2کلیات: ماهي مهره دار خونسرد آبزي است كه معمولاً با آبشش تنفس مي كند و با اندامي بنام باله شنا مي كند(استربا و هابیل 1989). موجودی که حداقل دارای آرواره و فک بوده اسکلت غضروفی یا استخوانی و باله دارد و با آبشش تنفس می کند. ماهيها متنوع ترين و پرتعداد ترين گروه از مهره داران مي باشند. طبق گزارش نلسون3 در سال 1994 تعداد كل گونه هاي ماهي شناخته شده 17000تا30000 گونه مي باشد كه وي آنها را در 50 راسته 445 خانواده و 4044 جنس قرار داده است،كه از بين آنها خانوادة كپورماهيان با 210 جنس و 2010 گونه بزرگترين خانواده مي باشد. بر طبق گزارش پايگاه اطلاعاتي ماهي تعداد كل گونه هاي شناخته شده گونه است كه به اين ترتيب بيش از نيمي از مهره داران امروزي را شامل مي شوند كه در آبهاي جهان گسترده شده اند. اين گسترش به لحاظ تنوع شگفت آوري است كه از نظر سازگاري هاي ريخت شناسي، فيزيولوژيكي و رفتاري از خود نشان داده اند (ستاري 1382) یونگ و هیلدبراند هردو ماهیان را رده هایی از زیر شاخه ورتبراتا به شمار آورده اند اما یونگ 4 رده ( شامل پلاکو درمی- الاسموبرانشی- اکتینوپتری جی- کراس آپتی جی)ولی هیلدبراند آنها را در سه رده (شامل پلاکودرمی – کندروایکتیس (ماهیان غضروفی)– استو ایکتیس (ماهیان استخوانی)) طبقه بندی می کند. در این درس بر مبنای طبقه بندی هیلدبراند مطالعه می کنیم.
3زیر شاخه ورتبراتا رده پلاکودرمی Class Placodermi Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches. A 380-million-year-old fossil of one species represents the oldest-known example of live birth.
4پلاکودرم ها، که اغلب "دایناسور دریایی" خوانده می شدند، به مدت 70 میلیون سال بر دریاچه ها و دریاهای جهان حکمرانی می کردند. اکثر ماهی های زرهدار، کوچک بودند ولی طول بعضی هایشان تا بیش از 6 متر می رسید. پلاکودرم ها مربوط به دوره دونین "Devonian Period" (از دوره های زمین شناسی که شامل 354 تا 417 میلیون سال قبل است) می باشند که در آن دوره بتدریج حیوانات خشکی از ماهی ها بوجود آمدند.
5زیر شاخه ورتبراتا رده ماهیان غضروفی Class Chondrichthyes
6برخی ویژگی های عمومی سرتاسر دریاها پهنه زیست آنهاست گوشتخوارند و یا لاشخوار (scavenger) باله دمی هتروسرکال دارند. نوتوکورد جای خود را به ستون مهره داده است مهره ها از نوع آمفی سلوس است. پولک ( scale) در آنها از نوع پلاکوئید است.
7Class ChondrichthyesAbout 1000 living species divided into two distinct groupsNeoselachii [also known as elasmobranchs] (sharks, skates and rays) about 950 species.Holocephalii (ratfishes). About 33 species.
8Class ChondrichthyesModern Chondricthyes include the sharks, rays and Chimeras.The Chondrichthyes’ well-developed jaws, highly developed sense organs, powerful swimming ability and streamlined shape have enabled them to thrive as marine predators for more than 350 million years, as other groups have come and gone.There are just under 1000 living species, all of which have cartilaginous skeletons, even though they are descended from ancestors that had bone.
9Class ChondrichthyesThe Chondrichthyes are an ancient group that although not as diverse as the bony fishes have persisted largely unchanged for hundreds of millions of years.The oldest unambiguous Chondrichthyans are found in the Early Devonian although there are older fossils of scales.
10SharksA typical shark is about 2m long, but they range in size from a few miniature forms that are 25 cm long up to perhaps 18m in length.Despite their range of sizes all modern sharks share a suite of characteristics.
11Characteristics of sharks The cartilaginous vertebral centra of sharks are distinctive.Adjacent vertebrae have depressions in their faces into which fit spherical remnants of the notochord.This arrangement of a rigid vertebral column of calcified cartilage swivelling on bearings of notochord allows the axial skeleton to swing from side to side.
12SharksUnlike earlier sharks, living species have their skin entirely covered in dermal placoid scales, which are small tooth-like structures (with enamel, dentine and pulp just like real teeth).These scales give sharkskin a tough, leathery and abrasive feel. The skin is also very streamlined.
21سیاهرگ کاردینال یا کووییرداکت قلب و گردش خونسیاهرگ کاردینال یا کووییرداکتقلب دارای چهار بخش است اولین بخش سینوس سیاهرگی است از انجا به دهلیز و از دهلیز به بطن راه می یابد از بطن به ائورت شکمی و از آنجا به ابششها پمپ می شود.
22آبششها از 5 تا 7جفت ابشش دارند. آبششها از 5 تا 7جفت ابشش دارند.A holobranch (gill) from a shark.
