Tardigrades: animals with superpowers

The smallest bears in the world have almost superhero abilities. Actually, they are not bears: water bears is the popular name of tardigrades. They are virtually indestructible invertebrates: they can survive decades without water or food, to extreme temperatures and they have even survived into outer space. Meet the animal that seems to come from another planet and learn to observe them in your home if you have a microscope.

WHAT IS A TARDIGRADE?

Water bear (Macrobiotus sapiens) in moss. Colored photo taken with a scanning electron microscope (SEM). Photo by Nicole Ottawa & Oliver Meckes

Tardigrades or water bears, are a group of invertebrates 0.05-1.5 mm long that preferably live in damp places. They are especially abundant in the film of moisture covering mosses and ferns, although there are oceanic and freshwater species, so we can consider they live anywhere in the world. Even a few meters away from you, in the gap between tile and tile. In one gram of moss they have find up to 22,000 individuals. They are found in Antarctica under layers of 5 meters of ice, in warm deserts, hot springs, in mountains 6,000 meters high and abyssal ocean depths: they are  extremophiles. It is estimated that over 1,000 species exist.

MORPHOLOGY

Its popular name refers to their appearance, and the scientific name to their slow movements. Their bodies are divided into five segments: cephalic, with its tube-shaped mouth (proboscis) with two internal stilettos and sometimes simple eyes (ommatidia) and sensory hairs, and the remaining 4 segment with a pair of legs per segment. Each leg has claws for anchoring to the ground.

Bottom view of a Tardigrade where the five segments of the body are observed. Colored photo taken with a scanning electron microscope (SEM). Photo by Eye Of Science/Science Photo Library

Tardigrade (Echiniscus sp.) In which you can see the claws. Colored photo taken with a scanning electron microscope (SEM). Photo de Eye Of Science/Science Photo Library

Look at this video of Craig Smith to see tardigrade’s movements in more detail:

FEEDING

With its mouth stilettos, tardigrades perforate plants and absorbe the products of photosynthesis, but they can also feed absorbing the cellular content of other microscopic organisms such as bacteria, algae, rotifers, nematodes… Some are predators too and can eat whole microorganisms.

Their digestive system is basically the mouth and a pharynx with powerful muscles to make sucking motions that opens directly into the intestine and anus. Some species defecate only when they shed.

Detail of the mouth of a tardigrade. Colored image of scanning electron microscope (SEM). Photo by Eye Of Science/Science Photo Library

INTERNAL ANATOMY

They have no circulatory or respiratory system: gas exchange is made directly by the body surface. They are covered by a rigid cuticle which can be of different colors and is shed as they grow. With each moult, they lose oral stilettos, to be segregated again. They are eutelic animals: to grow they only increase the size of their cells, not their number, that remains constant throughout life

REPRODUCTION

Tardigrades generally have separate sexes (are dioecious) and reproduce by eggs (are oviparous), but there are also hermaphrodites and parthenogenetic species (females reproduce without being fertilized by any male). Fertilization is external and development is direct: they don’t have larval stages.

Tardigrade egg. Colored image of scanning electron microscope (SEM). Photo by Eye of Science/Science Photo Library

TARDIGRADE’S RECORDS

The tardigrades are incredibly resilient animals that have survived the following conditions:

• Dehydration: they can survive for 30 years under laboratory conditions without a single drop of water. Some sources claim that resist up to 120 years or have been found in ice 2000 years old and have been able to revive, although it is likely to be an exaggeration.
• Extreme temperature: if you boil one tardigrade survives. If you put it to temperatures near the absolute zero (-273ºC), survives. Their survival rate ranges from -270ºC to 150ºC.
• Extreme pressure: they are capable of supporting from vacuum to 6,000 atmospheres, ie 6 times the pressure in the deepest point on Earth, the Mariana Trench (11,000 meters deep).
• Extreme radiation: tardigrades can withstand bombardment of radiation at a dose 1000 times the lethal to a human.
• Toxic substances: if they are immersed in ether or pure alcohol, survive.
• Outer space: tardigrades are the only animals that have survived into space without any protection. In 2007 the ESA (European Space Agency) within the TARDIS project (Tardigrades In Space) left tardigrades (Richtersius coronifer and Milnesium tardigradum) for 12 days on the surface of the Foton-M3 spacecraft and they survived the space travel. In 2011 NASA did the same placing them in the outside of the space shuttle Endeavour and the results were corroborated. They survived vacuum, cosmic rays and ultraviolet radiation 1,000 times higher than that of the Earth’s surface. The project Biokis (2011) of the Italian Space Agency (ASI) studied the impact of these trips at the molecular level.

