Arxiu d'etiquetes: thermoregulation

How do fishes survive in hot and cold waters?

I have decided to talk about the methods that fishes use to regulate body temperature. So, this week I will focus on cold-blooded fishes (ectotherms or poikilotherms) and the next one, on the totally or partially warm-blooded (endotherms). 

INTRODUCTION

According to physiologist, thermal relationships between animals and their environment might be of different types:

  • Endotherms: in this case, animals warm up their body tissues by metabolic production of heat (they are popularly know as warm-blooded animals).
Los animales de sangre caliente (endotermos) mantienen la temperatura independientemente de la del ambiente (Foto de Sheppard Software).
In warm-blooded animals, body temperature stays the same when its cold or hot outside (Picture from Sheppard Software).
  • Ectotherms (or poikilotherms): in this case, environmental conditions determinate the body temperature of the animal (they are popularly known as cold-blooded animals).
Los animales de sangre fría (ectotermos o poiquilotermos) tienen la misma temperatura que el ambiente (Foto de Sheppard Software).
In cold-blooded animals, body temperature depends on whether its cold or hot outside (Picture from Sheppard Software).

Cold-blooded animals, like most of the fishes, have to have the ability to tolerate a wide range of temperatures (eurithermal organisms). The reason is that they have to function in several body temperatures in order to survive environmental temperature changes.

Another concept is termoregulation, that consists on the maintenance of body temperature relatively constant. So, independently of their endotherm or ectotherm condition, at the same time, animals can be thermoregulator or not. This thermoregulation can be explained by its behaviour (for example, avoiding specific temperatures) or by physiologic methods (in this case, they are called homeotherms).

FISHES WITH BEHAVIOURAL THERMOREGULATION

At sea, water masses present different temperatures: the shallower waters has higher temperatures than the deeper. Fishes can choose to stay in a specific water mass or another and they maintain the same temperature. This is a single example of behavioural thermoregulation.

HOW DO FISHES PREVENT TO DIE TO A EXCESS OF HEAT?

When a poikilotherm organism is under high temperatures (but not lethal) produce Heat Shock Proteins. This strategy is not exclusive of fishes. In fact, this response is produced by all the animals. Most of these proteins are synthesised just when a body temperature increase happens or by other factors. The increase of the body temperature is a risk of death because proteins get denatured and they lose their function.

So Heat Shock Protein are responsible of compensate denaturation of proteins helping them to fold again. This process spends so many energy.

WHY DO FISHES NOT FREEZE IN COLD WATERS?

If fishes didn’t have mechanisms to avoid freezing, their body fluids would freeze from -0.1 to -1.9ºC. We have to keep in mind that internal freezing of cells produce their death, what can produce the death of the animal. In any case, in a freezing process, the first that freezes are body fluids outside the cells, what suppose a less risk.

Si los peces no tuvieran mecanismos para evitar la congelación, sus líquidos corporales se congelarían a partir de los -0,1 a -1,9ºC (Foto de Kitami City).
If fishes didn’t have mechanisms to avoid freezing, their body fluids would freeze from -0,1 to -1,9ºC (Picture from Kitami City).

In general, organisms that under the risk of freezing have several methods to face this situation, like the production of antifreeze or supercooling.

PRODUCTION OF ANTIFREEZE

Antifreeze are dissolved substances present in body liquids to decrease the freezing point (temperature from which a liquid freeze).

This substances can work in two ways. On the one hand, their presence in a liquid increases the concentration of substances and this produce a decrease of freezing point, but it is not for their chemical properties.

On the other hand, these substance can present chemical attributes that produce a reduction of the freezing point. In specific, they join to ice crystals and avoid their growing. This is the case of most of Teleostei fishes.

In the case of polar fishes, despite some species maintain antifreeze during all the year, most of them produce this substances only in winter.

To give an example, winter flounder (Pleuronectes americanus) is one of the most known species that produce antifreeze. This animal has so many copies of the gene that encode the synthesis of the antifreeze protein and these are synthesised before the winter thanks to the induction by the reduction of sun light.

Lenguado de invierno (Pleuronectes americanus) (Foto de Bio Umass)
Winter flounder (Pleuronectes americanus) (Picture from Bio Umass)

SUPERCOOLING

Supercooling is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid. Aqueous solutions are progressively cooled and don’t freeze not even below its freezing point. But this is an unstable state and the supercooled solution can suddenly freeze.

