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16.6: Fish

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    There are many varieties of fish (too many to discuss here!). Here are important facts about fish.
    Fish are found in nearly all aquatic environments (land & sea), and all depths of the oceans.
    Fish are all aquatic, gill-bearing, craniate (head-bearing) animals that lack limbs with digits.
    Fish groups account for more than half of all vertebrate species.

    At 32,000 species, fish exhibit greater species diversity than any other group of vertebrates.
    * almost 28,000 known extant (not yet extinct) species, ~27,000 are bony fish, ~970 sharks, rays, and chimeras.
    * over 100 hagfish and lampreys.
    * many extinct varieties.

    Most fish are ectothermic (cold-blooded), allowing their body temperatures to vary as ambient temperatures change some of the large active swimmers (examples white shark and tuna can hold a higher core temperature).

    Fish are abundant in most bodies of water, all parts (depths) of the oceans.

    Fish Evolution

    Amphibians, reptiles, birds and mammals) and fish share a common evolutionary ancestry.
    The earliest fish-like organisms appeared during the Cambrian period. (However, they lacked a true spine, but possessed notochords.)
    Fish evolve through the Paleozoic era, diversifying into a wide variety of forms.

    Many Paleozoic fishes developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just prey.

    Osteichthyes Fish with bony skeletons
    Chondrichthyes a class of fishes that have the cartilaginous skeletons, rather than bone—they have jaws, paired fins, scales, a heart with its chambers in series. Subclass Elasmobranchii (includes sharks, rays, skates, sawfish), and subclass Holocephali (include chimaeras). Some lineages are up to 280 million years old.

    Examples of Osteichthyes (bony fish)

    Giant Oarfish Anchovies in a school Marlin Schooling tuna
    Figure 16.36. Oarfish (a species with ancient roots) Figure 16.37. Anchovies Figure 16.38. Marlin Figure 16.39. Blue fin tuna
    Examples of Chondrichthyes (Sharks and Rays)
    Great White Hammerhead Whale Shark (Gulf of Mexico) Manta rays
    Figure 16.40. Great white shark Figure 16.41. Hammerhead shark Figure 16.42. Whale shark Figure 16.43. Manta Rays

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