakers interlock to close the gill slits.

External gills are used for respiration,

Although buccal pumping,

Gulping air from the surface,

May also be used to provide oxygen to their lungs.

Buccal pumping can occur in a two-stroke manner that pumps air from the mouth to the lungs,

And with four-stroke that reverses this pathway with compression forces.

Axolotls have four pigmentation genes.

When mutated,

They create different color variants.

The normal wild-type animal is brown or tan with gold speckles and an olive undertone.

The most common mutant colors are listed below.

1.

Leucistic – pale pink with black eyes.

2.

Xanthic – gray with black eyes.

3.

Albino – pale pink or white with red eyes,

Which is more common in axolotls than in other species.

4.

Melanoid – all black or dark blue with no gold speckling or olive tone.

In addition,

There is wide individual variability in the size,

Frequency,

And intensity of the gold speckling,

And at least one variant develops a black and white piebald appearance upon reaching maturity.

Because pet breeders frequently cross the variant colors,

Double homozygous mutants are common in the pet trade,

Especially white-pink animals with pink eyes that are double homozygous mutants for both the albino and leucistic trait.

Axolotls also have some limited ability to alter their color to provide better camouflage by changing the relative size and thickness of their melanophores.

The axolotl is native only to the freshwater of Lake Xochimilco and Lake Chalco in the Valley of Mexico.

Lake Chalco no longer exists,

Having been drained as a flood control measure,

And Lake Xochimilco remains a remnant of its former self,

Existing mainly as canals.

The water temperature in Xochimilco rarely rises above 20 degrees Celsius,

Although it may fall to 6 to 7 degrees Celsius in the winter,

And perhaps lower.

Surveys in 1998,

2003,

And 2008 found 6,

000,

1,

000,

And 100 axolotls per square kilometer in its Lake Xochimilco habitat,

Respectively.

A four-month-long search in 2013,

However,

Turned up no surviving individuals in the wild.

Just a month later,

Two wild ones were spotted in a network of canals leading from Xochimilco.

The wild population has been put under heavy pressure by the growth of Mexico City.

The axolotl is currently on the International Union for Conservation of Nature's annual Red List of Threatened Species.

Axolotls are members of the tiger salamander,

Along with all other Mexican species of ambystoma.

Their habitat is like that of most neotenic species,

A high-altitude body of water surrounded by a risky terrestrial environment.

These conditions are thought to favor neoteny.

However,

A terrestrial population of Mexican tiger salamanders occupies and breeds in the axolotl's habitat.

The axolotl is carnivorous,

Consuming small prey such as mollusks,

Worms,

Insects,

Other arthropods,

And small fish in the wild.

Axolotls locate food by smell,

And will snap at any potential meal,

Sucking the food into their stomachs with vacuum force.

Today,

The axolotl is still used in research as a model organism,

And large numbers are bred in captivity.

They are especially easy to breed compared to other salamanders in their family,

Which are rarely captive bred due to their demands of terrestrial life.

One attractive feature for research is the large and easily manipulated embryo,

Which allows viewing of the full development of a vertebrate.

Axolotls are used in heart defect studies due to the presence of a mutant gene that causes heart failure in embryos.

Since the embryos survive almost to hatching with no heart function,

The defect is very observable.

The axolotl is also considered an ideal animal model for the study of neural tube closure due to the similarities between human and axolotl neural plate and tube formation.

The axolotl's neural tube,

Unlike the frog's,

Is not hidden under a layer of superficial epithelium.

There are also mutations affecting other organ systems,

Some of which are not well characterized,

And others that are.

The genetics of the color variants of the axolotl have also been widely studied.

The feature of the axolotl that attracts most attention is its healing ability.

The axolotl does not heal by scarring and is capable of the regeneration of entire lost appendages in a period of months,

And in certain cases,

More vital structures such as tail,

Limb,

Central nervous system,

And tissues of the eye and heart.

They can even restore less vital parts of their brains.

They can also readily accept transplants from other individuals,

Including eyes and parts of the brain,

Restoring these alien organs to full functionality.

In some cases,

Axolotls have been known to repair a damaged limb,

As well as regenerating an additional one,

Ending up with an extra appendage that makes them attractive to pet owners as a novelty.

In metamorphosed individuals,

However,

The ability to regenerate is greatly diminished.

The axolotl is therefore used as a model for the development of limbs in vertebrates.

There are three basic requirements for regeneration of the limb,

The wound epithelium,

Nerve signaling,

And the presence of cells from the different limb axes.

