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Sharks are a group of fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, andpectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. However, the term “shark” has also been used for extinct members of the subclassElasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus. Under this broader definition, the earliest known sharks date from more than 420 million years ago.

Since then, sharks have diversified into over 470 species. They range in size from the small dwarf lanternshark(Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (39 ft). Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive in both seawater and freshwater.] They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have several sets of replaceable teeth. Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark areapex predators—organisms at the top of their underwater food chain

Tetraodontidae is a family of primarily marine and estuarine fish of the order Tetraodontiformes. The family includes many familiar species, which are variously called pufferfish, puffers, balloonfish, blowfish, bubblefish, globefish, swellfish,toadfish, toadies, honey toads, sugar toads, and sea squab.[1] They are morphologically similar to the closely relatedporcupinefish, which have large external spines (unlike the thinner, hidden spines of Tetraodontidae, which are only visible when the fish has puffed up). The scientific name refers to the four large teeth, fused into an upper and lower plate, which are used for crushing the shells of crustaceans and mollusks, their natural prey.

Pufferfish are generally believed to be the second-most poisonous vertebrates in the world, after the golden poison frog. Certain internal organs, such as liver, and sometimes the skin, contain tetrodotoxin and are highly toxic to most animals when eaten; nevertheless, the meat of some species is considered a delicacy in Japan (as 河豚, pronounced as fugu), Korea (as 복bok or 복어 bogeo ), and China (as 河豚 hétún) when prepared by specifically trained chefs who know which part is safe to eat and in what quantity. The Tetraodontidae contain at least 120 species of puffers in 19 genera.[1] They are most diverse in the tropics, relatively uncommon in the temperate zone, and completely absent from cold waters. They are typically small to medium in size, although a few species can reach lengths of greater than 100 cm (39 in).

The paradise fish, paradisefish, or paradise gourami, Macropodus opercularis, is a species of gourami found in most types of fresh water in East Asia, ranging from the Korean Peninsula to northern Vietnam. This species can reach a length of 6.7 cm (2.6 in), though most are only about 5.5 cm (2.2 in). Paradise gouramis were one of the first ornamental fish available to western aquarium keepers, having been imported toEurope as early as the 19th century. The paradise fish is one of the more aggressive members of its family. It is more aggressive than the three spot gourami, yet less pugnacious in nature than the less commonly kept combtail.

Paradise fish are fairly combative, harassing and attacking each other, as well as potentially killing small fish. In the wild, they are predators, eating insects,invertebrates, and fish fry. The popularity of this species has waned in recent decades as much more colorful (and often less pugnacious) species of gouramishave become widely available to hobbyists. This species is one of the few fish that can change its color (lighter or darker) in response to stimuli.

Eels are elongated fish, ranging in length from 5 cm (2.0 in) in the one-jawed eel (Monognathusahlstromi)[dubious – discuss] to 4 m (13 ft) in the slender giant moray.[2] Adults range in weight from 30 grams (1.1 oz) to well over 25 kilograms (55 lb). They possess no pelvic fins, and many species also lack pectoral fins. The dorsal and anal fins are fused with the caudal or tail fin, forming a single ribbon running along much of the length of the animal. Most eels live in the shallow waters of the ocean and burrow into sand, mud, or amongst rocks. A majority of eel species are nocturnal, and thus are rarely seen. Sometimes they are seen living together in holes, or “eel pits”. Some species of eels also live in deeper water on the continental shelves and over the slopes deep as 4,000 m (13,000 ft). Only members of the Anguilla family regularly inhabit fresh water, but they too return to the sea to breed. Eels begin life as flat and transparent larvae, or leptocephali.

Eel larvae drift in the surface waters of the sea, feeding onmarine snow, small particles that float in the water. Eel larvae then metamorphose into glass eels and then becomeelvers before finally seeking out their juvenile and adult habitats. Freshwater elvers travel upstream and are forced to climb up obstructions, such as weirs, dam walls, and natural waterfalls. Lady Colin Campbell found, at Ballisodare, the eel fisheries were greatly improved by the hanging of loosely plaited grass ladders over barriers, enabling the elvers to ascend. The heaviest true eel is the European conger. The maximum size of this species has been reported as reaching a length of 3 m (10 ft) and a weight of 110 kg (240 lb). Other eels are longer but do not weigh as much, such as the slender giant moray which reaches 4 m (13 ft).

