“How do they see Cephalopods? "
(squid-cuttlefish-octopus)
In nature, eyes rule, and every living being has developed them based on their lifestyle and habitat.
The ancestors of modern cephalopods modified their visual system to survive. However, this didn't happen overnight, cephalopods diversified into today's octopuses, cuttlefish and squid in a period of great change in the marine world known as the Mesozoic Marine Revolution from 160 to 100 million years ago. Our cephalopod friends, as we know them today (more or less, I don't want to go into too much detail to avoid a paleontology lesson) have inhabited the seas around the world for over twenty-one million years. Only the testimony of very rare fossil finds like this one:
I say very rare because being equipped with a soft body (after the Marine Revolution of the Mesozoic) and only with an internal shell (cuttlefish,squid and octopus), generally the findings are reduced to the discovery of objects like this: a "belemnite", that is rostrum.
For example, the photo below was a small swarm of squid or cuttlefish that immediately after their death settled on the sandy bottom of the sea, and over the millions of years; they fossilized losing all traces of the soft part of their body, leaving us only the I remain petrified, the famous "bone"; the gladius.
Today, all anglers and marine biology enthusiasts would like to know more about the vision of cephalopods. During our "pilgrimages" on eBay or on sites specialized in the sale of squid, when we have to choose shapes and colors, we wonder if squid and cuttlefish see us well, if they distinguish colors, if they perceive day from night. Obviously, it must be premised that no one today knows the absolute truth, but it is based on scientific research and tissue analysis carried out in the laboratory, and then compared with real situations.
In eging fishing, therefore, the phase of visual identification of the egi by the cephalopod is extremely important, if not decisive. It follows that the angler must visually "present" the bait in the best possible way. To do this, it is of fundamental importance to understand how cephalopods see their prey, because it is not said - and in fact, it is not - that these animals see in the same way we see us.
So how do cephalopods see?
In this article I will reproduce analyzes carried out by teams of scientists, American, Japanese and not only, with the relative conclusions. Often while doing research, I read articles reporting that squid have good eyesight and this in various literatures. However, let us try to understand, taking a step back and analyzing both the sight of cephalopods and their system for perceiving dangers in particular conditions. There are about five hundred species of squid around the world and about a hundred species of cuttlefish that live in all the oceans surrounding the globe, making them a reliable food source for humans first and foremost, for large sperm whales, dolphins. , sharks, seabirds, fish and even other squid. 
Conversely, the squid themselves are fearsome ocean predators. But their most extraordinary adaptations are those that have evolved to help them thwart their predators. Squid, which are found primarily in estuarine, deep-sea, and deep-sea habitats, often swim together in shoals. Exposed outdoors with no place to hide they make themselves vulnerable, so as a first line of defense, they rely on large, well-developed eyes. In the colossal squid, these are about the size of dinner plates, the largest eyes known in the animal kingdom.
Cephalopods have a highly capable visual system, with prominent eyes and dominant optic lobes, useful for detecting predators and initiating escape responses.
Cephalopod vision has been considered the dominant sensory modality used in predator detection due to the complex and well-developed nature of their eyes. Cephalopods have a large field of view that can extend beyond 360 degrees in the horizontal plane, allowing them to detect predators within a broad sensory sphere.
Let's start with the fact that the eyes of cephalopods are very different from human eyes.
Let's see if we can explain further details regarding the differences.
Japanese tests have measured that the highest level of visual acuity is 1.2 - 1.5 tenths for squid and cuttlefish. For their kind, these values are very high; obviously, they are not comparable with humankind. The result is acute vision, but only from the point of "focused position". Focused position refers to the squid's and cuttlefish eye ability to distinguish things only in the position in front of him. The ability of the squid eyes to distinguish or not distinguish things that are not in a focal position makes no sense, therefore everything that passes by the side of the eyes is not taken into consideration by its optical system. When a squid or cuttlefish "hunt an EGI "and finally touch it with its tentacles, it means that the bait has been attacked at the focal point in front of it, therefore in a visible position for it.
At that moment, the view of the squid is at its maximum-.
After the "View of the squid", the next question is the "perception" of sight. It is rumored that squid cannot distinguish colors, but this topic is a bit more complex, so I will try to explain it using data I have found in various researches. Let us try to illustrate it in detail. Each of us can remember his biology lesson at school.
The human eye is like the lens of a camera, the lenses of the eyes are like the lenses of a camera, and the retina is the film.