26تغذیهگوشتخوار(شکارگر) – لاشخوار هستند در برخی گیاهخواری هم گزارش شده است روده از یک بخش پرپیچ و خم ابتدایی و یک بخش صاف انتهایی با غددی خاص بنام غدد رکتالی که نقش دفع نمک را دارند تشکیل شده است
27TeethThe placoid scales are modified in the mouth to produce the rows of replaceable teeth characteristic of sharks.Each tooth in a shark can be rapidly replaced as it becomes worn or damaged. Teeth are arranged on a spiral or whorl shaped cartilaginous band in which replacement teeth are always developing behind the functional tooth.Teeth in young sharks may be replaced as often as once every 8 days.
30Shark JawsA shark’s jaws can open in a variety of different positions depending on the prey.This is because the upper jaw is attached flexibly to the chondocranium in two locations (front and back) both of which can move. This is called a hyostylic jaw suspension.(Movement of parts of the head skeleton is called cranial kinesis.)
31Shark JawsWhen the upper jaw is protruded, the hyomandibular cartilage which braces the rear of the upper jaw (the palatoquadrate) swings to the side and anteriorly which increases the distance between the right and left jaw articulations and the volume of the mouth.
32Shark JawsThe increase in volume is possible because the upper jaw attachment to the chondocramnium at the front is by elastic ligaments and so the upper jaw can move.The increase in volume powerfully sucks water and food into the mouth.
36Prey detectionSharks use a series of methods to detect prey related to distance.Chemoreception is used to detect prey from a distance and sharks appear to be able to detect odors as dilute a 1 part in 10 billion.
37Prey detectionVibrations can also be detected from a distance using the lateral line system.Once a shark gets relatively close, vision takes over.Sharks have very good vision at low light intensities. There is a high density of rods in the retina and a tapetum lucidum just behind the retina, which reflects light back through the retina.
38Prey detectionIn low light conditions the tapetum lucidum is beneficial, but in bright light is not.In bright light melanin containing cells expand to cover the tapetum lucidum.
39Prey detectionIf a familiar prey item is located an attack may occur quickly.If the prey is unfamiliar (e.g. a person) the shark may circle to gather more information.Such a shark may bump the potential prey with its rostrum presumably to gather extra sensory information.
40اکثرا زنده زا هستند و لی تخمگذاری هم در برخی دیده می شود. تولید مثلاکثرا زنده زا هستند و لی تخمگذاری هم در برخی دیده می شود.
41ReproductionReproduction in all Chondrichthyes is internal and the male uses modified pelvic fins called claspers to insert sperm.The presence or absence of claspers makes it easy to distinguish male from females.
45ReproductionDuring copulation a clasper is inserted into the female’s cloaca and hooked in place by spines at the tip.Sperm is ejaculated into a groove in the clasper and a muscular siphon sac filled with seawater is squeezed which washes the sperm down the groove into the cloaca from where the sperm swim up the female’s reproductive tract.
46ReproductionThe sharks use of internal fertilization is coupled with their use of a reproductive strategy in which a few young are invested in heavily.The energy investment is provided by the female who retains and nourishes a small number of offspring within her body.
47ReproductionAll skates and some sharks are oviparous and lay eggs soon after fertilization. The eggs hatch later.Most oviparous sharks produce large eggs with big yolks and a proteinaceous case is secreted around the fertilized egg.
48ReproductionOther sharks are ovoviviparous. The eggs develop within the mothers body and hatch either in her or just after being released from her.
49Embryo of deep sea cat shark. There is a very large yolk sac to Egg case of cat sharkEmbryo of deep sea cat shark.There is a very large yolk sac tosupport the embryo’s growth.
50ReproductionThe remaining species of shark are viviparous and the offspring are nourished by a placenta, unfertilized eggs or smaller siblings.These forms of food supply are collectively referred to as matrotrophy.
51Placental feeding of young Some sharks develop long stringy extensions of the oviduct. These secrete a milky substance into the mouths and gill openings of the young.The commonest form of viviparity in sharks uses a yolk sac placenta which allows the developing baby to obtain nutrition from its mothers blood stream.
52Life history strategy of sharks Sharks because they invest heavily in individual offspring produce relatively few young.This reproductive strategy is similar to that of humans and elephants.
53Fossil history of Chondrichtyes One of the best known extinct genera is Cladoselache a pelagic marine predator from the Devonian.It was shark-like in appearance. About 2 meters long with a large gape and three-pronged teeth. As in modern sharks the teeth were arranged on a ligamentous band in a whorl-shaped arrangement.
54Fossil history of Chondrichtyes Cladoselache had two dorsal fins, each preceded by a large spine.It also possessed paired pelvic and pectoral fins as in modern sharks, but the fins were much more broad based than in later sharks.The tail was symmetrical externally, but internally asymmetrical with the notochord extending into the upper lobe of the tail.
56Fossil history of Chondrichtyes Cladoselache’s skin had few scales found on the fins and around the eyes.In addition, Cladoselache lacked the rostrum of modern sharks.
