Category Environtal Studies

          All sharks are carnivorous (meat-eaters), and a few species, such as the white shark, which can grow to 9m (30ft), have been known to attack humans or even boats. But 90% of all shark species are not dangerous to humans at all.

          The United States averages just 16 shark attacks each year and slightly less than one shark-attack fatality every two years. Meanwhile, in the coastal U.S. states alone, lightning strikes and kills more than 41 people each year.

          Each year there are about 50 to 70 confirmed shark attacks and 5 to 15 shark-attack fatalities around the world. The numbers have risen over the past several decades but not because sharks are more aggressive: Humans have simply taken to coastal waters in increasing numbers.

          Over 375 shark species have been identified, but only about a dozen are considered particularly dangerous. Three species are responsible for most human attacks: great white (Carcharodon carcharias), tiger (Galeocerdo cuvier), and bull (Carcharhinus leucas) sharks.

          While sharks kill fewer than 20 people a year, their own numbers suffer greatly at human hands. Between 20 and 100 million sharks die each year due to fishing activity, according to data from the Florida Museum of Natural History’s International Shark Attack File. The organization estimates that some shark populations have plummeted 30 to 50 percent.

          The shortfin mako (Isurus oxyrinchus) is often recognized as the world’s speediest shark. It has been clocked at speeds of up to 20 miles an hour (32 kilometers an hour) and can probably swim even faster than that. Makos are fast enough to catch even the fleetest fish, such as tuna and swordfish.

Picture Credit : Google

          Of course, it is the female seahorse that is the real mother, producing and laying eggs. The difference is that she lays the eggs in a special pouch on the male seahorse’s body. The babies develop inside the pouch and emerge when they are fully developed. As they emerge, it looks as though they are being born from the male seahorse.

          Seahorses and their close relatives the pipefish and the seadragons are very unusual, because it is the males that get pregnant and give birth to the babies. Instead of growing the baby seahorses inside their belly in a uterus, like human mums do, the seahorse dads will carry the babies in a pouch, a bit like a kangaroo’s pouch.

          To produce babies, seahorses have to mate first. Seahorse mating is really beautiful. Males and females dance around one another and flutter their fins, and they may dance together over several days before they actually mate.

          When they’ve decided they like each other, the seahorse females swim towards the surface of the water, and the males follow. The females then put their bright orange eggs into the pouch of the males through the hole at the top of the pouch. Once the eggs are safely inside, the males will add their sperm and shut the opening. The eggs are fertilized by the sperm, and then start developing into baby seahorses.

          With that, the job of the seahorse mum is done! She swims off, and leaves the father to take care of the growing babies. Inside the pouch, the babies grow eyes, tiny snouts, and little tails. It takes about 20 days for the babies to develop, safely tucked away from other animals that might want to eat them.

Picture Credit : Google

          The salmon hatches in freshwater streams and rivers but then begins an incredible journey of up to 5000km (3000 miles), first to the open sea and then to return to the same river in which it was spawned in order to breed. The salmon only makes the journey once —after spawning, it dies. The European eel makes the reverse journey. It spawns in the Sargasso Sea, in the western Atlantic, and its tiny larvae swim to the shores of Europe and North America, becoming lever’s (small eels) on the journey. They then spend several years in freshwater rivers and lakes before returning to the Sargasso Sea to breed. Whales also travel huge distances, this time in search of food. The tiny plankton that they eat is found more abundantly in certain areas during the year.

          Salmon mostly spend their early life in rivers, and then swim out to sea where they live their adult lives and gain most of their body mass. When they have matured, they return to the rivers to spawn. Usually they return with uncanny precision to the natal river where they were born, and even to the very spawning ground of their birth. It is thought that, when they are in the ocean, they use magnetoreception to locate the general position of their natal river, and once close to the river, that they use their sense of smell to home in on the river entrance and even their natal spawning ground.

          A whale shark has made the longest migration journey ever recorded travelling 12,000 miles across the Pacific Ocean. The large fish, named Anne by scientists, was tracked making the mammoth migration from near Panama in the south eastern Pacific, to an area close to the Philippines in the Indo-Pacific. Experts at the Smithsonian Tropical Research Institute followed her signal from Panamanian waters, past Clipperton Island and Costa Rica’s Cocos Island, en-route to Darwin Island in the Galapagos, a site known to attract groups of sharks. The trip was the first recorded evidence of a trans-Pacific migration route for the species of the largest living fish.

