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

What do insects tell us about the health of our rivers?

Nowadays, concern about the health of inland waters (rivers, lakes, etc.) is growing, mainly due to increased use (and abuse) of these for human consumption. A few years ago, an expansion of the use of biotic indices took place, which allow us to determine the health of aquatic ecosystems; these indices usually use data such as presence, absence or/and abundance of different organisms known as ‘bioindicators’, that is, species that can be used to monitor the health of an environment or ecosystem. Among these organisms, there are a lot of arthropods.

Along this article, I will briefly explain what bioindicators are, the main role of arthropods as bioindicators and also introduce some of the most used bioindication indices to monitor the quality of riverine ecosystems in the Iberian Peninsula.

What is a bioindicator?

The term ‘bioindicator’ is used to refer to those biological processes, species or/and communities of organisms which can be used to assess the quality of an ecosystem and also how this ecosystem evolves over time, which is especially useful when changes take place due to anthropogenic disturbances, such as pollution.

Thus, in accordance with the above, a bioindicator can be:

• A particular species, whose presence/absence or abundance rate informs us of the state of health of a studied ecosystem, or
• A population or a community composed of various organisms which varies functionally or structurally according to the conditions of  environment.

Example: Lecanora conizaeoides lichen is highly resistant to pollution. Its presence on the studied ecosystem, coupled with the disappearance of another lichens, is indicative of high air pollution.

Lecanora conizaeoides (Picture by James Lindsey).

What do we consider a ‘good bioindicator’?

Do all the organisms have the necessary traits to become bioindicator subjects? The answer is no. Even though there is not a bioindicator prototype (because all depends on the studied ecosystem), we can resume here some of the traits that scientists take into account to select good bioindicator organisms:

• They have to respond to disturbances that take place on their ecosystem to a greater or lesser degree. This response should be comparable to that emitted by the rest of the organisms of the same species, and this response also has to be well correlated with the studied environment disturbances.
• Their response have to be representative of all the community or population.
• They must be native of the studied ecosystem and also be ubiquitous (that is, to be present in almost all ecosystems of the same or similar characteristics).
• They have to be abundant (rare species aren’t optimum subjects).
• They must be relatively stable to moderate climate changes (i.e. a storm or a natural temperature change does not affect them more than normal).
• They should be easy to detect and, as possible, they have to be sedentary.
• They have to be well studied, both from an ecological point of view as taxonomic (to know, therefore, their tolerance to environmental disturbances).
• Finally, they should be easy to manipulate and monitor in the laboratory.

The use of bioindicators will be optimized if we use entire communities or populations instead of using a single or a couple species, because this allows us to cover a wide interval of environmental tolerances: from organisms with a narrow tolerance range (that is, stenotopic) and sensitive to pollution, to very tolerant organisms that can survive in very polluted environments.

Thus, we will be able to know if an ecosystem is highly altered if we find only a very tolerant species and none of the considered sensitive species.

Bioindicator animals from inland waters

Nowadays, scientists use a lot of animals as bioindicators: from microorganism and microinvertebrates to terrestrial and aquatic vertebrates (micromammals, birds, fishes, etc.). In inland waters, and especially in the context of studies of riverine water quality, scientists mostly use aquatic macroinvertebrates to assess the quality of these ecosystems. Next, let’s see what a macroinvertebrate is.

What are macroinvertebrates?

The term ‘macroinvertebrate’ does not correspond to any taxonomic classification, but with an artificial concept that includes different aquatic invertebrate organisms.

Generally, is said that macroinvertebrates are organisms that can be trapped by a net with holes about 250μm.

Collecting macroinvertebrates by using a kick seine (Picture by USFWS/Southeast , Creative Commons).

Macroinvertebrates are mainly benthic, that is, animals that inhabit the substrate of aquatic ecosystems at least during some stage of their life cycle (although there are some that swim freely in water column or on its surface).

We can find a lot of macroinvertebrate groups in rivers and lakes, which can be classified in two main groups:

Picture sources: (1) Luis Silva Margareto ©, (2) DPDx Image Library, (3) Oakley Originals, Creative Commons, (4) Ryan Hodnett, Creative Commons, (5) Will Thomas, Creative Commons, (6) Duncan Hull, Creative Commons.

Among these groups, there are both tolerant organisms to environment distrubances (i.e. leeches) and sensitive organisms (i.e. a lot of larvae insects).