Among the cells in the retina, there are cells that function in bright places, and other cells that function in places without light. If you go quickly from a bright place to a dark one, you temporarily see nothing, but gradually you are able to see - This is because there are two types of cells in the retina: Cone cells that function in the lighted places, and rod cells. That work in dark places. The cone cells are activated in bright places and are involved in color discrimination. The rod cells are activated in the dark, they are involved in the sensitivity of black and white.
Now comes the slightly more complex part of the explanation.
Proteins contain photoreceptor cells within them. The type of color-sensitive protein in "cone cells" that distinguishes colors depends on the animal. There are also proteins that detect UV rays, but humans don't have them. In addition, in the "rod cells" there is a representative protein, "rhodopsin", which allows the view in black and white. Vision depends on which of these proteins a visual apparatus possesses. By examining the protein, we can tell which color the animals are able to distinguish. Therefore, by examining the proteins, we will be able to establish which colors may or may not be seen / detected by squid or cephalopods in general. In addition, if we examine this protein, we can connect that type of cephalopod to a specific area of the ocean and we will be able to understand at what depth it usually stays.
Squids and cuttlefish cannot distinguish colors, because they only have Rhodopsin.
Let's try to explain it in comparison with the human being. Do you know the frequencies of the "three primary colors of light"? Man can distinguish between red, blue, and green colors.
By combining them, we can distinguish colors in bright places. We also have Rhodopsin, so we can also distinguish black and white.
This photo shows the seabed at -40 meters.
It is a bit dark and difficult to see. However, the human being can distinguish colors, as you can see the seaweed, the rocks, and the wreck. On the contrary, here is the color table perceived by the cephalopods.
Consequently, we see that only Rhodopsin exists. This means that squids and cuttlefish cannot distinguish between red, blue, green, etc., because there is no protein that makes it possible to distinguish colors. Thus, we can conclude that they can only distinguish black and white. The sight of the squid we are talking about should perceive this image.
The squid and cuttlefish almost certainly lives in a black and white world.
From a scientific point of view, we can explain it up to a certain point why they have evolved in this sense, but later on, we venture a hypothesis. Let's take these frames underwater.
The color image is what we see.
The black and white image is what it is assumed the squid and cuttlefish can see.
Inevitably, the intensity of light in the water decreases with increasing depth. The more an EGI sinks, the more the color gets lost. Although the coating of this EGI is a bright pink color, in depth, the color fades, but the color of the algae was also green, now the color has changed to gray. The colors are gradually lost as the depth increases - The difference in the light and dark colors of the EGI is ideal for squid and cuttlefish that live in the most diverse situations of light and turbidity of seawater.
As an attentive reader, you will say: “So why are there a large number of Egi colors produced by the most disparate companies and present in stores, if then the deeper they go, the more the colors are lost? Technically, I can answer that your observation is correct, but the colors of the Egi are visible on the ground, but they are not the same thing seen underwater, but above all, they do not have the same visibility for the human eye and for cephalopods. The structure of the squid's and cuttlefish eyes is different from ours. The same color can look different to the squid, cuttlefish and us. We can ask ourselves why squids have abandoned the need to discriminate colors and have specialized in being able to perceive light and dark.
But the answer is immediately ready: "simple colors are depleted in water with little light, so they have specialized in knowing how to perceive light and dark. We imagine all this as a process of evolution and adaptation”.
This image represents a situation at sea, as a squid or cuttlefish can see it.
We have changed the image to black and white, to be able to make you understand that squid and cuttlefish cannot discriminate colors. Therefore, we see a speckled coating, of course. Egi that have a lot of light and dark are well visible to squid and cuttlefish. When the Egis move and are in the squid's or cuttlefish field of view in front of them, they can recognize that something is moving. Not only the colors, but also the coating have a great influence, on what the squid and the cuttlefish can see. Both of them can see the difference in the light darks of the colors between the pattern (design) and the color of the fabric or base. Give it a try, take the image of your favorite Egi and look at it in black and white. I really recommend that you do this while imagining the place where you generally go fishing. The color contrast of the EGI differs from situation to situation. For example if the seabed is rocky, sandy or with algae. The background is not always a clean (clear) white. We need to imagine the place where we generally fish or the time we go fishing. For example, in a stretch of sea at night, a rocky area in broad daylight, an area with algae in the morning and so on. It is assumed that the squid and cuttlefish cannot see the colors, but they can see the light, indeed, they perceive it rather than see it. The "Egi" of unique color.
Below is a table used by the Japanese to show how squid and cuttlefish probably perceive colors underwater at different depths compared with what the human eye perceives. Even if the writings are partly in Japanese, it is easy to understand what they refer to given the abbreviations and numbers contained therein.