A wound epidermis is quickly formed by the cells to cover up the site of the wound.

In the following days,

The cells of the wound epidermis divide and grow quickly forming a blastema,

Which means the wound is ready to heal and undergo patterning to form the new limb.

It is believed that during limb generation,

Axolotls have a different system to regulate their internal macrophage level and suppress inflammation,

As scarring prevents proper healing and regeneration.

However,

This belief has been questioned by other studies.

The axolotl's regenerative properties leave the species as the perfect model to study the process of stem cells and its own neoteny feature.

Current research can record specific examples of these regenerative properties through tracking cell fades and behaviors,

Lineage tracing skin triploid cell graphs,

Pigmentation imaging,

Electroporation,

Tissue clearing and lineage tracing from dye labeling.

The newer technologies of germline modification and transgenesis are better suited for live imaging the regenerative process that occur for axolotls.

The 32 billion base pair long sequence of the axolotl's genome was published in 2018 and was the largest animal genome completed at the time.

It revealed species-specific genetic pathways that may be responsible for limb regeneration.

Although the axolotl genome is about 10 times as large as the human genome,

It encodes a similar number of proteins,

Namely 23,

251.

The human genome encodes about 20,

000 proteins.

The size difference is mostly explained by a large fraction of repetitive sequences,

But such repeated elements also contribute to increased median intron sizes,

Which are 13,

16,

And 25 times that observed in human,

Mouse,

And Tibetan frog,

Respectively.

Most amphibians begin their lives as aquatic animals which are unable to live on dry land,

Often being dubbed as tadpoles.

To reach adulthood,

They go through a process called metamorphosis,

In which they lose their gills and start living on land.

However,

The axolotl is unusual in that it has a lack of thyroid-stimulating hormone,

Which is needed for the thyroid to produce thyroxine in order for the axolotl to go through metamorphosis.

Therefore,

It keeps its gills and lives in water all its life,

Even after it becomes an adult and is able to reproduce.

Neotony is the term for reaching sexual maturity without undergoing metamorphosis.

The genes responsible for neotony in laboratory animals may have been identified.

However,

They are not linked in wild populations,

Suggesting artificial selection is the cause of complete neotony in laboratory and pet axolotls.

The genes responsible have been narrowed down to a small chromosomal region called MET1,

Which contains several candidate genes.

The axolotl's body has the capacity to go through metamorphosis if given the necessary hormone,

But axolotls do not produce it and must be exposed to it from an external source,

After which an axolotl undergoes an artificially induced metamorphosis and begins living on land.

In laboratory conditions,

Metamorphosis is reliably induced by administering either the thyroid hormone thyroxine or a thyroid-stimulating hormone.

The former is more commonly used.

In animals with functioning thyroid glands,

Iodine in the form of iodide is selectively gathered into the colloid of the thyroid.

Inside the colloid,

Iodide is reduced to eliminate iodine,

Which reacts with the tyrosyl residues of thyroglobulin.

Two iodinated tyrosyl residues are conjugated together.

When they are cleaved from the thyroglobulin chain,

Thyroid hormone is obtained.

In the absence of induced metamorphosis,

Larval axolotls start absorbing iodide into their thyroid glands at 30 days post-fertilization.

Larval axolotls do produce thyroid hormone from iodide,

But the amount appears highly variable.

Adult axolotls do not produce thyroid hormone unless metamorphism is triggered.

Diodotyrosine,

An analog of the iodinated thyroglobulin precursor in thyroxine biosynthesis,

Causes metamorphosis in axolotls that have their thyroids removed.

Lugol's solution,

Which contains both iodide and I2,

Triggers metamorphosis when injected.

This is because diodotyrosine and thyroxine is produced when iodine reacts with proteins other than thyroglobulin.

If given in a bath instead of injected,

Iodine has no effect on axolotls.

Iodide,

Which does not react with proteins,

Does not trigger metamorphosis.

It does speed up the rate of metamorphosis once it has been triggered by thyroid hormone extract.

An axolotl undergoing metamorphosis experiences a number of physiological changes that help them adapt to life on land.

These include increased muscle tone in limbs,

The absorption of gills and fins into the body,

The development of eyelids,

And a reduction in the skin's permeability to water,

Allowing the axolotl to stay more easily hydrated when on land.

The lungs of an axolotl,

Though present alongside gills after reaching non-metamorphosed adulthood,

Develop further during metamorphosis.

