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Fish are a group of cold blooded (ectothermic) vertebrates which most all are gill-bearing, lack limbs with digits, have external fertilization of soft shelled water borne egg (anamniotes) and most have skin covered in scales.
Most fish allow their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature.
Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams to the abyssal and even hadal depths of the deepest oceans. With the largest number of described vetebrate species, fish exhibit greater species diversity than any other group of vertebrates.
As of July 2020, there are about 34,310 species of fish divided up into 64 orders, 572 families and 5,148 genera, however, the number of new species being identified each year has been increasing at the rate of about 200 over the last decade.
Fresh water or Salt water?
As we know, the fish of this earth live in either fresh water or salt water, but a few can transverse from one to the other such as salmon. Born in upriver stream, they travel down river to live in the salt water of the ocean. When ready, the salmon returns to it′s birthplace to spawn. However in comparison to most fish, those that spend there lives in both fresh and salt are few.
Ocean Waters
Those creatures living in the oceans of the earth, or Pelagic water also have a variety of zones and most sea creatures chose one of these zones and lives out it′s entire life, seldom to move out of there comfort zone. The pelagic zones are as follows:
Epipeagic Zone
is from the surface down to 200m (656 feet), or the photic zone, where there is abundant light for photosynthesis, well mixed water and temperatures above 53° F. (12° C.)
Life:
dolphin, jellyfish, tuna, turtles whales.
Mesopelagic Zone
is from 200m down to 1000m (3280 feet), or the aphotic twilight zone, where there is insufficient light for photosynthesis, a muddy bottom and a rapidly decreasing temperatures below 50° F. (10° C.)
Life:
catshark, crab, krill, squid
Bathypelagic Zone
is from 1000m down to 2000m (6560 feet), or the aphotic midnight zone, where there is a constant salinity, muddy bottom continues and food quantities decrease, temperature 40° F. (4° C) and below.
Life:
octopus, sea star, viperfish, sponge Bob
Abyssopelagic Zone
is from 2000m down to 6000m (19685 feet), or the aphotic no light zone, where temperatures fall to 28° F. (-2° C.)
Life:
anglerfish, black swallower. Many of the creatures that live in this zone and below live near hot-water vents called black smokers.
Hadalpelagic Zone
is in trenches (6000-10000m) below the ocean floor (or Abyssal plain), where the water is cold, dark and under high pressure.
Life:
liparid fish, sea cucumber, tube worms
Coastal Waters
At the shore on coastlines is what is called the intertidal zone where the high and low water reaches. Sloping gently from the shoreline is the continental shelf, a relativity shallow area where much life lives. At the edge of this shelf the bottom drops off quickly, and from here outward from the edge of the continental shelf is referred to as the open ocean.
Open Ocean
The bottom past the edge of the continental shelf is normally below the Epipeagic layer of the ocean and consists of the continental slope which continues into the bathyal zone.
Then, the bottom continues into the continental drop of the abbyssal zone to the bottom levels of the ocean called the abyssal plain or ocean basin.
Here on this abyssal plain is where there are deep trenches down into the hadal zones of the ocean bottom to depths as low as thirty-five thousand, eight hundred forty feet at the bottom of the Mariana trench.
Ocean Average Depths
Antarctic ocean, depth: 13-16,000 feet, (4000-5000 m) Deepest: 23,737 feet
Arctic ocean, depth: about 4,000 feet (1200 m) Deepest: 17,881 feet
Atlantic ocean, depth: about 10,800 feet (3300 m) Deepest: 28,231 feet
Indian ocean, depth: about 13,000 feet (4000 m) Deepest: 25,344 feet
Pacific ocean, depth: about 14,000 feet (4300 m) Deepest: 36,840 feet
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