          Marine biologist Dr Héctor Guzmán, who first tagged Anne near Coiba Island in Panama, said: “We have very little information about why whale sharks migrate. “Are they searching for food, seeking breeding opportunities or driven by some other impulse?” Genetic studies show that whale sharks across the globe are closely related, suggesting they must travel long distances to mate. An adult female can travel around 40 miles per day and can dive more than 1,900 metres.

          The largest groups of fish are bony fish. Most of these, making up 95% of fish species, are known as teleosts. They have skeletons made of bone and are usually covered with small overlapping bony plates called scales. They also have swim bladders, filled with gas, to help them remain buoyant. Cartilaginous fish include sharks, skates and rays. Their skeletons are made of flexible cartilage but, as they do not have swim bladders, they must keep moving all the time to keep their position in the water. They usually have tough, leathery skins and fleshy fins.

          Bony fish, also known as Osteichthyes, is a group of fish that is characterized by the presence of bone tissue. The majority of the fish in the world belong to this taxonomic order, which consists of 45 orders, 435 families, and around 28,000 species. This class of fish is divided into two subgroups: Actinopterygii (ray-finned) and Sarcopterygii (lobe-finned).

          Cartilaginous fish, also known as Chondrichthyes, is a group of fish that is characterized by the presence of cartilage tissue rather than bone tissue. This class of fish is divided into two subgroups: Elasmobranchii and Holocephali. Common names of cartilaginous fish include sharks, skates, sawfish, rays, and chimaeras.

          The principal difference between bony fish and cartilaginous fish is in the skeleton makeup. As previously mentioned, bony fish have a bone skeleton whereas cartilaginous fish have a skeleton made of cartilage. There are, however, several other differences between these two classes of fish. These differences are listed below.

          The vast majority of cartilaginous fish survive in marine, or saltwater, habitats. These fish can be found throughout the world’s seas and oceans. Bony fish, in contrast, are found in both saltwater and freshwater habitats.

          Fish gills are tissues located on the either side of the throat. These tissues ions and water into the fish’s system, where oxygen from the water and carbon dioxide from the fish are exchanged. In other words, fish gills act as lungs. In bony fish, the gills are covered by an external flap of skin, known as the operculum. In cartilaginous fish, the gills are exposed and not protected by any external skin. The majority of fish, whether bony or cartilaginous, have five pairs of gills.

          Bony and cartilaginous fish are also different in their reproductive behaviors. Bony fish reproduce in what is considered a primitive form of reproduction. These fish produce a large number of small eggs with very little yolk. These eggs are released into the open waters, among rocks on the river or seabed. Male fish then swim over the laid eggs, fertilizing them with sperm which may or may not reach all of the eggs. The eggs hatch into larvae, which are essentially defenseless. The larvae must then develop in the wild, where they are vulnerable to external threats. In this method, the survival rate is low.

          In cartilaginous fish, reproduction occurs internally. The sperm is deposited inside of the female in order to fertilize a small number of large sized eggs with a significant amount of yolk. Cartilaginous fish embryo may develop in one of two manners. In one, the embryo develops within a laid egg, relying on the large yolk for nutrients. In the second, more advanced manner, the embryo are able to develop in the secure and protected environment of the mother’s uterus. These fish are born as fully functional organisms, rather than as developing larvae. After delivery or hatching, baby cartilaginous fish are able to hunt and hide from predators. This development process ensures a higher rate of survival.

          In both classes of fish, the heart is divided into 4 chambers. In the hearts of cartilaginous fish, one of these chambers is known as the conus arteriosus, a special contracting heart muscle. In place of this chamber, bony fish have a bulbous arteriosus, a non-contracting muscle.

          Another difference between the bony and cartilaginous fish is in how each class produces red blood cells. In bony fish, the red blood cells are produced in the bone marrow, the central part of the bone. This process is known as hemopoiesis. Cartilaginous fish lack bone marrow for hemopoiesis. Instead, these fish produce red blood cells in the spleen and thymus organs.