Despite animals can voluntarily produce their supercooling, they can modify the probability of freeze spontaneously. To do it, they eliminate the nucleating agents of ice, substances that are the focus of the development of freezing.

Some deep fishes can swim, although the freezing point is at -1ºC, in waters of -1.9ºC.

REFERENCES

  • Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Integrated principles of Zoology. McGraw Hill (13 ed).
  • Hill, Wyse & Anderson (2006). Animal physiology. Editorial Medica Panamericana (1 ed).

Difusió-anglès

Hands-free in the Pliocene

In the previous post we discovered the anatomical changes associated with bipedalism in early hominids and the relationship of the selection of this feature with climate change. Is bipedalism a trait that makes us human? What are the advantages over other quadruped animals?

WHAT IS THE PLIOCENE?

Since the origin of our planet, geologists have divided time into different divisions of millions of years: the eons (Archean, Proterozoic and Phanerozoic), which in turn are divided into different eras. The Phanerozoic (from 542 Ma to present) is divided into three eras, from oldest to newest: Paleozoic, Mesozoic and Cenozoic. In this link you can see  the major biological milestones for each epoch.

cenozoic
Cenozoic detail. Full image

The Miocene is the time when the hominoids appear, (Proconsul is the most famous genus) and in the Pliocene appears, among others, Australopithecus. Homo sapiens do not appear until the Holocene, a blink in the planet’s history, as they say.

Usually the climate changes that have been happening throughout the history of Earth, represent extinction, diversification and new species, and so does our evolutionary branch: many authors relate climatic fluctuations with milestones of hominins. If you are interested in this interactive you can investigate this issue.

slideshow_plate_tectonics_02
Position of the continents in the Miocene after the collision between the Eurasian and Indica plates. (Photo by The Burgess Shale)

One of these climatic changes (caused by the collision of the Eurasian and Indica tectonic plates,  giving rise to the Himalayas and changing wind currents) was responsible for the disappearance of large tracts of rainforest, giving way to a landscape shrub or savanna. Hominoids who stayed in the forest, led to the current nonhuman apes, while those who occupied the savannatrees mosaic led to hominins, our lineage. What are the advantatges of bipedalism in that landscape?

ADVANTATGES OF BIPEDALISM

  • Handsfree: the two free limbs can be used to transport food and offspring. You can reach fruit trees without stepping on them and later, will allow the manipulation of tools, hunting and cultural events.
  • Less heat: without offering the entire back surface to the sun, and separating the body from the hot ground, it allows cope better with high temperatures and survive with less water.
  • More energy: walking on two legs consumes less energy than walking on four. This allow walking longer distances with less food, which is important in an environment where you have to flee or find food constantly. We have a great strength to walk or run many kilometers compared with quadrupeds.
  • Best visual field: the eyes have a higher position and can detect potential predators over shrubs or drive them away with stones if necessary. It is also easier to spot food sources.
  • Intimidating appearance: upright posture appears to increase body size and can avoid confrontations with certain predators.
  • Better communication: the insertion of the skull with the spine, leaving enough space for the vocal cords allow, over time, the appearance of articulate speech. Although other apes had the same brain capacity to talk, morphologically it is impossible because of the structure of their vocal apparatus.
Algunas ventajas del bipedismo. (Ilustración de Karen Carr Studios)
Some advantatges of bipedalism. (Illustration by Karen Carr Studios)

DISADVANTAGES OF BIPEDALISM

  • Low speed: for short distances, running on two legs is slower than four, in case of an unexpected attack by a predator, the chance to escape decreases.
  • Back pain: the stress that suffers our spine and legs throughout life due to upright posture, is the most likely cause of back pain, knees, hips and feet that suffer a large part of the world population.
  • Birth complications: our birth canal is narrower due to the structure of our pelvis, plus the large size of the skull of the young, it causes more pain and complications in human births compared to other mammalian quadrupeds.
Canal del parto en una mujer (izquierda) y una chimpancé (derecha). Foto tomada de Jose Mª Bermúdez de Castro
Birth canal in a woman (left) and a chimpanzee (right). (Photo taken of Jose Mª Bermúdez de Castro)

Thus, despite the disadvantages, in a warm environment, rather arid and with few trees for shelter from predators, who survived were bipedal hominoids. We consider our bipedalism as a trait that makes us human, as it is unique among animals: only birds are fully bipedal -like some extinguished dinosaurs, and except the penguin -with clumsy gait, their spine is not perpendicular to the ground, like ours.

REFERENCES

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