Chondrichthyes
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The Chondrichthyes or cartilaginous fish
- Subclass Elasmobranchii (sharks, rays and skates)
- * Superorder Batoidea (rays and skates), containing the orders:
- *# Rajiformes (common rays and skates)
- *# Pristiformes (Sawfishes)
- *# Torpediniformes (electric rays)
- * Superorder Selachimorpha (sharks), containing the orders:
- *# Hexanchiformes Two families are found within this order. Species of this order are distinguished from other sharks by having additional gill slits (either six or seven). Examples from this group include the cow sharks, frilled shark and even a shark that looks on first inspection to be a marine snake.
- *# Squaliformes Three families and more than 80 species are found within this order. These sharks have two dorsal fins, often with spines, and no anal fin. They have teeth designed for cutting in both the upper and lower jaws. Examples from this group include the bramble sharks, dogfish and roughsharks.
- *# Pristiophoriformes One family is found within this order. These are the sawsharks, with an elongate, toothed snout that they use for slashing the fishes that they then eat.
- *# Squatiniformes One family is found within this order. These are flattened sharks that can be distinguished from the similar appearing skates and rays by the fact that they have the gill slits along the side of the head like all other sharks. They have a caudal fin (tail) with the lower lobe being much longer in length than the upper, and are commonly referred to as angel sharks.
- *# Heterodontiformes One family is found within this order. They are commonly referred to as the bullhead, or horn sharks. They have a variety of teeth allowing them to grasp and then crush shellfishes.
- *# Orectolobiformes Seven families are found within this order. They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs and the largest of all fishes, the whale sharks. They are distinguished by having barbels at the edge of the nostrils. Most, but not all are nocturnal.
- *# Carcharhiniformes Eight families are found within this order. It is the largest order, containing almost 200 species. They are commonly referred to as the groundsharks, and some of the species include the blue, tiger, bull, reef and oceanic whitetip sharks (collectively called the requiem sharks) along with the houndsharks, catsharks and hammerhead sharks. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
- *# Lamniformes Seven families are found within this order. They are commonly referred to as the mackerel sharks. They include the goblin shark, basking shark, megamouth, the thresher, mako shark and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes contains the extinct Megalodon (Carcharodon megalodon), which like most extinct sharks is only known by the teeth (the only bone found in these cartilaginous fishes, and therefore are often the only fossils produced). A reproduction of the jaw was based on some of the largest teeth (up to almost 7 inches in length) and suggested a fish that could grow 120 feet in length. The jaw was realized to be inaccurate, and estimates revised downwards to around 50 feet.
- Subclass Holocephali (chimaera)
Characteristics
Not only does this class have internal fertilization and a reproduction strategy that reminds about what is seen in amniotes, they have also a relative brain development of its major divisions which reminds about what is found in birds and mammals. Their relative brain weight comes close to that of mammals, and is about ten times of bony fishes at the same size. There are not surprisingly some exceptions; the bony fishes mormyrids have a relative brain size to be compared with the brain size of humans, while the primitive Megamouth have a brain of only 0.002 percent of its body weight. One of the explanations why they can have such large brains is because they are using much less energy than one would expect. The density of nerve cells are much lower than in the brain of bony fishes, making it less energy demanding and allows it to be bigger.Their digestive systems have spiral valves, and with the exception of Holocephali they also have a cloaca.
Because they don't have any bone marrow, the red blood cells must be produced somewhere else. The spleen and special tissue around the gonads is where the production of red blood cells can be found, as well as in a special organ called Leydig's Organ that is only found in cartilaginous fishes, even if some have lost it. Another unique organ is named the epigonal organ, and probably has a role in the immune system. The subclass Holocephali, which is a very specialized group, lacks both of these organs.
Originally the pectoral and pelvic girdles, which do not contain any dermal elements, didn't connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and pubioischiadic bars evolved. In rays, the pectoral fins have connected to the head and are very flexible.
A spiracle is found behind each eye on most species, even if Holocephali and some pelagic sharks have lost it
Their tough skin is covered with dermal teeth (again with Holocephali as an exception as the teeth are lost in adults, only kept on the clasping organ seen on the front of the male's head), also called placoid scales or dermal denticles, making it feel like sandpaper. It is assumed that their oral teeth evolved from dermal denticles who migrated into the mouth. But it could also be the other way around as a teleost bony fish named Denticeps clupeoides, the only member in the family Denticipitidae, has most of its head covered by dermal teeth (as do probably Atherion elymus, another species of bony fish). This is most probably a secondary evolved characteristic. Which means there is not necessary a connection between the teeth and the original dermal scales. The old placoderms didn't have teeth at all, but had sharp bony plates in their mouth. So what came first, the oral teeth or the dermal teeth, nobody can for the moment tell for sure. Neither is it sure how many times it has happened if it turns out to be the case. Someone has even suggested that the original bony plates of all the vertebrates are gone and that the present scales are just modified teeth, even if both teeth and the body armour have a common origin a long time ago. But for the moment there is no evidence of this.
References
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