IL MIMETISMO: L'ARTE DI APPARIRE DIVERSI DA QUELLO CHE SI E'

 

Il mimetismo: l’arte di apparire diversi da quello che si è.

 

 Molti animali sono diventati abilissimi nel mimetismo criptico cioè nella capacità di confondersi con  l’ambiente in cui vivono.

Il mimetismo criptico può essere difensivo od offensivo

 

 Quello difensivo rappresenta probabilmente un grosso vantaggio per evitare la predazione infatti, come potete vedere  dalle foto, rende gli organismi che lo praticano estremamente poco visibili.

                                                       INSETTO FOGLIA SECCA

                                       INSETTO STECCO

                                                     PESCE SUL FONDO MARINO

 

   Mimetismo criptico  offensivo.

 In questo caso è il predatore che si nasconde, usando colori, forme e comportamenti che lo fanno diventare invisibile, permettendogli così di avvicinarsi alla preda senza essere notato come questo gufo!!!.

 

 

Molte volte il mimetismo criptico somma le due finalità. 

E' il caso dell'ermellino o della volpe artica che sono in grado di adattarsi all’ambiente nelle sue variazioni stagionali in modo da essere sempre meno visibili sia per la prede, che per i predatori uomo compreso. Quando infatti arriva l'inverno e il terreno si ricopre di neve, un mantello bianco diventa un travestimento perfetto contro eventuali predatori più grandi, come volpi e rapaci, ma anche un camuffamento ideale per avvicinarsi alle prede senza essere visti.

                                          ERMELLINO

                                   estate                                                   inverno

 

Altro tipo di mimetismo che può essere sia offensivo che difensivo è il camuffamento. 

Il camuffamento consiste nell'assumere un aspetto che fa scambiare per un altro. Le possibili prede possono assumere  caratteristiche simili a organismi potenzialmente pericolosi per i predatori.  Ad esempio questa farfalla  con le ali aperte simula il muso di un piccolo  mammifero che può spaventare gli uccellini che la potrebbero mangiare.

Oppure le possibili prede potrebbero mimare specie affini ma pericolose. Più raramente un animale pericoloso mima una specie innocua.     Comunque in ambedue i casi tutte e due le specie ne traggono vantaggio mettendo in difficoltà sia le possibili prede che i possibili predatori.

IL CASO DEL SERPENTE CORALLO. 

U Un serpente molto velenoso (il corallo) imita la livrea di un serpente solo moderatamente velenoso (Erythrolamprus), aiutando così entrambi a sopravvivere, perché i predatori imparano a evitare entrambi i tipi.. 

Per distinguerli bisognerebbe guardare la sequenza dei colori: Vero Corallo (velenoso): Rosso a contatto con il giallo (es. nero-giallo-rosso-giallo-nero).

·                            Falso Corallo (innocuo): Rosso a contatto con il nero (es. giallo-nero-rosso-nero-giallo). 

·                            "Red on yellow, kill a fellow; red on black, friend of Jack (or venom lack)".

·                            "Rosso su giallo, serpente corallo; rosso su nero, amico sincero". 

·                   

 

                                 Corallo            falso corallo      

Dubito però che chi si imbattesse in uno dei due serpenti abbia il tempo di osservare la disposizione dei colori!

 

Mimetismo “intelligente” del POLPO

 Il polpo cambia colore per rendersi invisibile ai predatori che, nel caso dei cefalopodi, sono per lo più pesci, uccelli, e mammiferi marini. L’invisibilità d’altro canto è un vantaggio anche durante la predazione dell’animale stesso. Ma il polpo vive nel basso fondale marino ambiente estremamente variabile per il movimento continuo dell’acqua che lo sovrasta, cambiamenti di luce ecc. quindi è necessario un continuo adattamento. Questo è possibile grazie ai tre strati di cellule particolari tipici della sua pelle che contengono granuli di pigmento che si mescolano in modi diversi sotto l'azione del sistema nervoso centrale, come risposta agli stimoli prodotti dal variare dell'ambiente.

 

 

Infine il mimetismo emotivo del camaleonte.

 Probabilmente qualcuno si è meravigliato di non aver sentito nominare il camaleonte, il piccolo rettile diventato proverbiale per la sua capacità mimetica. Infatti il suo nome viene spesso applicato alle persone che cambiano parere o comportamento.

