Category Science

          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.

          There are almost, as many different ways in which insects protect themselves from enemies as there are different insects. Some insects, such as wasps and ants, have powerful stings or are able to shower their attackers with poisonous fluid. The hoverfly does not sting, but its colouring is so like that of a wasp or bee that enemies are very wary of it! Other insects, such as stick insects and praying mantises use camouflage. They look like the leaves and twigs among which they feed.

          In the insect community there exist many different methods of hunting and killing. Some of these methods are short and quick, and others seem to be slow and painful. Some insects do not even have to fight by virtue of their spectacular camouflaged bodies. However, other insects are nearly always vulnerable to predators. Many insects sport particular colors that scare predators away and some insects use venom in order to subdue their prey before feasting on it. There are many more methods of attack and defense to be observed in the insect world, and even the few methods named above do not begin to touch upon the great variety of ways that insects attack others and defend themselves.

          Some insects use irritating sprays to subdue their enemies. For example, ladybugs, bombardier beetles, and blister beetles are just a few insects that are capable of deterring predators with unpleasant fluids. The bombardier beetle keeps a caustic substance within its abdomen at all times. When this beetle’s life is threatened by a predator, it will spray the invader with its caustic fluid. While the injured predator is occupied with the caustic substance, the bombardier beetle will make its getaway.

          Another interesting, and largely unheard of defense tactic employed by some arthropods involves the sacrifice of a limb. Many long-legged insects, such as katydids, walkingsticks and craneflies have easily detachable legs, which they are more than happy to give up to a predator if it means getting away alive. These legs have “fracture points” located at certain joints on the legs. When a leg is pulled by a predator, the leg will become detached, leaving the insect alive and the predator with a modest meal.

          This is different than mimicry or camouflage, though it uses the same principle. Some insects “hide in plain sight” by resembling objects in their environment. A thorn could really be a treehopper; a twig might be a walkingstick, an assassin bug, or a caterpillar; and sometimes a dead leaf turns out to be a katydid, a moth, or even a butterfly. Some caterpillars resemble bird droppings, and others have false eyespots on their wings or body to create an imitation of a predator’s head. Often, these guys are the coolest-looking… the details in their appearance astonishing in their accuracy and creativity.

          If there is one thing most of us have in common, it’s distaste for foul smells. And the really bad ones can be enough to make you recoil. Ever been at the epicenter of a skunk attack? It’s like someone is burning tires directly in your NOSE. Stink bugs have special glands that produce a foul-smelling reek. The caterpillar form of some swallowtail butterflies have glands just behind their heads that, when disturbed, will rear up and release a terrible stench. Darkling beetles will raise their big, black butt in warning when they are threatened, and if you don’t pay attention to the warning – will expel acrid, foul-smelling fluid.

          When stink and burning isn’t enough, some bugs will hit their attackers with sticky compounds that harden like glue and incapacitate. Some kinds of cockroaches guard their backsides with a slimy anal secretion (those are three words that are just terrible together) that cripples any ants that launch an attack. And there are types of soldier termites that have nozzle-like heads that can spays sticky, immobilizing toxic fluids at attackers as varied as ants, spiders, centipedes, and other predatory arthropods.