Most inland aquatic macroinvertebrates (≃80%) are arthropods (of which I will discuss in the next section), among which there are many insects and, especially, their larvae (which are generally benthic), whose study and observation play an essential role on calculating indices of water quality.

Importance of insects in bioindication

As I’ve said above, about 80% of macroinvertebrates of inland waters are arthropods and, mostly, different orders of insects in its larval or nymphal form. Let’s see some of the most common groups we can find in rivers and lakes:

Trichoptera (or caddisflies)

They are insects closely related to the Lepidoptera order (butterflies and moths). Their aquatic nymphs can build a shelter around their bodies made of substrate materials. We can distinguish them from other aquatic insect larvae because they have a couple of anal filaments provided with strong hoofs. They usually inhabit clear and clean waters with a lot of currents.

Trichoptera nymph (inside its shelter, left) and adult (right). Picture of the nymph by Matt Reinbold (Creative Commons) and picture of the adult by Donald Hobern (Creative Commons).

Ephemeroptera (or mayflies)

One of the most ancient orders of flying insects. Their aquatic nymphs, which usually inhabit rivers, are characterized for having three long anal filaments. Adults, which fly over the water surface, are very fragile and have a short life cycle in comparison with nymphs (the name Ephemeroptera is derived from Greek ‘ephemera’ meaning sort-lived, and ‘ptera’ meaning wings).

Ephemeroptera nymph (left) and adult (right). Picture of the nymph by Keisotyo (Creative Commons) and picture of the adult by Mick Talbot (Creative Commons).

Plecoptera (or stoneflies)

Flying insects very similar to Ephemeroptera order. Like these, they have anal filaments, but they differentiate from them because they have two apical hooks in each leg. They usually inhabit lakes and streams.

Plecoptera nymph (left) and adult (right). Picture of the nymph by Böhringer (Creative Commons) and picture of the adult by gailhampshire (Creative Commons).

Other groups of insects with aquatic larvae or nymphs

Among the most common insects inhabiting rivers and lakes we can also find species of Odonata order (dragonflies and damselflies), Coleoptera (beetles), Diptera (mosquitoes and flies), etc.

Among all the organisms mentioned above, there are very tolerant species to pollution (i.e. some Diptera larvae; this is the case of some species of Chironomidae family, which are very tolerant to organic and inorganic pollution due to the presence of heavy metals in their environment) and also very sensitive species (i.e. some species of Trichoptera order).

Depending on their tolerance to environment disturbances, scientists group these organisms (plus the rest of macroinvertebrates) into different categories that are assigned a value. This values, at the end, allow us to calculate water quality indices.

Biotic indices for riverine waters

The different pollution tolerance degrees among macroinvertebrates of a community allow us to classify them and to assign them a qualitative value (the bigger the number is, more sensitive are organisms to pollution). Thanks to these values, we can calculate different biotic indices, which are no more than qualitative values assigned to a community in order to classify it according to its quality: the greater the value is, better is the water quality.

One of the most used indices on the assessment of ecological state of rivers from the Iberian Peninsula is the IBMWP index (Iberian Bio-Monitoring Working Party), an adaptation by Alba Tercedor (1988) of the British index BMWP. In rough outlines, the greater the value is, better is the water quality. On this website you will find more details about this index, and also the pre-established values assigned to each macroinvertebrate (available in Spanish only).

In additions, there is also used the IASPT index, a complementary index which is the result of divide the IBMWP value by the number of identified taxa. This index give us information about the dominant community in the studied location. You can see more details on this website (available in Spanish only).

.      .      .

As you probably have seen while reading of this article, macroinvertebrates, and insects especially, play an important role in the study of inland water quality. Furthermore, their presence or absence is extremely important for the rest of the organisms of their ecosystem, because of what we must become aware of the problems deriving from the reduction of their number or diversity.