          Fish are the oldest vertebrates on Earth. They are cold blooded and spend all their lives in water. They breathe by taking in oxygen dissolved in the water. Most fish breathe by using gills. They gulp in water through their mouths and pass it out through the gills, which are rich in blood and extract oxygen from the water as it passes through them.

          Despite living in water, fishes need oxygen to live. Unlike land-dwellers, though, they must extract this vital oxygen from water, which is over 800 times as dense as air. This requires very efficient mechanisms for extraction and the passage of large volumes of water (which contains only about 5% as much oxygen as air) over the absorption surfaces.

          To achieve this, fishes use a combination of the mouth (buccal cavity) and the gill covers and openings (opercula). Working together, this form a sort of low-power, efficient pump that keeps water moving over the gas absorption surfaces of the gills. The efficiency of this system is improved by having a lot of surface area and very thin membranes (skin) on the gills. However, these two features also increase problems with osmoregulation, as they also encourage water loss or intake. Consequently, every species must trade off some respiratory efficiency as a compromise for proper osmoregulation.

          Blood passing through the gills is pumped in the opposite direction to the water flowing over these structures to increase oxygen absorption efficiency. This also ensures that the blood oxygen level is always less than the surrounding water, to encourage diffusion. The oxygen itself enters the blood because there is less concentration in the blood than in the water: it passes through the thin membranes and is picked up by hemoglobin in red blood cells, then transported throughout the fish’s body.

          As the oxygen is carried through the body, it diffuses into the appropriate areas because they have a higher concentration of carbon dioxide. It is absorbed by the tissues and used in essential cell functions. The carbon dioxide is produced as a by-product of metabolism. Since it is soluble, it diffuses into the passing blood and is carried away to eventually be diffused through the gill walls. Some of the carbon dioxide may be carried in the blood as bicarbonate ions, which are used as part of osmoregulation by trading the ions for chloride salts at the gills.

          Insects’ extraordinary compound eyes are made up of hundreds of tiny lenses. The images from all the lenses are made sense of by the insect’s brain. Like us, insects can see colour, although in a different way. Flowers that seem dull to us may seem very bright to an insect. As well as having good vision, many insects have sensitive hearing and an acute sense of smell. A female moth, for example, gives off a smell that can be detected by male moths several kilometres away.

          Scientists have long believed insects would not see fine images. This is because their compound eyes typically consist of thousands of tiny lens-capped ‘eye-units’, which together should capture a low-resolution pixelated image of the surrounding world.

          In contrast, the human eye has a single lens, which slims and bulges as it focuses objects of interests on a retinal light-sensor (photoreceptor) array; the megapixel “camera chip” inside the eye. By actively changing the lens shape, or accommodating, an object can be kept in sharp focus, whether close or far away. As the lens in the human eye is quite large and the retinal photoreceptor array underneath it is densely-packed, the eye captures high-resolution images.

          However, researchers from the University of Sheffield’s Department of Biomedical Science with their Beijing, Cambridge and Lisbon collaborators have now discovered that insect compound eyes can also generate surprisingly high-resolution images, and that this has much to do with how the photoreceptor cells inside the compound eyes react to image motion.

          Unlike in the human eye, the thousands of tiny lenses, which make the compound eye’s characteristic net-like surface, do not move, or cannot accommodate. But the University of Sheffield researchers found that photoreceptor cells underneath the lenses, instead, move rapidly and automatically in and out of focus, as they sample an image of the world around them. This microscopic light-sensor “twitching” is so fast that we cannot see it with our naked eye. To record these movements inside intact insect eyes during light stimulation, the researcher had to build a bespoke microscope with a high-speed camera system.

          Remarkably, they also found that the way insect compound eye samples an image (or takes a snapshot) is tuned to its natural visual behaviours. By combining their normal head/eye movements – as they view the world in saccadic bursts – with the resulting light-induced microscopic photoreceptor cell twitching, the insects, such as flies, can resolve the world in much finer detail than was predicted by their compound eye structure, giving them hyperacute vision. The new study, published in the journal e-Life, changes our understanding of insect and human vision and could also be used in industry to improve robotic sensors.