PART ONE OF AMAZING SPECIES: INDESTRUCTIBLE ORGANISMS

Hey everybody so its been a while since my last post, but I'm back now. There are millions of species on planet earth, some big,

African Elephant and Blue Whale

some small

Ladybug and Humming Bird

some fast, and some slow.

Galapagos Tortoise, Cheetah, Peregrine Falcon

For today however, lets focus on some lesser known species. These organisms may not be as flashy as others but to paraphrase a certain famous Osteichthyes, they just keep swimming (or crawling or whatever).


These are the creatures who will still be doing their thing long after all of us "higher" lifeforms have kicked the bucket. (and no I'm not talking about the cockroach)

Lungfish

Queensland Lungfish (Neoceratodus forsteri)

There are several extant orders of lungfish scattered across most of the world. Lepidosireniformes in particular are split between the families, Lepidosirenidae (South American lungfish) and Protopteridae (African lungfish). These fish are capable of some incredible feats of endurance. As they are primarily found in shallow, anoxic swamps which can experience periods of drought these fish have evolved to breathe air. Their air bladder has become essentially a "lung" and their heart is even capable of circulating oxygenated and deoxygenated blood separately to separate parts of their bodies. The South African lungfish has taken this resistance to drought to the extreme, and has been recorded as surviving 4 YEARS out of water! It achieves this miraculous feat by burrowing itself into the mud at the bottom of its swamp and wrapping itself in a cocoon of mud. It then enters a hibernation like state of suspended animation during which its metabolism is reduced to almost 1/60th of its normal rate to reduce waste production.

Clip of Lungfish from BBC Documentary

Bdelloidea

Bdelloidea

Bdelloidea is a class of microscopic, asexual organisms which inhabit a range of freshwater habitats around the world. There are more that 450 classified species of Bdelloidea which comprise a genetic lineage approximately 25 million years old. These organisms have survived that long in large part due to their remarkable durability. They, like the lungfish, are capable of surviving prolonged desiccation; in their case Bdelloideas have been shown to "resurrect" following 9 years without water. They do this by preemptively dehydrating themselves in a process known as anhydrobiosis. Bdelloideas remain in a dormant state until the surrounding conditions are favorable enough to awaken them. Not only does this state allow them to endure long droughts, the stress of desiccation and re-hydration actually improves their fitness and fecundity.

As asexual organisms, Bdelloidea are susceptible to a major evolutionary hurdle not experienced by sexually reproducing creatures. According to a hypothesis of co-evolution known as the Red Queen Hypothesis, asexual species are especially vulnerable to rapidly evolving parasites and pathogens. This is due to asexual species being unable to quickly alter their genotypes to combat rapid parasite evolution. However, Bdelloidea have several characteristics which circumvent this. Firstly they can survive desiccation which would kill most parasites and pathogens. Secondly, while in their dehydrated form they are easily carried away by winds, and can form new colonies elsewhere if favorable conditions are present. Finally Bdelloidea have been shown to utilize horizontal gene transfer upon re-hydration. This has allowed them to steal genetic material from bacteria, fungi, and plants nearby (~10% of their genome is comprised of this stolen DNA). There is no consensus at this point as to how or why Bdelloidea perform this gene theft.

Bdelloidea rotifers

One final benefit of the Bdelloideas incredible anhydrobiotic state is the remarkable defense it provides from harmful radiation. Studies performed on two species of Bdelloideas (Adineta vaga and Philodina roseola) have shown they posses an extraordinary resistance to ionizing radiation and the ability to efficiently repair double stranded DNA breaks. Researchers have suggested that these abilities are likely a result of the desiccation resistance adaptations.

Hydra

Hydra

Remember those stories you heard as a kid about how if you cut a worm in half it would grow into two worms? Pretty cool right? Unfortunately for worms it isn't true so we should probably stop telling people that. There are creatures with tremendous regenerative powers. Some sea stars can regrow their entire bodies as long as at least 1/5th of their head remains intact, and sponges/sea cucumbers/flat worms can in fact reproduce by being divided into small sections. However, the Hydra surpasses them all. This genus of tiny, freshwater creatures possess regenerative capabilities rivaling its namesake.

The Hydra fight from Disney's Hercules

While the Hydra of legend would grow two heads to replace each that was lost, the real Hydra goes even further in its ability to survive dismemberment. These organisms have the ability to completely reconstitute their bodies after being chopped, diced, sliced, and smashed into a pile of goo. The key to their incredible regeneration is their specialized "command cells". Rather than a traditional brain Hydra use these "command cells" to instruct other cells in their bodies. Following being turned into a puddle all they need to reform is for 5-20 of these "command cells" to remain intact in order for them to instruct the destroyed cells to reform into the Hydra. Technau et al. (2000) showed that this small group of cells would signal the surrounding cells to reform into a new Hydra in less than 96 hours. While that may seem like a long time remember its still faster than you could reform after being turned into hamburger.