The table is divided into two main parts, at the top; we see the perception of color by humans under water, which varies with varying depth, a range of basic colors. The nm for each color is shown on the horizontal line at the top of the diagram, while the depth is marked in the vertical part on the left of the image. Therefore, by crossing color with depth we have how the human eye perceives color underwater up to about 21 meters. The table below represents the perception of colors at different depths by cephalopods.
The concept of "Zone Squid" was born, that is the squid of a color that is basically the most popular in terms of catches in that specific area (not necessarily large). This, as already said, is due to the underwater environment in which the cephalopods feed on both biological and morphological conditions which can also be influenced by the lights of the port areas, the presence of fresh waters, and other elements that determine their characteristics.
I publish below other studies that still have a common basis, according to what has already been said, but this is the science, you must have a starting point (which in this case is the "Camera" structure of the vertebrate eye, and in particular of man.
It has been documented that several types of cephalopods, notably squid and octopus, and potentially cuttlefish, have eyes that can distinguish the orientation of polarized light. The precise function of this ability has not been demonstrated, but it is speculated that it is for prey detection, navigation, and possibly communication between color-changing cephalopods.
Put simply, sensitivity to polarized light allows you to see contrasts, a dark fish on a light background, or a light fish on a dark background, this is how they identify their "targets", distinguish the contrasts.
These two images represent vision sensitive to polarized light.
Even if a fish were transparent it wouldn't be a problem for them. Squid have a weapon for any obstacle and neutralize it anyway.
This is what the same image would look like when viewed from mammals or fish.
In the documentary "Super Senses: the secret power of the animals", the researchers argue that cephalopods are "color blind", so much so that they do not see the differences between colors. Scientists have discovered that it is not the color in the realm they live in, often made of darkness, in which they hunt for most of their lives, but the physical quality of the light and how it is polarized. When light waves are emitted from the sun or other source, they travel in all directions, and at any angle, but are not polarized. Instead, when the light hits some objects, only the waves traveling at a certain angle are reflected, this is the polarized light.
Man cannot perceive polarization without special sunglasses unlike cephalopods. They can detect the slightest variation in the angle of light reflected from an object including other similar ones. While we see for example a male cuttlefish that changes color, the female and other cuttlefish will see a mosaic of extremely detailed drawings, relying on polarized mink.
In vertebrate eyes, nerve fibers pass before the retina, blocking some light and creating a blind spot where the fibers pass through the retina. In cephalopod eyes, nerve fibers pass behind the retina and do not block light or interrupt the retina. 1 is the retina and 2 the nerve fibers. 3 is the optic nerve. 4 is the vertebrate blind spot.
Cephalopods, as active marine predators, possess specialized sensory organs for use in aquatic conditions. They have a "camera" eye, which consists of an iris, a circular lens, a vitreous cavity (eye gel), pigmented cells, and photoreceptor cells that translate light from the photosensitive retina into nerve signals that travel along the nerve optic to the brain. Over the past 140 years, the "camera type" cephalopod eye has been compared to the vertebrate eye as an example of convergent evolution, in which both types of organisms have evolved independently, the eye-camera trait and both share similar functionality. There is a controversy as to whether this is truly convergent evolution or parallel evolution. Unlike the camera eye of vertebrates, folds of tissue (invagination) of the body surface (rather than growths of the brain) form in cephalopods, and as a result, the cornea is located above the top of the eye instead of being a structural part of the eye. Unlike the vertebrate eye, a cephalopod eye is focused through motion, much like the lens of a camera or telescope, rather than changing shape as the lens does in the human eye. The eye is approximately spherical, as is the lens, which is completely internal. The eyes of cephalopods develop in such a way that they have retinal axons that pass on the back of the retina, so the optic nerve does not have to pass through the photoreceptor layer to exit the eye and does not have the natural, central, physiological blind spot vertebrates.
The crystallines used in the lens appear to have developed independently of the vertebrate crystallines, suggesting a homoplastic origin of the lens. Most cephalopods possess complex extra-ocular muscular systems that allow for very precise control over the gross positioning of the eyes. Octopuses possess an autonomous response that maintains the orientation of their pupils in such a way that they are always horizontal.