Però i camaleonti non cambiano colore per mimetizzarsi con l’ambiente.  In realtà a provocare i mutamenti di pigmentazione è l'umore di questi animali.

Nella loro pelle, infatti, avvengono delle reazioni fisiche legate alle emozioni che provano  e sono queste che fanno mutare la livrea. Le cellule di uno strato della pelle contengono minuscoli cristalli di guanina che spostandosi, quando entrano in contatto con la luce, riflettono colori diversi. Quando il camaleonte è rilassato i cristalli sono vicini tra loro e riflettono la luce blu che, in considerazione della naturale pigmentazione gialla del rettile, danno il tipico colore verde. Se però l'animale è eccitato, durante l'accoppiamento o uno scontro o per catturare la preda, i cristalli si allontanano e viene modificata l'angolazione della luce riflessa, che è percepita ai nostri occhi di tonalità giallo-arancione. Quando ha paura ad esempio in un combattimento si gonfia e assume colori vivaci per intimorire l’avversario. In caso di resa invece assume un colore scuro.

Biodiversity is life!

Biodiversity ensued from a long history that started billions of years ago, just when life appeared on the earth.

From that time, life evolved according to the variations of the environment and, through critical periods and particularly productive periods, reached the spectacular species richness now existing.

However, at the present time, a single species, the species often considered the top of evolution, i.e. HOMO SAPIENS is giving rise to a new, very rapid, crisis: a number of scientists think that at least half of the species at present still existing will disappear in the next 50 years|!

 Why is biodiversity extremely important? What would happen with its drastic reduction?

 Three levels must  be considered

I. Genetic biodiversity.

Genetic biodiversity refers to the variety of genes within a species or population. It's the total number of genetic characteristics consisting of identical individuals!

This genetic variability is so much important that in the majority of the species evolved and established, sexual reproduction, that involves the fusion of two different genetic apparatuses creating a new apparatus with also new genes combinations! Even in most microorganisms, prokaryotic or eukaryotic, that usually reproduce by binary fission there are sexual phenomena allowing intraspecific genetic variability.

 

                 IN ALL THE SPECIES EACH MEMBER IS

                 GENETICALLY DIFFERENT, IS UNIQUE      

 

 

              Try to find perfectly identical shells of the same species!|

Since they started to raise domestic animals and  to grow edible plants humans applied artificial selection i.e. an evolutionary process in which they consciously select for or against particular features in organisms.

 In this way, for example, humans have bred food crops from wild plants and animals.

Now most human activities are performed on a large scale often  causing the reduction of  intraspecific biodiversity. For example we all experienced that flies and mosquitoes are at present more resistant than some years ago to the most common insecticides. What happened? Not necessarily mutations: simply only members with more resistant genes survived, reproduced and transmitted their genes to the following generations.

  

                                                        A mosquito in action

 

           II. Number of species diversity 

.

The present biodiversity is the fruit of the  billion years interaction  between life and earth. The earth provided the environment and the materials and life used fantasy creating organisms able to make good  fuse of every ecological niche.

Probably even at present we do no know the real number of the species living with us on the planet!

First of all a countless number of species exist among unicellular prokaryotic organisms. For example bacteria ere very similar in their morphology but greatly differ in their metabolism and, for this, are able to use different, special resources.

Eukaryotic unicellular and pluricellular organisms  differ  instead from each  other in morphology, sizes, metabolism and behavior.

                                                                                                                                                                                                                                                            

                                    

S                                           Shells of different Phoraminiphera.

Why did so many species evolve and why their maintenance is vital  for us? Because biodiversity is the most precious and important thing we have without which our entire as well as support system for human, animal life, would collapse.

    Indeed the elements available to maintain life are limited  on the hearth and the living organisms provide to recycle them each one using its specificity.

 

So there are:

       1) Producer organisms, also known as autotrophs, i.e. living things able to create their own food, typically through photosynthesis or chemiosynthesis.  To form new cells they need: energy provided by the sun, hydrogen and oxygen present in the water, carbon they take from carbon dioxide present in the atmosphere or dissolved in the water. Anyway some molecules present and necessary in every living being like proteins and nucleic acids contain also sulphur and phosphorus so producer organisms need also nutrient salts like phosphate and nitrates they find in the ground.

Producer organisms include plants, algae and some bacteria.