An axolotl that has gone through metamorphosis resembles an adult plateau tiger salamander,

Though the axolotl differs in its longer toes.

Six adult axolotls were shipped from Mexico City to Paris in 1863.

Unaware of their neoteny,

Auguste Dumerot was surprised when,

Instead of the axolotl,

He found in the vivarium a new species,

Similar to the salamander.

This discovery was a starting point of research about neoteny.

It is not certain that Ambostoma veloci specimens were not included in the original shipment.

Phelum Laufberger in Prague used thyroid hormone injections to induce an axolotl to grow into a terrestrial adult salamander.

The experiment was repeated by Englishman Julian Huxley,

Who was unaware the experiment had already been done using ground thyroids.

Since then,

Experiments have been done often with injections of iodine or various thyroid hormones used to induce metamorphosis.

Many other species within the axolotl's genus are also either entirely neotenic or have neotenic populations.

Sirens and necturus are other neotenic salamanders,

Although unlike axolotls they cannot be induced to metamorphose by an injection of iodine or thyroxine hormone.

Neoteny has been observed in all salamander families in which it seems to be a survival mechanism in aquatic environments only of mountain and hill with little food,

And in particular with little iodine.

In this way,

Salamanders can reproduce and survive in the form of a smaller larval stage,

Which is aquatic and requires a lower quality and quantity of food compared to the big adult,

Which is terrestrial.

If the salamander larvae ingest a sufficient amount of iodine,

They quickly begin metamorphosis and transform into bigger terrestrial adults with higher dietary requirements.

In fact,

In some high mountain lakes there live dwarf forms of salamanoids that are caused by deficiencies in food and in particular iodine,

Which causes cretinism and dwarfism due to hypothyroidism,

As it does in humans.

The species is named after the Aztec deity Xolotl,

The god of fire and lightning,

Who transformed himself into an axolotl to avoid being sacrificed by fellow gods.

They continue to play an outsized cultural role in Mexico.

Axolotl also means water monster in the Nahuatl language.

They appear in the works of Mexican muralist Diego Rivera.

In 2021,

Mexico released a new design for its 50 peso banknote,

Featuring an axolotl along with maize and chinampas on its back.

It was recognized as banknote of the year by the International Banknote Society.

HD 224693,

A star in the equatorial constellation of Cetus,

Was named axolotl in 2019.

The Pokémon Mudkip and its evolutions added in Pokémon Ruby and Sapphire 2002 take some visual inspiration from axolotls.

Additionally,

The Pokémon Wooper,

Added in Pokémon Gold,

Silver,

And Crystal 1999,

Is directly based on an axolotl.

The looks of the dragons Toothless and the Night Fury in the How to Train Your Dragon movies are based on axolotls.

They were also added to the video game Minecraft in 2020.

It is following Mojang Studios' trend of adding endangered species to the game to raise awareness.

They were also added to its spin-off Minecraft Dungeons in 2022 and are available in LEGO Minecraft.

An anthropomorphic axolotl named Axo was also added as a purchasable outfit in Fortnite Battle Royale on August 9,

2020.

Lake Xochimilco is an ancient endorheic lake located in the present-day borough of Xochimilco in southern Mexico City.

It is the last remaining habitat of the axolotl.

The lake is within the Valley of Mexico Hydrological Basin in central Mexico.

Lake Xochimilco is part of a series of historical lakes,

Which included 1.

Lake Texcoco,

Brackish,

2.

Lake Zumpango,

3.

Lake Chaltocan,

4.

Lake Chalco,

Freshwater.

Lake Xochimilco was originally a part of an even larger lake,

Lake Texcoco,

During the last glacial period.

Between 12,

000 and 6,

000 years ago,

The climate in central Mexico warmed and the snowmelt that once fed Lake Texcoco virtually disappeared.

This caused the lake to drop hundreds of feet over the next several thousand years.

By 2,

000 years ago,

Xochimilco became a bay in the southern portion of Texcoco.

Then around the 13th or 14th century,

The Aztecs built large causeways,

Effectively creating a new lake.

These lakes were the home of many Mesoamerican cultures,

Including the Teotihuacanos,

The Toltecs,

And the Aztecs.

Due to its shallow waters and the freshwater springs that line the south shore of the lake,

Lake Xochimilco was the center of Chinampa agriculture in the centuries of the pre-Columbian era.