HOW DO THEY DO THAT?

The tardigrades are able to withstand such extreme conditions because they enter cryptobiosis status when conditions are unfavorable. It is an extreme state of anabiosis (decreased metabolism). According to the conditions they endure, the cryptobiosis is classified as:

• Anhydrobiosis: in case of environmental dehydration, they enter a “barrel status” because adopt barrel shaping to reduce its surface and wrap in a layer of wax to prevent water loss through transpiration. To prevent cell death they synthesize trehalose, a sugar substitute for water, so body structure and cell membranes remain intact. They reduce the water content of their body to just 1% and then stop their metabolism almost completely (0.01% below normal).

  

  Tardigrade dehydrated. Photo by Photo Science Library

• Cryobiosis: in low temperatures, the water of living beings crystallizes, it breaks the structure of cells and the living being die. Tardigrades use proteins to suddenly freeze water cells as small crystals, so they can avoid breakage.
• Osmobiosis: it occurs in case of increase of the salt concentration of the environment.
• Anoxybiosis: in the absence of oxygen, they enter a state of inactivity in which leave their body fully stretched, so they need water to stay perky.

Referring to exposures to radiation, which would destroy the DNA, it has been observed that tardigrades are able to repair the damaged genetic material.

These techniques have already been imitated in fields such as medicine, preserving rat hearts to “revive” them later, and open other fields of living tissue preservation and transplantation. They also open new fields in space exploration for extraterrestrial life (Astrobiology) and even in the human exploration of space to withstand long interplanetary travel, ideas for now, closer to science fiction than reality.

ARE THEY ALIENS?

The sparse fossil record, the unclear evolutionary relatedness and great resistance, led to hypothesis speculating with the possibility that tardigrades have come from outer space. It is not a crazy idea, but highly unlikely. Panspermia is the hypothesis that life, or rather, complex organic molecules, did not originate on Earth, but travelled within meteorites in the early Solar System. Indeed, amino acids (essential molecules for life) have been found in meteorites composition, so panspermia is a hypothesis that can not be ruled out yet.

Photo by Eye Of Science/Photolife Library

But it is not the case of tardigrades: their DNA is the same as the rest of terrestrial life forms and recent phylogenetic studies relate them to onychophorans (worm-like animals), aschelminthes and arthropods. What is fascinating is that is the animal with more foreign DNA: up to 16% of its genome belongs to fungi, bacteria or archaea, obtained by a process called horizontal gene transfer. The presence of foreign genes in other animal species is usually not more than 1%. Could be this fact what has enabled them to develop this great resistance?

DO YOU WANT TO SEARCH TARDIGRADES BY YOURSELF AND OBSERVE THEM IN ACTION?

Being so common and potentially livIng almost anywhere, if you have a simple microscope,  you can search and view living tardigrades by yourself:

• Grab a piece of moss of a rock or wall, it is better if it is a little dry.
• Let it dry in the sun and clean it of dirt and other large debris.
• Put it upside down in a transparent container (such as a petri dish),  soak it with water and wait a few hours.
• Remove moss and look for tardigrades in the water container (put it on a black background for easier viewing). If lucky, with a magnifying glass you’ll see them moving.
• Take them with a pipette or dropper, place them on the slide and enjoy! You could see things like this:

REFERENCES

• Pequeños pero invencibles
• Revelan más secretos del indestructible tardígrado
• Meet the miniature water bear: nature’s ultimate survivor or an alien from another planet?
• Tardígrados, los animales que desafían toda preconcepción biológica
• Tardigrada.es
• Los tardígrados, ositos de agua
• Tiny animal survive exposure to space
• Anhydrobiosis in invertebrates
• “Alien” water bears amaze scientists
• Tardigrada (Astronoo)
• La criptobiosis en el phylum Tardigrada
• El animal “más resistente de la Tierra” se prueba en el espacio
• Cover photo by Oliver Meckes

Danger, poisonous mammals!