 

2. Consumer organisms, or heterotrophs, are living things that cannot produce their own food and instead obtain energy by consuming other organisms, whether plants or animals. There are different levels of consumers.  Dietary specialization i.e. the adaptation of certain species to primarily consume specific types of food resources, has a very important ecological role. For example even at the level of Protists there are species eating bacteria, species eating algae (able to chose their pray on the basis of size, cathabolites emission, type of movement) and species able to pray other protists often using special hunting  “devices” . 

A ciliate full of ingested algae

             

                                                       

                          The voracious predator ciliated Didinium attacking the ciliate                                                         Paramecium with poisonous extrusomes.

 

                                             

                                           Didinium ingesting Paramecium.  

 

 

  Both producer and consumer organisms release organic waste an leave dead bodies. To avoid the  transformation of the earth in a dump nature evolved also:

3. Saprophitic and decomposer organisms. In particular bacteria and mushrooms.  They are true chemical laboratories! They transform organic materials coming from dead bodies and waste in mineral salts again available for producers. Different species evolved for each element and environment and the life cycle so closely integrated can be interrupted by the lost of even  a single species.

 

                           Food chains in the sea and in the forest

 

 

II. Habitat biodiversity

 

The main habitats of the world are forests, grasslands, deserts, mountains, polar regions, and aquatic habitats (freshwater and marine). These diverse biomes are defined by their unique environmental conditions and are home to a vast array of plant and animals. However each one is subdivided in a lot of different sub-habitats that, with the organisms they host, form various ecosystems in which  each species plays a role.

But humans are modifying ecosystems very rapidly without a global vision of their characteristics.

Consider for example the massive destruction of  the forests!

A forest is a habitat that provides food, water, and shelter for a wide variety of plants and animals, structured in vertical layers including the forest floor, herb, shrub, understory, canopy, and emergent layer, which support different species and create a complex food web.

In forests productivity is very high. The large amount of organic substances produced fall down on to the ground where are rapidly decomposed by saprophitic organisms. The nutrients are as much rapidly absorbed by plant’s roots. So in any moment there are few mineral salts in the soil. When nutrients are insufficient, productivity decreases. Moreover removing trees leads to severe soil erosion, which washes away the topsoil. The exposed soil then loses its fertility, and can become barren and unable to support crops effectively. 

So deforestation causes direct damages in the climate without important increase in agricultural production.

                           

       *****************************************

 

While the traditional model of growth has often resulted in environmental degradation, new approaches such as sustainable development, green technologies, and circular economies are proving that it is possible to achieve economic prosperity without harming the planet

To reconcile economy and climate would be the challenge for modern policy. 

 

 

 

 

 Protists: cells and organisms.

Although the kingdom Protists is no more recognised as a real systematic category, the term, written with a small “p” ,   may be still useful to indicate in general eukaryotic unicellular organisms to which this post is dedicated.

  Protists have only one cell and this unique cell not only performs all vital functions typical of eukaryotic cells but also realize and recognize the external stimuli coming from the environment and from every other organism they may contact.  Then, the cell has to work out a suited answer, to get the required energy and finally to reproduce. ……to say the least!!!

As an example I will, thereafter, illustrate what Paramecium, the most famous ciliate protist, often reported even on primary school books, is able to do.

1)   Paramecium is able to move thank to its cilia. Cilia are slender protuberances that project from the cell body. These organelles “invented” by Ciliates, to which Paramecium belongs, are anyway present in many cells of our body with the same structure but used for different purposes. Cilia beat the water like little flexible paddles. Their beating is variable in frequency and direction. Thus the ciliate is able to vary the speed of its motion and to reverse its motion once an obstacle is present.

2)  Like all ciliate protists, Paramecium is an heterotrophic organism,    requiring organic compounds for its principal source of food. In other words like animals  Paramecium eats. “But a mouth is needed to eat”, you will say, well Paramecium has a  mouth!!!

 It is called cytostome ( i.e. cellular mouth) and is localized at the end of a funnel shaped   depression of the surface all covered by cilia. Obviously the mouth is not completely open, otherwise cytoplasm would go out, it simply  is a little zone delimited by the plasmamembrane (the typical membrane by which every cell is covered) alone while additional membranes are present on the rest of the cell surface.

When the food, forced by the ciliary beating, reaches the cystostome the membrane blows up like a balloon toward the interior, gathering food and water. When the balloon is swollen enough, it detaches and begin to move in the cytoplasm. There it is reached by small vesicles containing digestive enzymes and is called “food vacuole”.  

