Number 10 Horror Frog While most frogs are known for being able
to regenerate, the horror frog is arguably in a league of its own. This creature, also
known as the Wolverine frog, has some truly outstanding adaptations. Much like its namesake
Marvel mutant, this creature has claws that project through the skin. The claws are made
out of bone, unlike true claws which are made of keratin. Each claw is sheathed and anchored,
with strands of collagen to a small bony nodule found in the tissue beyond the frog’s fingertips.
Whenever threatened or attacked, the frog will intentionally break the nodule, propelling
its sharp bones through the skin. British naturalist Gerald Durrell reportedly experienced
the defensive behavior firsthand, when he tried grabbing one of the creatures. The retraction
mechanism for the claws isn’t known but the working theory is that they retract passively,
over time, and that the damaged tissue regenerates. Number 9 Shark
Sharks have remarkable regenerative abilities to go with their reputation as fearsome predators.
They’re teeth are constantly regrown and that’s mainly because they’re developed
within a bony cavity and not actually attached to a bone. It’s believed that a shark will
lose up to 40,000 teeth throughout its lifetime. Leopard sharks, for example, constantly replace
their teeth every 9 to 12 days. While redoubtable weapons, teeth are not the only important
body parts that sharks can repair. Some species can regenerate skin and scales after they’ve
taken damage. Within a few days, mucus will start seeping into the wound thus initiating
the healing process. One study revealed an outstanding example of organ regeneration.
A shark had had two thirds of its liver removed and it took less than 24 hours for more than
half of the organ to increase in volume. Number 8 Lizard
Some lizard species will take some truly drastic steps in order to avoid predation. When caught
by a predator the lizard will shed part of its tail in order to escape. The process is
called autotomy but is commonly referred to as self-amputation. The detached tail will
often continue to wriggle, maintaining the idea of continuous struggle and giving the
lizard enough time to scurry away. Over the course of a few months, the lizard will regrow
its tail. The new appendage will generally be shorter, different in color or texture
and composed of cartilage rather than bone. Scientists from Arizona State University,
who have studied lizard regeneration, claim these creatures “share the same toolbox
of genes as humans”. They’ve successfully identified the genes responsible for regeneration,
a breakthrough that may, one day, enable the regrowth of human muscle, cartilage or even
spinal cord. Number 7 Starfish
Starfish are marine invertebrates that have a central disc and five or more limbs attached
to it. They’re opportunistic feeders that mainly prey on the invertebrates found in
the benthic zone, the region at the lowest level in any body of water. Some starfish
are counted among the world’s worst invasive species while others can bring about great
natural destruction. The crown-of-thorns starfish, for example, is a voracious consumer of coral
and has caused significant damage to the Great Barrier Reef. Most starfish can regenerate
lost arms and regrow new ones over time. In the early stages of limb loss, however, these
creatures are vulnerable to infection. Starfish can even regrow a completely new disc from
a separated limb, although some species do require at least part of the former disc to
still be attached to the arm. The limb will live off stored nutrients until a new disc
and mouth are regrown so that it can resume feeding. The division of the body doesn’t
only occur as a means of reproduction as the starfish may also actively shed a body part
in response to a predatory attack. Number 6 Axolotl
The axolotl genome, published in 2018, is ten times larger than the human genome. An
examination of it revealed specific genetic pathways, likely responsible for its outstanding
regenerative ability. It’s the main reason that this rare salamander is used by scientists
as a model organism. Axolotls can regenerate entire lost appendages within months, including
the tail and limbs or tissues of the heart and eyes. They’ve even been observed regenerating
some less vital parts of their brains. They’ll readily accept any transplants within their
species and restore organs to full working order. Their regenerative powers are so great
that, in some cases, they’ll grow an extra limb while repairing a damaged one, ending
up with an extra appendage. Number 5 Sea Cucumbers
Like starfish, sea cucumbers are also echinoderms. The differences are that they don’t have
a calcium exoskeleton and their capacity for regeneration is greater than that of others
in their phylum. Since they’re largely sedentary, they’ve developed alternative means of deterring
predators, that don’t involve much movement. In a process called evisceration, sea cucumbers
will actually jettison their organs to scare off predators. Evisceration, for some cucumber
species, may also be accompanied by the release of a toxin called holothurin. Its chemical
properties are similar to those of soap and it may kill other animals in the sea cucumber’s
proximity. The marine creature is capable of regenerating its expelled viscera within
weeks. The secret is proprietary genes, which aren’t found in other echinoderms and that
are highly expressed during regeneration.
