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Cartilaginous and Bony Fishes

Chondryichthyans

Figure 61. Shark_fish_chondrichthyes.jpg

Figure 4. Shark fish (chondryichthyes)
"Diversas lampreas.1 - Aquarium Finisterrae" by Knepp Timothy, U.S. Fish and Wildlife Service, Wikimedia Commons is in the Public Domain

 

The Chondryichthyes (cartilaginous fish) are one of the three extant major clades of jawed vertebrates and comprise two sister groups, the elasmobranchs (sharks, skates and rays), and the holocephalans (chimeras and ratfish). Although relatively species-poor by comparison to their osteichthyan (bony vertebrate) relatives (about 970 species of chondrichthyan compared to about 55,600 species of bony vertebrates), they occupy an important phylogenetic position as the sister group to all other jawed vertebrates and as one of the first lineages to diverge after the two rounds of whole-genome duplications that occurred early in vertebrate evolution.

 

Cartilaginous Fish

First appearing on Earth almost 450 million years ago, cartilaginous fish today include both fearsome predators and harmless mollusk eaters. Members of chondryichthyes all lack true bone and have a skeleton made of cartilage. Only their teeth, and sometimes their vertebrae, are calcified; this calcified cartilage has a different structure from that of true bone.

 

Sharks

Sharks are found in all seas. They generally do not live in fresh water, with a few exceptions such as the bull shark and the river shark which can swim both in seawater and freshwater. Sharks possess a full cartilaginous skeleton and a highly streamlined body, and are perhaps the most successful member of the chondrichthyes (Figure 61). Evidence for the existence of sharks dates from the Ordovician period, more than 420 million years ago, before land vertebrates existed and before many plants had colonized the continents. Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks. The oldest generally accepted shark scales are from about 420 million years ago, in the Silurian period.

Sharks practice internal fertilization. Unlike most bony fish, sharks are K-selected reproducers, meaning that they produce a small number of well-developed young as opposed to a large number of poorly developed young. 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 squalene and the fact that cartilage is about half as dense as bone. The liver constitutes up to 30 percent of a shark's body mass. The liver's effectiveness is limited, so sharks employ dynamic lift to maintain depth, sinking when they stop swimming. Sand tiger sharks store air in their stomachs, using it as a form of swim bladder. Most sharks need to constantly swim in order to breathe and cannot sleep very long, if at all, without sinking. Certain shark species, like the nurse shark, however, are capable of pumping water across their gills, allowing them to rest on the ocean bottom.

 

Rays and Skates

Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in 13 families. Batoids are flat-bodied, and, like sharks, are a species of cartilaginous marine fish, meaning they have a boneless skeleton made of a tough, elastic substance. Most batoids have five ventral slot-like body openings called gill slits that lead from the gills, but the Hexatrygonidae have six. Batoid gill slits lie under the pectoral fins on the underside, whereas a shark's are on the sides of the head. Most batoids have a flat, disk-like body, with the exception of the guitarfishes and sawfishes, while most sharks have a streamlined body. Many species of batoid have developed their pectoral fins into broad, flat, wing-like appendages. The anal fin is absent. The eyes and spiracles are on top of the head. Batoids have a ventrally located mouth and can considerably protrude their upper jaw (palatoquadrate cartilage) away from the cranium to capture prey. The jaws have euhyostylic-type suspension, which relies completely on the hyomandibular cartilages for support. Most species live on the sea floor, in a variety of geographical regions, many in coastal waters, and a few live in deep waters to at least 3,000 meters (9,800 feet).

 

Osteichthyes

Guiyu

Figure 5. Guiya
"Guiyu BW" by Arthur Weasley, Wikimedia Commons is licensed under CC BY 3.0y

 

Osteichthyes, also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species. It is the largest class of vertebrates in existence today. Osteichthyes is divided into the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii).

All bony fish possess gills. For the majority of them, this is their sole or main means of respiration. Lungfish and other osteichthyan species, are capable of respiration through lungs or vascularized swim bladders. Other species can respire through their skin, intestines, and/or stomach.

The Actinopterygii, or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii which also possess lepidotrichia. These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles). In terms of numbers, actinopterygians are the dominant class of vertebrates, comprising nearly 96% of the 25,000 species of fish. They are ubiquitous throughout fresh water and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 millimetres (0.31 in), to the massive Ocean Sunfish, at 2,300 kilograms (5,100 lb), and the long-bodied Oarfish, to at least 11 metres (36 ft).

 


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