We usually associate snakes, spiders, jellyfish, etc. as venomous animals par excellence, but did you know that there are poisonous mammals? In this article we will discover who are they and the nature and use of their poisons.

THE PLATYPUS

The platypus (Ornithorhynchus anatinus) is the most famous among the poisonous mammals, and not just for this feature. With a peak like a duck and oviparous (laying eggs), when it was discovered some scientists thought it was a fraud.

Platypus (Ornithorhynchus anatinus). Photo by Jonathan Munro

They belong to the order monotremes, which means “one hole” in reference to the cloaca, the end of the digestive and reproductive systems. Some evolutionary biologists refer to them as the “missing link” between reptiles and mammals, as they have characteristics of both groups. Monotremes are the only mammals that lay eggs, but his body is covered with hair and the young are fed with breast milk. They are distributed by Australia, Tasmania and New Guinea.

Platypuses have a spur on the hind legs, which only in the case of males, release poison produced by femoral glands (located in the leg). The male uses it mainly to defend their territory and establish their dominance during the mating season, although if it is bothered also uses it as a defense. This poison can kill small animals, including dogs, and cause severe pain and swelling in humans. This pain can last days or months.

Spur on the hind leg of a platypus. Photo by E. Lonnon

Toxins are four proteins, three of which are unique to the platypus. They are like the defensins (DLP, defensin-like proteins). These are globular proteins, small and compacted, involved in the activation of pain receptors. Understanding how these toxins act it has special interest because they cause a lasting and severe pain; it may open new chances in the synthesis of analgesic drugs.

Short-beake echidna (Tachyglossus aculeatus). Photo de Tony Britt-Lewis

Echidnas (family Tachyglossidae) complete the order of monotremes with the platypus; consequently they are also oviparous. The family consists of four species, with the common characteristic of having the body covered with dense hair and spines. They are mainly insectivores specializing in ants and termites.

Like the platypus, they also have spurs behind the knees, but their secretions are not poisonous. The substances are used to mark their territory, according to the recent studies.

SLOW LORIS

As we saw in a previous post, lorises are primates in the prosimians suborder. They are nocturnal, arboreal and feed primarily on insects, vegetables and fruits. The slow lorises (Nycticebus) living in Southeast Asia, are the only poisonous primate. They possess poison glands on the elbows (brachial gland), and poison their body with arms and tongue, which can also join saliva and be transmitted by bitting.

Pygmy slow loris (Nycticebus pigmaeus). Photo by Ch’ien C. Lee

In this case the poison is used as a defense against predators, causing them pain, inflammation, necrosis (cell death) in the area of the bite, hematuria (blood in urine) or in some cases anaphylactic shock (allergic reaction) which can lead to death, even in humans (some are threatened by the illegal pet trade and traditional Chinese medicine). The poison also serves as protection for the young, they are licked by their parents and the poisonous secretion is distributed throughout the coat. Being poisonous, unusual among primates, can help counteract the disadvantages of its slow movements. Exudate from glands, as in echidnas, can also give olfactory information of range and territory between individuals of loris (Hagey et al., 2007).

Kayan loris (Nycticebus kayan). Photo by Ch’ien C. Lee

Toxins are polypeptides (generated when glandular secretion is mixed with saliva) and an unidentified steroid. Secretion is similar to the allergen Fel d 1 which is in the domestic cat and cause allergies in humans (Hagey et al., 2006; Krane et al., 2003).

It is believed that slow lorises even have converged evolutionarily with cobras, for his defensive behavior when threatened, whistling and raising his arms around his head. (Nekaris et. al, 2003).

Mimicry between loris and cobras. 1. Javan slow loris, 2 y 3. Spectacled cobra, 4. Bengal slow loris. Photo by Nekaris et. al.

In the following video a lazy lori is disturbed and hisses like a snake while trying to bite:

SOLENODON OR ALMIQUI

They are small and nocturnal mammals, basically insectivores, that live in the West Indies. The Hispaniolan solenodon (Solenodon paradoxus), also known as the Dominican solenodon, Haitian solenodon or agouta, lives on the island de La Española (Dominican Republic and Haiti) while The Cuban solenodon or almiqui (Solenodon cubanus) is distributed throughout Cuba. They are considered living fossils because they have similar characteristics to primitive mammals of the end of the Mesozoic Era (kingdom of the dinosaurs).