The guppy (Poecilia reticulata), also known as million fish and rainbow fish,[1] is one of the world’s most widely distributed tropical fish, and one of the most popularfreshwater aquarium fish species. It is a member of the Poeciliidae family and, like all other members of the family, is live-bearing.[2] Guppies, whose natural range is in northeast South America, were introduced to many habitats and are now found all over the world. They are highly adaptable and thrive in many different environmental and ecological conditions.[3] Male guppies, which are smaller than females, have ornamental caudal and dorsal fins, while females are duller in colour. Wild guppies generally feed on a variety of food source including benthic algae and aquatic insectlarvae.[4] Guppies are used as a model organism in the field of ecology, evolution, and behavioural studies.[3]

They are the most ferocious fish in the world. Even the most formidable fish, the sharks or the barracudas, usually attack things smaller than themselves. But the piranhas habitually attack things much larger than themselves. They will snap a finger off a hand incautiously trailed in the water; they mutilate swimmers—in every river town in Paraguay there are men who have been thus mutilated; they will rend and devour alive any wounded man or beast; for blood in the water excites them to madness. They will tear wounded wild fowl to pieces; and bite off the tails of big fish as they grow exhausted when fighting after being hooked. But the piranha is a short, deep-bodied fish, with a blunt face and a heavily undershot or projecting lower jaw which gapes widely. The razor-edged teeth are wedge-shaped like a shark’s, and the jaw muscles possess great power.

The rabid, furious snaps drive the teeth through flesh and bone. The head with its short muzzle, staring malignant eyes, and gaping, cruelly armed jaws, is the embodiment of evil ferocity; and the actions of the fish exactly match its looks. I never witnessed an exhibition of such impotent, savage fury as was shown by the piranhas as they flapped on deck. When fresh from the water and thrown on the boards they uttered an extraordinary squealing sound. As they flapped about they bit with vicious eagerness at whatever presented itself. One of them flapped into a cloth and seized it with a bulldog grip.

Another grasped one of its fellows; another snapped at a piece of wood, and left the teeth-marks deep therein. They are the pests of the waters, and it is necessary to be exceedingly cautious about either swimming or wading where they are found. If cattle are driven into, or of their own accord enter, the water, they are commonly not molested; but if by chance some unusually big or ferocious specimen of these fearsome fishes does bite an animal—taking off part of an ear, or perhaps of a teat from the udder of a cow—the blood brings up every member of the ravenous throng which is anywhere near, and unless the attacked animal can immediately make its escape from the water it is devoured alive.[28]

A tuna is a saltwater finfish that belongs to the tribe Thunnini, a sub-grouping of the mackerel family (Scombridae) – which together with the tunas, also includes the bonitos, mackerels, and Spanish mackerels. Thunnini comprises fifteen species across five genera,[1] the sizes of which vary greatly, ranging from the bullet tuna (max. length: 50 cm (1.6 ft), weight: 1.8 kg (4 lb)) up to theAtlantic bluefin tuna (max. length: 4.6 m (15 ft), weight: 684 kg (1,508 lb)). The bluefin averages 2 m (6.6 ft), and is believed to live for up to 50 years. Their circulatory and respiratory systems are unique among fish, enabling them to maintain a body temperature higher than the surrounding water. An active and agile predator, the tuna has a sleek, streamlined body, and is among the fastest-swimming pelagic fish – the yellowfin tuna, for example, is capable of speeds of up to 75 km/h (47 mph).[2] Found in warm seas, it is extensively fished commercially and is popular as a game fish. As a result of over-fishing, stocks of some tuna species, such as the Southern bluefin tuna, have been reduced dangerously close to the point of extinction.[3]

The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a “hammer” shape called a “cephalofoil”. Most hammerhead species are placed in the genus Sphyrna while the winghead sharkis placed in its own genus, Eusphyra. Many not necessarily mutually exclusive functions have been proposed for the cephalofoil, including sensory reception, maneuvering, and prey manipulation. Hammerheads are found worldwide in warmer waters along coastlines and continental shelves. Unlike most sharks, hammerheads usually swim in schools during the day, becoming solitary hunters at night. Some of these schools can be found near Malpelo Island in Colombia, Cocos Island off Costa Rica, and near Molokai Island in Hawaii. Large schools are also seen in southern and eastern Africa.

Starfish or sea stars are star-shaped echinoderms belonging to the classAsteroidea. Common usage frequently finds these names being also applied toophiuroids, which are correctly referred to as “brittle stars” or “basket stars”. About 1,500 species of starfish occur on the seabed in all the world’s oceans, from the tropics to frigid polar waters. They are found from the intertidal zonedown to abyssal depths, 6,000 m (20,000 ft) below the surface. Starfish are marine invertebrates. They typically have a central disc and five arms, though some species have more than this. The
aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic systemand a mouth at the centre of the oral or lower surface. They are opportunisticfeeders and are mostly predators on benthic invertebrates.

Several species have specialized feeding behaviours including eversion of their stomachs andsuspension feeding. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts or lost arms and they can shed arms as a means of defence. The Asteroidea occupy several significant ecological roles. Starfish, such as the ochre sea star (Pisaster ochraceus) and the reef sea star (Stichaster australis), have become widely known as examples of the keystone species concept in ecology. The tropicalcrown-of-thorns starfish (Acanthaster planci) is a voracious predator of coralthroughout the Indo-Pacific region, and the northern Pacific sea star is considered to be one of the world’s 100 worst invasive species.

The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather poor, as starfish tend to disintegrate after death. Only the ossicles and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture. They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.

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