REFERENCES

• Bioindicadors com a detectors de la contaminació ambiental. Diari Apícola Ecolluita (Associació Col·lectiu Abellaire).
• Bioindicadors. GESNA (Estudis Ambientals S.L.).
• Estudio de determinación de índices bióticos en 87 puntos de los ríos de Navarra (Informe Final, 2005). Ekolur, Asesoría Ambiental.
• Holt, E. A. & Miller, S. W. (2010) Bioindicators: Using Organisms to Measure Environmental Impacts. Nature Education Knowledge 3(10):8.
• MEDPACS (MEDiterranian Predictions And Classifictions System).
• Prat N., Ríos B., Acosta R., Rieradevall M. (2009). Los macroinvertebrados como indicadores de calidad de las aguas.
• Protocolo de cálculo del índice IBMWP. Ministerio de Agricultura, Alimentación y Medio Ambiente (Gobierno de España).
• http://www.cricyt.edu.ar/enciclopedia/terminos/Bioindic.htm
• https://ilbca.wordpress.com/bioindicadores/
• http://xtec.cat/cda-delta/pdf/rv_invertebrats.pdf

Head photography by U.S. Fish and Wildlife Service Southeast Region.

Maratus sp.: The spider that wants to be a peacock

If I told you that there exists a 5 millimeters Australian peacock, would you believe me? Although we can find a large number of incredible animals in this country, scientists haven’t yet discovered such a small bird. However, we can find a small peacock-like animals: the peacock spiders (Maratus sp. Salticidae Family, also known as jumping spiders), whose ‘abdomen’ or opisthosoma (the posterior part of the body in some arthropods, including arachnids) have a flap-like extensions that they can unfold to the sides of its body as real peacocks do.

The last month we introduce you these organisms at our different websites (Facebook and Twitter). Through this article, you will learn its most relevant characteristics and you’ll find out the hidden function of its drop down opisthosoma.

JUMPING SPIDERS

Peacock spiders are a part of Salticidae family, whose members are also known as ‘jumping spiders’. This family has up to 5000 species (probably, they form the largest and diverse group of spiders known nowadays), and they’re located all over the world (they can be found even at the top of Mount Everest; this is the case of Euophrys omnisuperstes). Even so, most of them inhabit tropical forests.

¿HOW CAN WE DISTINGISH THEM FROM OTHER SPIDERS?

MAIN TRAITS

Usually, spiders from Salticidae family get to be a size of few millimeters as adults (normally they don’t exceed 10mm long). From an anatomical point of view, the members of this group are characterized by its two big, simple front eyes flanked by two smaller ones, plus four eyes more located over them. The size and the position of these eyes give them an excellent vision in comparison with other spiders, and even compared to other group of arthropods its vision is extraordinary.

Look at these big eyes! Can you resist them?

Specimen of Paraphidippus auranticus (Picture by Thomas Shahan (c)).

Besides its excellent vision, these organisms have the ability to cover a distance of 50 times its length in one jump, because of what they received the nickname ’jumping’. Thus, their ability to travel long distances in just one leap and their extraordinary vision are the main traits that make these spiders being excellent predators: they hunt by stalking their prey without making spider webs or silk traps. Moreover, some of their legs tend to be longer than the others, letting them to catch preys way better.

Jumping spider predating a specimen of Diaea evanida or pink flower spider (Picture by James Niland on Flickr, Creative Commons).

Spiders of this family usually present a noticeable sexual dimorphism (that is, remarkable physiognomic differences between males and females). Jumping spider males have bigger oral appendixes (or pedipalps) than females, which they use during mating dance and copulation as much for attracting the attention of females as for giving females their spermatophore (capsule or mass containing spermatozoa) during mating.

Sitticus fasciger male (with dark big pedipalps) (Picture by sankax on Flickr, Creative Commons).

Sitticus fasciger female (Picture by sankax on Fickr, Creative Commons).

In addition to these developed pedipalps, males of some species of jumping spiders have a colorful, and even iridescent, opisthosoma (the posterior part of the body in some arthropods, including arachnids). Some of them even have an opisthosoma that can reflect UV radiations which are detected by females thanks to their extraordinary vision (as some studies suggest). In contrast, females use to be more cryptic or darker colored than males (but not always).

Male (right) and female (left) of Maratus mungaich. Male has a blue and red colored ‘abdomen’ or opisthosoma (Picture by Jurgen Otto on Flicker, Creative Commons).

The flashy opisthosoma of males attracts the attention of females, and this is complemented with mating dances composed by different dance moves: they vibrate their opisthosoma and their pedipalps or move their anterior legs, they move from one side to the other, etc. Usually, these mating dances tend to be very complex.