Unlike our eyes, the eyes of cephalopods (cuttlefish, octopus, and the like) contain only one type of color-sensitive protein, apparently limiting them to a black-and-white view of the world. Nevertheless, a new study, according to the US scientific journal Proceedings of the National Academy of Sciances, shows how, in fact, they could get by. By quickly focusing their eyes at different depths, cephalopods could take advantage of an optical property called "chromatic aberration". Each color of light has a different wavelength - and because lenses flex some wavelengths more than others do one color of light passing through one lens may be in focus while another is still blurry. Thus, with the right type of eye, a rapid movement of the focus would let the observer understand the real color of an object based on when it blurs. According to a study published in Proceedings of the National Academy of Sciences, the off-center pupils of many cephalopods - including the W-shaped pupils of cuttlefish - make this blur effect more extreme. In this study, the scientists built a computational model of an octopus eye and showed that it can determine the colors of the object simply by changing the focus.
Chromatic aberration is nothing more than color-dependent blurring and, by focusing across different colors; it is theoretically possible for cephalopods to use their eyes as a spectrophotometer capable of focusing each wavelength sequentially to distinguish color. It is believed that focusing individual light frequencies on the retina allows cephalopods to distinguish colors based on the level of blur. This translates into a vision that allows the identification of color contrasts very well. Cephalopods could ultimately see with a sequence of images like the ones below that rotate quickly creating contrasts.
Since all of this is still theoretical, the next step will be to test live whether cephalopods actually see color this way - and whether any other "colorblind" animal could as well.
Perceiving colors requires specialization of the eyes and the brain. For example, humans have three types of cone cells, each associated with a different wavelength range. These cells transmit their readings to the brain, which interprets them with a color vision. This ability has a price: in low light conditions, as already mentioned, our vision becomes less precise. Conversely, in general, organisms capable of seeing in low light "sacrifice" the perception of the infinite nuances present in nature. Cephalopods have only one type of optical cells and have distinguished themselves in laboratory tests for their apparent inability to perceive tones according to standard methods (therefore designed for eyes like ours). Yet the world of these animals is made of colors: they camouflage themselves until they become invisible or communicate with a wide range of colors (and with great risk of being eaten) during their breeding season. The big question that scholars asked themselves was therefore: how is it possible that animals unable to see colors use them in such an exceptional way?
Two researchers, father and son, A. Stubbs and C. Stubbs, have developed a model that explains their color vision.
Basically, they have an eye that perceives tints based on the distance at which they are in focus, a bit as digital cameras do by maximizing contrast over focal length.
An absolutely new mechanism in nature which, however, is probably also used by spiders and dolphins and which once again demonstrates how much evolution has indulged itself on our planet.
The following text was taken partially from an article written by Dr. Ferruccio Maltagliati of the University of Pisa, Department of Biology, Marine Biology and Ecology Unit, in: "The cuttlefish and the rainbow: implications for eging "
In it he reports studies carried out by famous biologists, and from his personal experiment.
“…… ..Who doesn't know Paul Newman, Keanu Reeves, Bill Clinton and Mark Zuckenberg? These are four celebrities who do not distinguish (or distinguished in the case of the late Paul Newman) the colors in their lives. This does not mean that it cannot be said that they have failed to be successful men. Well yes, even our cuttlefish friends have the same "problem", that is, they are unable to distinguish colors! This has by no means prevented these marine creatures from having great evolutionary success. Perhaps the color blindness of cephalopods will sound strange to many angler friends and to many others it will even appear nonsense. Even today, specialized sport fishing magazines continue to publish articles that highlight the great importance of the colors of the egi.
Recently I also happened to be a spectator of television services, on channels dedicated to recreational fishing, which highlight the importance of the colors of the egi for catching cuttlefish or squid. Then, among eging sport anglers, who has never heard phrases such as: "today they eat green!", "I took them all with orange", or "early in the morning you need blue, then you have to change with red ". All these statements are based on the personal feelings of the anglers, who, many times inform us about certain truths, but in other cases, it happens that they lead to misleading conclusions. In addition, certainly the case of the importance of the coloring of artificial baits in cephalopod fishing falls into the second category!
As a marine biologist and sport angler I would like to emphasize that, the old professional and recreational anglers often know more than we biologists about certain aspects of the behavior, ecology and biology of many marine species not yet sufficiently studied experimentally by science. Let me give you an example: professional trawler anglers know perfectly which are the breeding areas and periods of hake or Norway lobster.
Marine biology is in fact a relatively modern science, extremely heterogeneous and complex, in which there are still many gaps to be filled (which, in my opinion, is its beauty!).
Consider that by marine biologist we mean both the marine microbiologist, who treats marine organisms invisible to the naked eye, but also the ketologist, who studies the fin whales, that is animals over 15 meters in length and more than 100 tons in weight!