57Fossil history of Chondrichtyes A contemporary genus of Cladoselache was Xenacanthus a freshwater shark.A bottom dweller with robust fins and a heavily calcified skeleton.Xenacanths appeared in the Devonian and died out in the Triassic.
60Extant radiation of Chondrichthyes By the Jurassic sharks of modern appearance had evolved. Several genera from that era are still extant.The most distinctive feature of modern sharks is the rostrum or snout that overhangs the mouth.Less prominent, but also of major importance was the evolution of solid calcified vertebrae.Finally, the teeth are covered with thicker more complex enamel than in earlier sharks.
70Great White SharkGreat White sharks specialize in feeding on colonial seals and sealions, but also take a wide variety of other prey including dolphins, other sharks, turtles and other fish.Around sea lion nursery areas sharks attack the mammals as they come and go. They remain deep in the water until a victim passes within range above and then rocket to the surface like a trout after a mayfly often exploding out of the water and flinging the prey in the air.
72Shortfin makoBecause it hunts such fast prey, makos have to be fast and athletic. Its speed has been recorded at 50km/h (31 mph), but in bursts it can accelerate to 74 km/h (46 mph).Makos often leap high out of the water in pursuit of prey and there have been several instances of hooked makos landing on the decks of fishing boats.
74Thresher SharkA Thresher shark is instantly identifiable thanks to the enormously elongated upper lobes of its tailfin.The tail plays a central role in their hunting strategy. Either working alone or in groups threshers surround groups of pelagic fish and stun or disorient them using their tails.
76Tiger sharkTiger sharks are indiscriminate consumers and will eat almost anything.Their powerful jaws allow them to crack turtles shells and clams.Stomach contents of captured sharks have included seals, sea snakes, birds, fish, squid and even old tires.
77Tiger sharkTiger sharks trail only great whites in numbers of attacks on people, but because they will eat almsot anything they rarely leave after biting a human, as great whites often do.
79Cookiecutter sharkCookiecutter are bizarrely specialized predators that bite disk-shaped pieces of tissue out of much larger animals.Cookiecutter sharks attach to their prey with their lips and then quickly spin using their proportionally enormous teeth to carve out a piece of flesh.Cookiecutter sharks feed on megamouth, basking and whale sharks as well as fish such as tuna and marlin as well as dolphins and whales.
80Cookiecutter sharkCookiecutters are bioluminescent and appear to use this ability to attract victims.On the ventral surface cookiecutter’s glow along their whole length except for a dark patch of skin under the jaw.The bioluminescent areas hide the shark against the light of the surface water, but the dark patch stands out and acts as a lure for predatory fish, which when they attack end up being bitten by the shark.
82Whale SharkWhale sharks are filter feeder that sieve plankton, krill and other small prey from the water.The prey is trapped using 10-cm long gill rakers, which are bristle-like structures that sieve the water before it passes through the gill slits.Whale sharks filter about 1500 gallons (6000 liters) of water each hour. Basking sharks and megamouth sharks also filter feed.
84Skates and raysMore than half of all elasmobranchs are skates and rays.More species (about 534) than there are sharks.They have characteristically dorsoventrally flattened bodies and greatly enlarged pectoral fins, which they swim with using a wavelike motion.
85Skates and raysSkates and rays should not be confused with flatfishes (e.g. sole and halibut), which are bony fishes.Skates and rays have gill slits placed ventrally and eyes dorsally placed.In flatfish the body is twisted during development to bring both eyes and gills to the dorsal surface, but not symmetrically.
86Skates and raysThe group is specialized for bottom dwelling and feeding on hard foods (e.g. molluscs and crustaceans) that have to be ground up.Teeth are flat crowned plates that form an arrangement like paving stones.The mouth is located underneath the body and can be rapidly protruded to suck up prey.
87Differences between skates and rays Skates have an elongated but thick tail stalk, which has two dorsal fins and a caudal fin at the end.Skates are oviparous.Generally skates also have a rostrum a pointed nose-like extension of the braincase.Rays typically have a whip-like tail and the fins are replaced by serrated venom-containing barbs.Rays are viviparous and most lack a rostrum
89Skates and raysThe spiracles (openings behind the eye) are much larger in rays than in sharks because water for the gills enters exclusively through them as the mouth is usually buried in the sand.
90Skates and raysSkates and rays are usually well camouflaged and sit on the bottom. A few species are dangerous because of their sharp and barbed tail (stingrays) or because they can generate severe electric shocks (electric rays).Most species are bottom feeders that eat invertebrates. However, the largest species (e.g. manta rays) as in sharks are planktivores.
93Subclass Holocephali: Chimaeras Chimaeras are a small group (about 33 species) of deep sea (>80m) cartilaginous fishes known commonly as ratfish or ghostfish.Because they live mainly in deep water they are not a well known group.
95Subclass Holocephali: Chimaeras They have a large head, plate-like grinding teeth, a cover over the gills and lack both a spiracle and stomach.They appear to mostly feed on sea urchins, shrimp, and mollusks.The tail is thin and tapers to a point (hence the name ratfish) and not much use in swimming. Instead chimaeras depend on flapping their pectoral fins for much of their movement.