Shark - Wikipedia
Just after dawn in Kalyan, on the outskirts of Mumbai, India, trash pickers It's unclear how long it will take for that plastic to completely biodegrade into its Meanwhile, ocean plastic is estimated to kill millions of marine animals every year. . Dissecting the fish, he was surprised to find microplastics in the guts of more than. View a list of Things to Do in Laguna Beach at Little or No Cost. The Laguna Art Museum also offers free admission during this monthly event from The artists' village offers the complete spectrum of fine art in a casual These bungalows and cottages date as far back as and contribute to Laguna Beach's rich. The evolution of fish began about million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Fish may have evolved from an animal similar to a coral-like sea squirt (a.
Agnatha comes from the Greekand means "no jaws". Although a minor element of modern marine faunajawless fish were prominent among the early fish in the early Paleozoic. Two types of Early Cambrian animal apparently having fins, vertebrate musculature, and gills are known from the early Cambrian Maotianshan shales of China: They have been tentatively assigned to Agnatha by Janvier.
A third possible agnathid from the same region is Haikouella. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shale of British Columbia.
The first armoured agnathans—the Ostracodermsprecursors to the bony fish and hence to the tetrapods including humans —are known from the middle Ordovicianand by the Late Silurian the agnathans had reached the high point of their evolution.
Most of the ostracoderms, such as thelodontsosteostracansand galeaspidswere more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes.
Cyclostomes apparently split from other agnathans before the evolution of dentine and bone, which are present in many fossil agnathans, including conodonts. The agnathans as a whole are paraphyletic because most extinct agnathans belong to the stem group of gnathostomes.
Physiology Buoyancy Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver filled with oil that contains squaleneand their cartilage, which is about half the normal density of bone. Sand tiger sharks store air in their stomachs, using it as a form of swim bladder.
Bottom-dwelling sharks, like the nurse sharkhave negative buoyancy, allowing them to rest on the ocean floor. Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers use this condition to handle sharks safely. Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a spiracle lies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom—dwelling sharks.
Spiracles are reduced or missing in active pelagic sharks. While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are obligate ram ventilators and would presumably asphyxiate if unable to move.
Obligate ram ventilation is also true of some pelagic bony fish species. Here the shark pumps blood to its gills via the ventral aorta artery where it branches into afferent brachial arteries.
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Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses.
From there blood enters the heart ventricle and the cycle repeats. Members of the family Lamnidae such as the shortfin mako shark and the great white shark are homeothermic and maintain a higher body temperature than the surrounding water. In these sharks, a strip of aerobic red muscle located near the center of the body generates the heat, which the body retains via a countercurrent exchange mechanism by a system of blood vessels called the rete mirabile "miraculous net".
The common thresher and bigeye thresher sharks have a similar mechanism for maintaining an elevated body temperature. This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to marine environments.
A few exceptions exist, such as the bull sharkwhich has developed a way to change its kidney function to excrete large amounts of urea.
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The food moves from the mouth to a J-shaped stomach, where it is stored and initial digestion occurs. This short length is achieved by the spiral valve with multiple turns within a single short section instead of a long tube-like intestine. The valve provides a long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the cloaca.
Sharks have keen olfactory senses, located in the short duct which is not fused, unlike bony fish between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater. They are more attracted to the chemicals found in the intestines of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharkshave external barbels that greatly increase their ability to sense prey.
Sight Eye of a Bigeyed sixgill shark Hexanchus nakamurai Shark eyes are similar to the eyes of other vertebratesincluding similar lensescorneas and retinasthough their eyesight is well adapted to the marine environment with the help of a tissue called tapetum lucidum. This tissue is behind the retina and reflects light back to it, thereby increasing visibility in the dark waters.
The effectiveness of the tissue varies, with some sharks having stronger nocturnal adaptations. Many sharks can contract and dilate their pupilslike humans, something no teleost fish can do.
Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have nictitating membranes. This membrane covers the eyes while hunting and when the shark is being attacked. However, some species, including the great white shark Carcharodon carchariasdo not have this membrane, but instead roll their eyes backwards to protect them when striking prey.
The importance of sight in shark hunting behavior is debated. Some believe that electro- and chemoreception are more significant, while others point to the nictating membrane as evidence that sight is important. Presumably, the shark would not protect its eyes were they unimportant.
The use of sight probably varies with species and water conditions. The shark's field of vision can swap between monocular and stereoscopic at any time. The remaining seven species had in addition to rods a single type of cone photoreceptor sensitive to green and, seeing only in shades of grey and green, are believed to be effectively colorblind. The study indicates that an object's contrast against the background, rather than colour, may be more important for object detection.
If you date online, take precautions to protect yourself. Here are six things to keep in mind to help you spot and avoid scammers on online dating sites.
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If you fall into this category, be especially wary of people that you meet through dating websites. The AARP also says that seniors are a common target of these scams. Again, both men and women can and have fallen victim to online dating scammers, but women tend to be targeted more aggressively.
Interestingly, the AARP says that men fall victim to these scams more often, but that women are more likely to report the scam.
Profile Warning Signs The profiles of online dating scammers can exhibit some clear signs that something is off—you just need to know what to look for. Scammers also often list themselves as widowed especially with a childself-employed, or working overseas.
The photos used by scammers can also clue you in that something is off. The reverse search engine that usually gets most of the clicks is TinEye. If that image shows up on other profiles with different names, you should be suspicious. Read Morebut it could also be a scammer. If you receive other photos, and anything seems off, be wary.