The first scientific studies on the vision of cephalopods date back to the late 1950s, when two American scholars who worked at the Zoological Station of Naples, Paul K. Brown and Patricia S. Brown, published the results of their research in the prestigious journal Nature. These biologists observed that a single visual pigment, rhodopsin, was present in the retina of the eye of cuttlefish and octopuses. It should be noted that animals that distinguish colors have at least two pigments (man has three). The two biologists, however, in that work did not draw conclusions on the monochromatic vision of cephalopods. Subsequently, noteworthy is the study by two other biologists, N. Justin Marshall and John B. Messenger, also published in Nature, but about forty years later, in 1996. These scholars carried out experiments based on the mimetic abilities of cuttlefish. . The mimetic patterns shown by the cuttlefish on experimental substrates set with different color combinations were detected (Fig. X). In short, these two authors concluded by stating that cuttlefish do not distinguish colors, but are well capable of detecting color contrasts, if of a certain importance. A decade later, an American research team led by biologist Lydia M. Mäthger conducted research, the results of which were published in the journal Vision Research, also based on the mimetic abilities of cuttlefish in different combinations of substrate colors.
They confirmed color blindness in cuttlefish (Fig. Y) and added that these animals are able to resolve color contrasts of less than 15%. A couple of years later, in 2008, the same research group, led by Alexandra Barbosa published in Vision Research that the resolution of contrast in sepia is about 5%, a fairly low value, which indicates how this species manages to distinguish color contrasts very well. Consider that man is able to distinguish contrasts up to 2% and the owl, a nocturnal animal with exceptional visual abilities, up to 1%.
At this point I would like to report on a curiosity that I encountered while browsing the scientific literature on vision in cephalopods: to date the visual pigments of only some species of cephalopods have been studied; among those studied, all have only one visual pigment in the retina, which is in agreement with their monochromatic vision. However, there is an exception. The western Pacific 'firefly squid', Watasenia scintillans, which possesses three visual pigments in the retina. It is the only cephalopod species "suspected" of possessing distinctive color abilities.
Firefly squid, (Watasenia scintillans), the top of the photo is the bioluminescence in this species, the bottom is the squid under normal conditions.
Returning to the vision of cuttlefish, some time ago I came up with the idea of setting up an experiment aimed at verifying the "chromatic beliefs" of most eging fishermen, in other words, I intended to test the veracity of the aforementioned " hypothesis "on the differential efficacy of the colors of the egi.
Professionally I do not deal with the biology of vision and therefore, as ignorant in this field, the first thing I did was to document myself and then I went to sift through the databases of my university all the scientific literature produced on the vision of cephalopods.
To my surprise, I discovered that, from the late 1950s to the present, numerous experimental studies, carried out mainly on octopus (Octopus vulgaris) and cuttlefish (Sepia officinalis), have unequivocally demonstrated that cephalopods colors. To which, I said to myself: "Wow! This time the sport fishermen were really wrong!" I therefore abandoned the idea of carrying out a new experimental study, as the aspects that interested me had already been extensively investigated by colleagues much more experienced in that field than I am. However, driven by scientific curiosity, but above all by that of a sport angler, I downloaded the electronic formats of the various scientific articles and studied them carefully.
Below is a summary of the information I extracted from the articles that I considered most relevant for the purposes of eging fishing.
Fig. 1. Experiment by Marshall and Messenger (Nature 382: 408-9, 1996) carried out on a cuttlefish about 12 cm long on suitably prepared colored gravel bottoms (A, red on white, B, red on blue and C, yellow on blue). The principle on which the experiment is based is that the cuttlefish camouflages itself based on how it sees the substrate on which it is located. The images with index a show ambient light photographs of the sepia on the three different substrates. Images with index b indicate the details of the skin. The images with index C are photographs taken with a wavelength filter equal to that of the visual pigment present in the retina of the cuttlefish and simulate how the cuttlefish sees the substrate. Note that in A the cuttlefish tends to produce evident mimetic spots, in B the mottling is much less marked, while in C the skin of the cuttlefish shows a practically uniform color.
Fig. 2. Two egi with different colors and identical design with black streaks. A, vision of the egi by man; B, simulation of how they are seen by sepia. The representation in B was obtained with a slightly lower resolution of the contrasts and on the basis of the wavelength (λ = 492 nm, corresponding to the green color) absorbed by the pigment present in the cephalopod retina. Note that what appears chromatically different to man is identical for the cephalopod.