This made the region a prime target for the expansionist Aztecs,

Who obtained control over the lake in a series of campaigns,

From circa 1432 until 1440.

After the Spanish conquered the Aztec empire in 1521,

They destroyed the dams and sluice gates.

This,

Along with a sharp decline in the native population,

Led to the near abandonment of the Chinampa Gardens.

The five lakes within the Valley of Mexico have now largely disappeared,

Drained to reduce flooding.

Only the Xochimilco Canals remain from the original Lake Xochimilco.

Chinamperros work their Chinampa Gardens between the canals.

Xochimilco Lake remnants are part of large urban parks in Mexico City,

With water-based and land recreation.

Colorful trajineros,

Rafts,

Take groups of people on the remaining canals for pleasure.

Xochimilco Ecological Park and Plant Market was established as a nature reserve and park,

And is the largest park in Mexico City after Chapultepec.

There is also the 13-hectare Xochimilco-Cuamanco Plant Market,

The largest in Latin America.

The market is on Chinampaland.

The area was declared a biological reserve by the Mexican government in 1984.

It became part of a UNESCO World Heritage Site in 1987.

Lake Xochimilco is the last remaining native habitat for the axolotl,

A species of mole salamander endemic to Mexico.

Until Lake Chalco was drained,

The species had also been present there.

Given Lake Xochimilco's present extensively compromised and reduced state,

And the accelerating impact of Mexico City's urban growth,

As of 2008,

Axolotl in the wild are listed as a critically endangered species by the IUCN.

On Saturday,

May 20,

2023,

Google honored Lake Xochimilco with a Google Doodle.

The Doodle invited players to look for axolotls in the lake and take pictures by clicking on them.

The goal of the Doodle was to photograph five types of axolotls within the duration of the game.

Lake Texcoco was a natural lake within the Anhuac or Valley of Mexico.

Lake Texcoco is best known for an island situated on the western side of the lake,

Where the Mexica built the city of Mexico Tenochtitlan,

Which would later become the capital of the Aztec Empire.

After the Spanish conquest,

Efforts to control flooding led to most of the lake being drained.

The entire lake basin is now almost completely occupied by Mexico City,

The capital of the present-day nation of Mexico.

Drainage of the lake has led to serious ecological and human consequences.

The local climate and water availability have changed considerably,

Contributing to water scarcity in the area.

Subsequent groundwater extraction leads to land subsidence under much of the city.

Native species endemic to the lake region,

Such as the axolotl,

Have become severely endangered or extinct due to ecosystem change.

After the cancellation of Mexico City Texcoco Airport,

The government initiated a major restoration project of a significant part of the lake in the form of the Lake Texcoco Ecological Park,

14,

000 hectares of public space and ecological restoration.

The Valley of Mexico is a basin with an average elevation of 2,

236 meters above mean sea level.

Located in the southern highlands of Mexico's central Altiplano.

Lake Texcoco formally extended over a large portion of the southern half of the basin,

Where it was the largest of an interconnected chain of five major and several smaller lakes.

Much of the lake was fed from groundwater aquifers.

Fresh water poured in from Lake Chalco and Xochimilco's freshwater springs and the thermal springs of Zumpango and Xaltocan,

As well as some in Texcoco itself,

Provided saline water.

During periods of high water levels,

Typically after the May to October rainy seasons,

The lakes were often joined as one body of water,

At an average elevation of 2,

242 meters above mean sea level.

In the drier winter months,

The lake system tended to separate into individual bodies of water,

A flow that was mitigated by the construction of dikes and causeways in the late post-classic period,

1200 to 1521 CE of Mesoamerican chronology.

Lake Texcoco was the lowest line of all the lakes and occupied the minimum elevation in the valley,

So that water ultimately drained towards it.

The Valley of Mexico is a closed or endorheic basin.

Because there is no outflow,

Evapotranspiration is estimated to be 72 to 79 percent of precipitation.

Reviews

I couldn’t even pronounce the subject (okay - lakes I can pronounce! 😂😂) so this was guaranteed to knock me out, which it did. Thank you!! 😁😁😁

I always thought aoxolotls were smaller than that! Anyway, I was asleep before you got to the other topics 😆 Thanks!

I don’t know anything about the lakes except they were once huge but were filled in by the Spanish - or something like that - because I fell asleep with the rare salamanders. Thanks Benjamin.

Right to sleep 😴 something about salamanders???😉 Perfect 👌

I like the combinations I don't really get to listen to them all, but I think it's a great idea.