NOW, JUST AS A MATTER OF INTEREST…

There exist some jumping spider genera that mimic ants. This phenomenon is known as ‘myrmecomorphia’, term used to describe the fact that some animal imitate ants or also the organisms that accomplish the mimic (‘myrmecomorph’).

Some of the most famous species resembling to ants are inside the Myrmarachne genus.

Specimen of Myrmarachne sp. (Picture by Jean and Fred on Flickr, Creative Commons).

With this example, and what has been explained above, it becomes clear that the Salticidae family is a very diverse group at various levels.

MINIATURE PEACOCKS: THE PEACOCK SPIDERS

At the end of XIX century, the reverend and zoologist Octavius Pickard-Cambridge, who loved spiders, discovered a surprising organism: a small spider just over 5mm whose males have a flamboyant opistosoma.

Maratus volans male (Foto de Jurgen Otto a Flickr, Creative Commons).

According to this definition, there seems not be any difference between jumping spiders and these new organisms. But the true is that these miniature spiders have an ace in the hole: the opisthosoma of males has lateral and flexible flap-like extensions that they can unfold like wings to the sides of their bodies and over their prosoma (that is, the anterior part of the body in arachnids). Finally, when its function ends they can be folded again.

‘Abdomen’ or opisthosoma of a Maratus mungiach male (1,2,3-Different degrees of expansion of the lateral flaps) (Otto and Hill, 2011).

So, due to its colored and expansible opisthosoma this new spider was named ‘peacock spider’.

Probably, the most famous peacock spider is Maratus volans, one of the first species called as ‘peacock spider’. To date there have been identified up to 30 species of peacock spiders, and each presents subtle variations in the patterns of its opisthosoma. All of them are included in Maratus genus, whose members (but M. furvus) are endemic to Australia. Moreover, Jurgen Otto and David Hill have recently identified two new species of the Maratus genus in Queensland: Maratus jactatus and Maratus sceletus.

Maratus amabilis male (Picture by Jurgen Otto on Flickr, Creative Commons).

Maratus harrisi male (Picture by Jurgen Otto on Flickr, Creative Commons).

Maratus volans male (Picture by Jurgen Otto on Flickr, Creative Commons).

Although in the beginning it was been suggested that these flap-like expansions let them fly (because of what they were called ‘flying peacock spiders’ at first place), later studies revealed that these expansions have a more ‘romantic’ function. Let’s discover it in the next section.

A SURPRISING COURTSHIP

As well as male peacock do when they see a female, these Australian spiders use their opisthosoma in order to impress the females during courtship (that usually takes place on september-november).

Throughout this process, males perform a mating dance which is a little different from the ones performed by other jumping spiders: at the first place, they make vibrate their opisthosoma and their pedipalps; then, they raise their third pair of legs and shake them vigorously in order to attract nearby females; finally, they raise their opisthosoma and unfold the lateral flap-like extensions showing its colored pattern. Then, they make vibrate their opisthosoma to show its iridescence and they begin to move from one side to the other while shaking their legs.

But, as everybody knows, an image is better that any word, so I invite you to see the next video (made by Jürgen Otto) where you can see a male of Maratus volans dancing for his ladies:

As you have been able to appreciate while reading this article, two animals that belong to a not related groups (peacocks and spiders) have developed a quite similar courtship strategy. And it seems to yield good results in both cases. Nature is surprising us again!

 

REFERENCES

• Lim M.L.M, Li D. (2005). Extreme ultraviolet sexual dimorphism in jumping spiders (Araneae: Salticidae). Biological Journal of the Linnean Society 89: 397–406.
• Otto J.C., Hill D.E. (2015). Two new peacock spiders of the calcitrans group from southern Queensland (Araneae: Salticidae: Euophryinae: Maratus). Peckhamia 121.1: 1-34.
• Otto J.C., Hill D.E. (2014). Spiders of the mungaich group from Western Australia (Araneae: Salticidae: Euophryinae: Maratus), with one new species from Cape Arid. Peckhamia 112.1: 1-35.ç
• Otto J.C., Hill D.E. (2011). An illustrated review of the known peacock spiders of the genus Maratus from Australia, with description of a new species (Araneae: Salticidae: Euophryinae).
• Salticidae, Jumping spiders.
• Two New Species of Peacock Spiders Discovered in Australia. Sci-knews.com.
• The extraordinary courtship dance of Australia’s peacock spider. The Guardian.

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