Then should octopuses, cuttlefish, squid and squid be considered "visually impaired" organisms? Of course not, far from it! In an article published in 2011 in the Philosophical Transactions of the Royal Society B by a group of Australian researchers headed by N. Justin Marshall, it is reported how the aquatic environment is rich in polarized stimuli that can provide important information to animals that are sensitive to this type of electromagnetic radiation. Without going into the merits of the biology of the vision of polarized light, nor of the complex physics of this type of electromagnetic radiation, I report only that this type of sensitivity has been demonstrated in both cephalopods and fish. However, these two groups of animals use it for different purposes: the cephalopods use it both to communicate with each other and to identify prey and predators; while the fish use it mainly for navigation and orientation. In particular, in cephalopods it has been found that the responses to polarized stimuli are qualitatively comparable with those obtained from the strong contrasts detectable with "normal" vision. This suggests that the sensitivity to polarized light represents a self-sufficient visual channel which however increases the total visual abilities and therefore improves the perception of the surrounding environment by the cephalopod.
So what about the choice of colors for the egi in cephalopod fishing? What is the best color? Well, in light of the above, color is certainly not the most relevant variable, on the contrary...! Probably, the drawings of the egi have a certain importance (eg streaks or spots well contrasted with respect to the basic color of the egi), which are undoubtedly identifiable by the cephalopod. Much remains to be understood about the reflection of the egi towards polarized light, which could be a very important additional aspect to unleash the predatory instinct of the cuttlefish. In my humble opinion, the structure of the egi in the water and, above all, the movement imparted by the angler are the aspects to be taken into greater consideration in this type of fishing.
Recently an absurd article signed by several scholars (but no zoologists), including the "infamous" Chandra Wickramasinghe, suggested that cephalopods could be "alien", that is, genetically derived from something brought to Earth from space millions of years ago.
Perhaps it is no coincidence that in the movie "Arrival" the aliens visiting earth were described as cephalopods. After all, somehow they are. They seem to have evolved in a completely different way! For this, we need to free ourselves, in research, of the temptation to find similarities with our systems, or we will not understand anything about them, the fact remains that these animals continue to collect a science fiction series of primates.
An interesting starting point can arise from what has been described for giant squid. Large squid are of two types: giant ones and colossal ones. The most common, as adults, weigh as much as a large swordfish but their eyes have a volume 27 times greater; they are the size of a basketball.
The fact that creatures of similar size can have such different eyes has never convinced Sönke Johnsen, a scholar at Duke University, especially since the squid inhabits the depths of the ocean, into which, as we know, light almost cannot penetrate.
Johnsen then collaborated with a group of biologists to develop a model capable of reproducing the physical and biological conditions of the way these mollusks use their eyes; the results of their work have just been published in Current Biology.
The team of researchers first collected the eye sizes of specimens captured or photographed in recent years; then, he collected data on the amount of light present between 300 and 1000 meters deep, where the giant squid lives.
Only later did he elaborate a mathematical model capable of explaining the functioning of the eyes of the cephalopods in question.
It has been shown that squid eyes are able to capture more light than the smaller ones of other animals of similar size thanks to the size of their retina. This feature allows them to capture even small differences in contrast in the darkness. This ability, in itself, would not be of great importance in the depths of the ocean, but it becomes so if, as it happens, it allows them to escape from the most fearsome predator: the sperm whale.
During the hunt, the cetaceans emit typical sound signals to identify their prey; squid are unable to perceive them but there are other organisms in the ocean that are sensitive to these stimuli. On the other hand, they perceive the displacement of the masses of water.
As the sperm whale passes, the plankton's bioluminescent organisms react by producing more light. Thanks to the large eyes, and not only that, the mollusk is able to catch this faint glint even at great distances. Most likely the infallible radar of the sperm whale is able to identify the squid before it perceives the bioluminescence of the plankton. The large eyes of the squid, therefore, are not sufficient to avoid entering the range of action of the cetacean, but are essential to allow it to escape quickly.
Another clear example of how the ability to escape the predator represents one of the greatest evolutionary drives, even in the depths of the ocean.
Therefore, it is presumable that even the smallest forms of squid and squid perceive the bioluminescence’s generated by the movement of water, by large predators, not necessarily killer whales or sperm whales, but dolphins, swordfish, tuna, barracuda, greenhouse fish. It follows that the presence of these large predators, in particular light conditions, will alarm our cephalopod friends by making them choose safer areas to eat instead of being eaten.
An important advantage of squid in fleeing from predators is their dependence on multiple sensory systems for detecting them.