The food inside the vacuole is then digested and assimilated. Waste will be then eliminated through another specialized superficial zone called “Cytopige” i.e. cellular anus. 

So Paramecium possess a complete digestive apparatus, comprising mouth and anus, that can be formed every time is needed!!

Noticeably the membranes of disrupted vacuoles are then recycled to form the new ones.

A n excretory apparatus, consisting in the contractile vacuole system, is even present to eliminate excess water and salt.

Schematic drawing of Paramecium

3) What does Paramecium eats?  Bacteria and autotrophic protists often called microalgae. It can be considered in some way “erbivorous”.

     Most Paramecium species live in fresh water, a very variable habitat in which the food amount may be at times scarce.  To avoid starvation, a number of species are able to preserve the food: some food vacuoles are not reached by digestive enzymes and the microalgae they contain remain alive. They can also reproduce since their host will provide to keep them exposed to the light to allow photosynthesis. Then, when the environmental food is lacking, our provident protozoon can digest the algae it grew up or, simply, utilize their photosynthetic products. In a certain way these Paramecia become authotrophs.







Paramecium growing microalgae




4)  As erbivorous animals Paramecium is a potential victim of carnivorous predators (many ciliates are predators).  Many of these predator ciliates  attack and immobilize the pray ( whose presence they detect trough membrane receptors) by a sort of toxic “arrows” (toxicysts) and easily ingest the victim. And the victim is not able to escape? Yes, toxicysts discharge appear to evoke trichocyst discharge in Paramecium. Trichocysts are Paramecium defensive weapons: they are not toxic but their explosive extrusion causes a rapid backward movement by which the victim may escape predation.

Toxycists and trichocysts are two different types  extrusomes, cell organelles present only in protists. Different types of extrusomes exist but all of them can be extruded without damaging the cell.  

Paramecium after trichocysts extrusion

5)  Once the energy accumulated by feeding is enough, Paramecium reproduces. Like all Ciliates, Paramecium has a dual nuclear apparatus, consisting of a  macronucleus in which genes are present in many copies (polyploid)   and one or more diploid micronuclei. The macronucleus controls non-reproductive cell functions, expressing the genes needed for daily functioning. The micronucleus is the generative  nucleus, containing the genetic material that is passed along from one generation to the next.

Paramecia reproduce asexually, by binary fission. During reproduction, the macronucleus splits simply while the micronuclei undergo mitosis. The cell divides transversally after the replication of all cellular structures. In this way each new cell obtains a copy of the micronucleus and the macronucleus and is ready to live autonomously.






Paramecium during binary fission

6)  But Paramecium, like all Ciliates, has also sex!!!!!

We do not know exactly in which situation the “thing” called “conjugation” happens in the natural environment. In the laboratory it is generally induced by a light starvation. Conjugation is only realized between conspecific individuals but not all whit all !!! The two conjugants are of different mating types, in other words of different sex.
 We are not able to distinguish the different mating types but Paramecia are able to recognize each other, through membrane receptors. The recognition  induces a typical preconjugant behavior, that expert protozoologists can easily identify, and finally pair formation. The two conjugants remain attached by the cytostomial zone where a cytoplasmic bridge takes shape. During the process the old macronucleus disintegrates and the micronucleus of the cells undergo meiosis. 

Thus micronucleus is able to perform mitosis and meiosis. 

Then one of the aploid nuclei derived by meiosis pass trough the cytoplasmic bridge in the other partner and fuse with an aploid nucleus there stayed on.  At the end each conjugant has a new diploid nucleus, different from the nucleus they had before. The new macronucleus is formed by replication of the new diploid nucleus.Then the  two individuals separate. They were two at the beginning of the process and are still two at the end. Thus conjugation cannot be considered a kind of reproduction: it is however a sexual phenomenon causing genetic mixing to increase the species variability

A pair of Paramecia

The first great pollution of earth’s atmosphere

 

About 3.5 billions years ago the earth surface was very different  from now, particularly it was less hospitable. Oceans were larger, very often there were volcanic eruptions and meteor showers. Temperatures were probably similar to those registered now but the sunlight was less vivid. The most important feature of the ancient environment was the absence of free oxygen. Indeed it was almost lacking in the atmosphere whose main components were carbon dioxide, methane and  ammonia.

But life was already present!

Which kind of organisms could survive in a such hard environment?