Number 4 Mexican tetra Scientists working with Mexican tetra fish
variations might hold the key to the future regeneration of the human heart. From an evolutionary
perspective, the fish variations are fascinating. About 1.5 million years ago, floodwaters washed
tetra fish from the rivers of Northern Mexico into caves. They evolved and adapted to living
in complete darkness. Nowadays, the cave dwelling tetra variation has no eyes or pigment. The
tetra fish living in the river still have those features as well as the added ability
of regenerating their heart tissue. A 2018 study, funded by the British Heart Foundation,
looked at both tetra species hoping to better understand heart regeneration. The discovered
two genes, caveolin and lrrc10, that were more active in the river tetra fish, following
an injury to the heart. Both genes are present in humans and lrrc10 is actually associated
with a heart condition called dilated cardiomyopathy. Based on the finding, the researchers believed
that it would eventually be possible to restore damaged heart muscles by artificially modifying
how the isolated genes and others function. Number 3 Planarian Worms
The planarian is a flatworm that has earned the reputation of being “immortal under
the edge of a knife”. These creatures possess incredibly potent stem cells called neoblasts.
They have the ability to create all the necessary cell types if a piece of the planarian is
severed. As such, a complete new organism will form from as little as 1/279th of the
planarian it’s been removed from. Interestingly enough, the piece doesn’t have to be completely
severed in order for regeneration to commence. If the head of the planarian is cut in half
but each part is still attached to the body, the creature might regenerate and keep on
living with two heads. Neoblasts are truly remarkable, totipotent engines of life. Recent
work has revealed that a single neoblast can restore an entire animal that has been exposed
to radiation and rendered incapable of regeneration. Number 2 Hydra
Much like the mythological creature that inspired its name, this small, freshwater creature
has the ability of limitless regeneration. The hydra, whenever injured or severed, will
simply bud off a clone of itself. The hydrozoan’s stem cells have a capacity for indefinite
self-renewal. This would make the hydra, as first argued by Daniel Martinez in 1998, biologically
immortal. It seems that the driving factor behind the hydra’s limitless renewal is
a protein called forkhead box O, or FoxO. Current research supports Martinez’s claims
but, while scientists are optimistic about the implications for human aging, this still
has a long way to go. Aside from its remarkable regenerative ability, the hydra also has a
rather complex hunting mechanism for what’s essentially a simple nervous system. Its mouth
part is surrounded by one to twelve thin tentacles. The tentacles are covered in cnidocytes, explosive
cells that release microscopic, neurotoxin-laced harpoons into their prey. The hydra will thus
stretch its tentacles out and wrap them around aquatic invertebrates, which comprise the
bulk of its diet. Number 1 Lobster
Lobster regeneration is so advanced that they’re theoretically immortal. To understand how
that’s possible, it’s worth examining cellular aging in humans. Each strand of DNA
has a cap at the end, called a telomere, which protects the chromosome. During cell division,
the telomeres get progressively shorter until they’re worn out and can no longer do their
job properly. Cells have thus reached their Hayflick limit, the total number of times
that they can divide before division stops. This is how cellular aging takes place, which
is connected to the overall physical aging of an organism. Telomerase is an enzyme that
regenerates the protective caps so that division can occur indefinitely. It’s present in
most creatures, as an embryo develops, but is usually absent from adult stages of life.
Lobsters, however, express telomerase throughout their tissues into adulthood. They exhibit
no loss of appetite, fertility, energy, or metabolism as they get older. Size is suspected
as being the main factor preventing the lobster from achieving true immortality. The lobster’s
exoskeleton is hard and inflexible, and the creature must shed it in order to grow. Lobsters
expend a lot of energy during moulting and for older lobster the process ceases entirely.
As such, the exoskeleton either becomes deteriorated or collapses, killing them.
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