Despite the highly advanced visual system, there are many situations where visual cues are reduced and / or unreliable, such as in murky waters, at night, in complex environments where visual indicators are overwhelming, or in cases where predators are well camouflaged.
Under these conditions, cephalopods can benefit from other sensory systems, such as a similar lateral line system, which is sensitive enough to detect a 1m-long fish swimming at a distance of about 30m, even when vision is disabled.
Hydrodynamic stimuli provide important information for aquatic animals and, as a result, most taxa have developed a sensory system for detecting water movements and pressure fluctuations. Over the past two decades, many studies have revealed the functional significance of the fish's lateral line. Aquatic animals create flows and pressure fields when they swim, and sensing these hydrodynamic conditions can provide important insights into the movement behaviors of animals. Fish can use this hydrodynamic information to detect and avoid predators.
Biologists Carly A. York and Ian K. Bartol performed specific studies and noted that when it is dark or the water is cloudy, however, squid rely on a secondary sensory system, made up of thousands of tiny hair cells. Only about twelve microns long and running down the head and arms.
Drawing of hair cell, under microscope view
Each of some epidermal hair cells that is polarized is attached to axons in the nervous system. Hair polarization (i.e. sensitivity to polarized light) occurs anteriorly, posteriorly and laterally in both left and right directions. This allows cephalopods to detect water movements as low as 18.8 µms-1, which is comparable in sensitivity to that of the lateral lines of fish. Cuttlefish (Sepia officinalis) responds behaviorally to stimulation of their lateral line analog in the frequency range 10 to 600 Hz. Furthermore, York and Bartol have shown that ablation (only temporary elimination carried out with particular lights or total and final with of substances used in the bio-medical field) of hair cells leads to a reduced survival of young and adult squid (Lolliguncula brevis) when interacting with a predator.
Microscope images of the hair cells shown on Doryteuthis pealeii paralarvae. (A) Lines of hair cells on the head indicated with arrows. (B) Close-up views of sensory hair cells. (C) Sensory hair cells after treatment with a 500 μmol l-1 neomycin sulfate solution. Most of the hair cells were completely destroyed after treatment, with the remaining hair cells being porous and heavily damaged. Measurement scale: A, 400 μm; B, C, 5 microns.
Doryteuthis pealeii: adult form
Swimming animals create a wake, so when the hair on the squid's body detects this movement, it sends a signal to the brain, which helps it determine the direction of the water flow. In this way, a squid can sense an oncoming predator even in the darkest waters.
The results of an American study demonstrate for the first time that both the vision and the analogous lateral line system provide sensory information for the initiation of an escape response from the predator in squid during ontogenesis. Processes by which the biological development of a living organism takes place, from the fertilized ovarian cell to the embryo up to the complete individual).
We now speak of the mimicry of certain cephalopods, associated with the behavior of chromatophores, each consisting of black, brown, red or yellow pigments and ringed in the muscles. Most species of the class of cephalopods have chromatophores on their skin, cells that contain pigment granules, which contracting and expanding due to impulses sent by the nervous system produce color variations, this allows to create a contrast effect, between the lower part of the squid lighter than the darker upper part, to eliminate a visible shape to a predator who could spy on it from below, but also for communication reasons. Some predators, however, such as whales and dolphins, circumvent this ploy by using sound waves to detect a squid's mimetic shape. Desmond Ramirez and Todd Oakley of the University Of California (Santa Barbara, USA) have studied how octopuses in particular collect information about their environment through their eyes. By analyzing the skin of some squid and octopus, they discovered how the skin of these cephalopods reacted to light without any kind of stimulus from the brain or eyes. They collected various biopsies on the skin of one species of octopus in particular, Octopus bimaculoides: they hit the tissue with a white light and were impressed that when it was hit by the light, the chromatophores contracted, changing the color of the skin and relaxed. when instead the light was turned off, returning to the initial tonality. Then, they verified the effective wavelength useful for the chromatophores to expand and thus change the color of the skin from purple to orange, obtaining a value of 480nm, which is equal to the wavelength that allows perceiving the blue-azure. , color to which the eye of cephalopods responds most strongly. This revealed a certain sensitivity of the skin in relation to different wavelengths; the researchers called the cephalopod system to calculate the light intensity Light Activated Chromatophore Expansion (LACE). They found that the main cause of this phenomenon is to be found in the presence of rhodopsins, proteins belonging to the opsin family, usually produced in the eye and sensitive to light. In fact, in chromatophores, the same mechanisms that occur in the eyes seem to manifest themselves. Other researchers from the University of Maryland (Alexandra Kingston, Tom Cronin and R. Hanlon) have found the same presence in the chromatophores of other species of squid and cuttlefish. Is it therefore possible that in cephalopods the same structures, which show a behavioral response to some stimuli, are photosensitive?