They were only unicellular prokaryotes (like the bacteria and microalgae living today) able to gain energy without oxygen. They were very numerous and different from each other for their metabolism based on different energetic sources (methane, ammonia, sulphur and so on) predators were lacking: nobody was eaten. So they grew and spread everywhere.  

Then, about 2 billion years ago, a new  evolutionary metabolism appeared.

Aquatic organisms called blue-green algae began using energy from the Sun to split molecules of H2O and CO2 and recombine them into organic compounds and molecular oxygen (O2). This solar energy conversion process is known as photosynthesis.   

Yes, the same photosynthesis that practically support life on the earth even now.

According to modern hypothesis photosynthesis originated through the fusion of various evolutionary lines which pulled part of their genetic material.  This fusion of genes, at present referred to as horizontal gene transfer, created a metabolism much more productive than the others already existing, also considering that the sunlight was in the meantime increased. Some of the photosynthetically created oxygen combined with organic carbon to recreate CO2 molecules. The remaining oxygen accumulated in the atmosphere, touching off a massive ecological disaster with respect to early existing anaerobic organisms.

A true atmospheric pollution!

Most species died and disappeared for ever. 

Other species could survive in particular environments with a low oxygen concentration. 

Others “learned” to breathe oxygen while others were able to survive by the fusion with those breathing oxygen organisms.

 In this way the eukaryotic cell originated.

Thus, starting from a great atmospheric transformation, lethal for the majority of the forms of life at that time existing, new forms arose, from which during the time pluricellular organisms (man included) derived.

To cut a long story short: perhaps if the atmospheric pollution in progress now will increase beyond measure we, existing living beings, could succumb while new life forms could be originated on the earth.

Amoebae

 

I wrote some posts about Ciliates, the protists I studied directly for my research work. Then I wrote about Dinoflagellates and Foraminifera. All these protists have a well defined shape and their genus, often even species, can be identified by a simple observation at the light microscope (obviously with  a well-trained eye).

Amoebae are different. Indeed these protists are characterized by an amazing variability in shape. For this reason  the first specimens observed at microscope were called “little Protei” remembering the multiform Greek god Proteus.

Amoebae belong to the class Sarcodina and lack specialized structures for locomotion and sensation like cilia or flagella. Moreover their cell membrane is not reinforced by cuticles or other structures. Their locomotion is trained by extending and retracting pseudopodia, i.e. temporary extension of the protoplasm. In this way Amoebae change continuously their shape. Pseudopodia act as locomotor organelles adhering to the substrate and pulling the body itself. This locomotion is called indeed amoeboid.

Pseudopodia are the key feature of the organisms of Sarcodina.

 Indeed also Foraminifera that are included in this class  have pseudopodia but their are slim and elongate, to be let out trough the shell foramina and may split and rejoin each other. Moreover they contain a rigid internal structure. For this reason are called Actinopodia. Amoebae pseudopodia are instead various in  shape and size, lack of internal rigid structures, and extend from every part of the cellular body.  They are called rizopodia

ameba rizopodia

Foraminifera actinopodia

Amoebae are heterotrophic this means that to live and reproduce they must eat.

They haven’t a cytostoma like ciliates; they traps food particles with the help of pseudopodia that encircle them. After that, the food particle along with water is taken in and digested. This process is called phagocytosis . .

Like other characteristics described in protists amoeboid  motion and  phagocytosis were maintained during evolution and utilized in specialized cells of pluricellular organisms, even in human beings! 

For example in Macrophages, a type of white blood cell of our immune system, that engulf and digest pathogens, such as cancer cells, microbes, cellular debris.

                                               Engulfment of bacteria by macrophages

Free living amoebae are very common: they live in the sea, in fresh water and in damp soil. Their size varies from a few micrometers to millimetres according to the species. They can be “naked” that is without any recovering structure or covered by a rigid shell consisting of different materials (Calcium, Silicium or a conglomeration of environmental debris). These are “thecate”  

                           Arcella vulgaris tecamoeba

·                            Amoebae feed on bacteria, other protists and organic debris. They are primary or secondary consumers small in size that can be eaten by bigger consumers: thus they fit in the food chains.

Some amoeba species can be pathogenic, causing disease in humans and other organisms.  I willingly leave the study of those species to parasitologists. 

My  expertise concerns only free living protists. Those I treated in the course PROTIST ECOLOGY I held for many years at Pisa University.