Sound familiar? Yes. Hydras are elves.

This regenerative power of the Hydra gives rise to an additional remarkable ability. They do not age. That's right, Hydra are biologically immortal. Their stem cells have can renew themselves indefinitely. This is incredible because it means that unless some outside force kills them they will live forever.

Tardigrade

Tardigrade aka: Water Bear

While you may think you've seen some incredible, indestructible creatures so far, to quote Bachman-Turner Overdrive "You ain't seen nothing yet!". Here we come at last to the end, and what do we find here? The Tardigrades otherwise known as Water Bears. Tardigrades are an entire phylum of microscopic, eight legged, water dwelling, segmented creatures which are found in virtually every environment and habitat on the planet. From the bottom of the oceans to the tops of mountain ranges and the frozen wastelands of Antarctica to the sweltering jungles of the Amazon there is nowhere on Earth you can not escape the Tardigrades.
So far on this list we've covered organisms which can survive extraordinary periods of dehydration and being smashed into tiny pieces. We've met fish which bury themselves in mud for a decade and creatures which do not age. However, the Tardigrades put them all to shame.

Tardigrades can/are:

Perform cryptobiosis, essentially stopping their metabolism dead. Tardigrades can reduce their metabolism to below 0.01% of its normal function. Not only are they capable of basically stopping their bodies from functioning, but they can do it in multiple ways. They can undergo anhydrobiosis (caused by lack of water, like the Bdelloidea), cryobiosis (in response to reduced temperatures), osmobiosis (when their water becomes hyper-saturated with solute), or anoxybiosis (when there is a lack of oxygen). Tardigrades can survive in this state, referred to as a tun, for more than 5 years. They have also been reported to undergo chemobiosis (a response to high levels of toxins in the environment) but this has not been confirmed.

Survive a greater range of temperatures than any other creature on earth; from -272 C (almost absolute zero, ~1 K) to 151 C (-458 F - 304 F).

Withstand the vacuum of space as well as the crushing depths of the ocean floor. Some species can survive ~6000 atmospheres of pressure, which is almost six times greater than the pressure found at the bottom of the Marianas Trench.

Dehydrate from ~85% water to ~3% water body composition. They can survive in this dehydrated state for at least a decade, although there are reports of limb movement (not generally considered survival) after 120 years!

Incredibly resilient to radiation. They can survive doses of ionizing radiation nearly 1000 times the lethal dose for most other organisms. This has been attributed to their ability to dehydrate which reduces the reactants for the radiation. However, they have also been found to be remarkably resilient while hydrated, due in large part to their incredibly efficient ability to repair their DNA.

Live in the vacuum of space, unaided for up to 10 days. They are the only organism we know of that can survive the pressure, lack of oxygen, and immense radiation of space for a significant length of time.

Tardigrades, courtesy of Cosmos: A Space Odyssey

It seems that the only way to kill these unstoppable creatures is through mechanical pressure (squish them). That however becomes a little difficult as they are microscopic, and live in water. Have you ever tried stomping on something in a puddle? Unless its sitting on the bottom too dense/heavy for the water to lift its really freaking hard.

So how did Tardigrades become the unkillable monsters of the micro-world? Researchers believe that it is their ability to perform horizontal gene transfer (HGT). As we mentioned earlier with the rotifers, this is the ability to steal genes from other organisms in this case mainly bacteria. This most likely happens as a result of the Tardigrades dehydrating and re-hydrating themselves. When they dehydrate they break down their DNA, then upon re-hydration they are susceptible to sucking up foreign matter and DNA. Combine these facts with their previously stated remarkable capacity for DNA repair, and viola freaky new Frankenstein's Monster DNA. One species of Tardigrade had its genome sequenced by researchers from the University of North Carolina. They found that a full 6th of its genetic material (17.5%) was taken from other organisms mainly bacteria (Archea). They also found that the Tardigrades use this "stolen" DNA during times of stress which indicates that at least part of their legendary durability is due to their ability to absorb genetic material from others. Basically the more resilient you are to drying out, the more material you can absorb which in turn makes you more resilient to drying out, and so on etc. The researchers concluded by saying that the species they investigated, Hypsibius dujardini, is actually kind of a wuss compared with other species, and that the genomes of its hardier relatives may show even greater percentages of foreign DNA.

Cosmos: A Space Odyssey

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