Many years ago, the old fishermen told me that they could only catch squid from the boat at certain times, that is when the movement of the line from the bottom up did not generate a particular phenomenon, that is, it did not "burn". In fact, this bioluminous trail generated by the movement of the line from the bottom up, alarmed the squid, making it impossible to fish them, perhaps precisely because they associate that trail with the presence of a predator and therefore with a danger. Their escape for survival is inevitable. We can thus affirm that what unites all the theories put forward by various and authoritative scholars is that technically the cephalopods do not see colors, but seem to be able to perceive them in other ways. Just look at how their appearance changes to blend in with the color of the environment around them, a necessary condition for their survival given that they are not equipped with an external shell that can protect them. Being animals that spend most of their life in darkness, the evolution of the species has meant that the color perceived by the eyes was not decisive for staying alive, limiting the vision only in black and white, but they have entrusted their process of survival to the chromatophores, scattered over their body like infinite eyes capable of perceiving different wavelengths. In fact, in them, the same mechanisms that occur in the eyes seem to manifest themselves, ie they are photosensitive but "perceive colors". The sudden change in color is possible thanks to the presence of particular cells called CHROMATOPHORS: they have a vesicle full of pigment and are surrounded by very thin but very reactive muscle bundles. These muscles regulate the dispersion of the pigment inside the saccule in such a way that, the more the muscles are relaxed, the more the color of the cell and therefore the portion of the animal's body is dark since the pigment is uniformly distributed in the cell itself, vice versa more the muscles are contracted, the more the pigment is accumulated in the center of the cell, the clearer the color. The mimicry of the cephalopods works perfectly where the environments have “natural” colors and therefore chromatic scales of grays, browns and reds; when the animal is moved to environments where the colors are more flamboyant and intense; the latter cannot reproduce the color perfectly but will only reproduce the chromatic contrasts just think of a bottom of a colored bucket.
Nevertheless, in order not to be discovered, the squid, the cuttlefish, the octopus still have two other tricks up their sleeves.
The first concerns the ink, produced inside its mantle. The "black" is mainly composed of mucus and melanin, which produces its dark color. When squid eject ink, they use it either to create a large smoke screen that completely blocks the predator's view or a patch that roughly mimics the size and shape of the squid. This creates a ghost shape, called "pseudomorphic", which causes the predator to think that the inkjet is the real squid.
As a finishing touch, squid rely on jet propulsion to quickly disappear from their predators, reaching speeds of up to 25 miles per hour and moving meters away in seconds, making them the fastest invertebrates on Earth.
I conclude by saying that all this scientific information obtained through experimentation and not based on personal feelings, in addition to one's own experience, represent a good starting point on which to base when we organize our future outings to… "cephalopods".
When science provides us with more information, we will be happy to share it with all enthusiasts.
Paolo Gavazzeni
SPECIAL THANKS TO:
- University of Maryland researchers Alexandra Kingston, Tom Cronin and R. Hanlon.
- Desmond Ramirez and Todd Oakley of the University Of California (Santa Barbara, USA)
- Biologists Carly A. York and Ian K. Bartol & Paul S. Krueger
- Mr. Sonke Johnsen, Duke University Scholar,
- The Philosphical Transactions of the Royal Society B
- The Biologists Marshall and Messenger
- Biologist Lydia M. Mäthger & Vision Research journal,
- Biologist Alexandra Barbosa & Vision Research
- The Zoological Station of Naples, Paul K. Brown and Patricia S. Brown, & the prestigious Nature magazine.
- Biologists, N. Justin Marshall and John B. Messenger, & the journal Nature.
- Researchers, father and son, A. Stubbs and C. Stubbs.
- The US scientific journal "Proceedings of the National Academy of Sciances".
- Dr. Ferruccio Maltagliati of the University of Pisa, Department of Biology.
- The documentary "Super Senses: the secret power of the animals",
- Yamashita Lab.
- Research biologist Carly Anne York - Western Kentucky University. Doctor of Philosophy & Ecological Sciences
&
Ian Bartol (Director), Lisa Horth, Kent Carpenter,
Sara Maxwell, Paul Krueger, Joseph Thompson
- The Biologists Budelmann and Bleckmann
WE ALWAYS RESPECT THE SEA AND NATURE