Hispaniolan solenodon (Solenodon paradoxus). Photo by Eladio M. Fernández.

Unlike other poisonous mammals, toxic saliva is produced under the jaw (submandibular glands), which is transported by pipes to the front of the mouth. The second incisor teeth have a groove where toxic saliva accumulates to promote their entry into the wounds. They are the only mammals that inject venom through its teeth, similar to the way snakes do.

Paradoxus Solenodon lower jaw incisor showing the groove. Photo by Phil Myers

The main function of this venom is to immobilize prey, as well as insects they can hunt small vertebrates such as reptiles, amphibians and birds.

Cuban solenodon (Solenodon cubanus). Photo by Julio Genaro.

This poison may have been developed to keep alive but immobilized prey during times of shortage, to aid in digestion, minimize energy expenditure in the struggle for hunting and face prey even twice as big as them. This venom is not deadly to humans.

SHREWS

The northern short-tailed shrew (Blarina brevicauda), the Eurasian water shrew (Neomys fodiens) and the Mediterranean water shrew (Neomys anomalus) also have submandibular glands similar to solenodons. They are distributed by North America (northern short-tailed shrew) and Europe and Asia (water shrews), including the Iberian Peninsula.

The northern short-tailed shrew (Blarina brevicauda). Photo by Gilles Gonthier.

The short-tailed shrew can consume up to three times its weight in food per day. Their saliva is the most poisonous and uses it to paralyze their prey, to eat them or keep them alive in times of shortage. The water shrews also store its immobilized prey under rocks.

Mediterranean water shrew (Neomys anomalus). Photo by Rollin Verlinde.

These animals attack from behind and bite the neck of its prey so that the poison acts more quickly, affecting the central nervous system (neurotoxins). The respiratory and vascular system is also affected and causes seizures, incoordination, paralysis and even death of small vertebrates.

Eurasian water shrew (Neomys fodiens). Photo by R. Altenkamp.

Its teeth don’t have grooves as the solenodons do, but a concave surface to store the toxic saliva.

Lower jaw of Neomys anomalus. Photo by António Pena.

It is suspected that other mammals also produce toxic saliva similarly, as the European mole (Talpa europaea) and other species of shrew, but there are no conclusive studies.

MANED RAT

The maned rat or crested rat (Lophiomys imhausi), lives in Africa and  uses his poisoned hair to protect themself from predators.

Maned rat (Lophiomys imhausi). Photo by Kevin Deacon

Unlike other mammals that produce their own poison, the crested rat gets toxin (called ouabain) from the bark and roots of a tree (Acokanthera schimperi). Chews the bark and the mixture of saliva and toxins are distributed on the body. Their hairs are cylindrical whith a perforated microscopic structure, which favors the absorption of venom. In case of danger, it bristles and shows his brown coat with white stripes, warning of its potential danger. This strategy of persuasion based on brightly colored warning is known as aposematism present in many animals, such as bees.

In this BBC video you can see a crested rat and a hair under the microscope absorbing ink, showing its porous structure:

It is unknown how it is immune to the toxin, since it is the same substance used by some African tribes for hunting such large animals like elephants.

Ouabain is a glycoside which controls the heartbeat, causing infarcts if absorbed in large quantities. The study of the mechanisms that protect the crested rat of a substance that regulates the heartbeat, can help develop treatments for heart problems.

European hedgehogs (Erinaceus europaeus) have similar behavior (smearing the body with foreign poison), but it is not established whether the objective is defensive because it does not scare away predators.

In conclusion, strategies, practices and nature of the poison in mammals are varied and their study may have important medical implications for drug development and increase awareness of the evolutionary relationships between different groups of living animals (reptiles-mammals) and their ancestors.

REFERENCIAS

• Platypus poison
• Scientists discover the function of the echidna’s spur
• Los primates venenosos existen
• El único primate venenoso del mundo ya no está solo
• “Venom” of the slow loris: sequence similarity
  of prosimian skin gland protein and Fel d 1 cat allergen
• Venomous Slow Loris May Have Evolved To Mimic Cobras
• La rata que se unta con veneno para defenderse
• ¿Es cierto que los erizos se embadurnan de veneno?
• Pequeños pero feroces: mamíferos venenosos
• Are slow lorises really venomous?
• Encuentro con un “fósil viviente” en R. Dominicana
• Imagen de portada de Dave Watts