THE WORLD OF LIFE with Douglas Drenkow

Ray-finned fishes are marine or freshwater species. A few (such as mudskippers) are occasionally found on the land, and a few (such as "flying", actually, gliding fish) are occasionally found in the air.

DESCRIPTION

See also vertebrates in general.

Ray-finned fishes are the typical modern-day fishes. Ray-finned fishes characteristically have paired, fan-shaped, spiny fins (some of which are missing in eels).

The typically streamlined body of a ray-finned fish bears overlapping bony scales, "opercula" gill-covers, a jawed mouth with bony teeth, and a pair (or two) of nostrils on the snout (for the sense of smell, not for breathing). There are no eyelids or tear glands for these underwater creatures.

The body of a ray-finned fish is supported by the buoyancy of the water in which it lives and also by an "endoskeleton" (inside the body, as vs. the "exoskeleton" of an arthropod) composed of bone (only primitive ray-finned fishes have significant amounts of rubbery cartilage in their skeleton, as do sharks etc.).

Primitive ray-finned fish (such as the Nile bichir or the sturgeon) have a large mouth, a usually "heterocercal" (shark-like) tail, and scales that are "ganoid" (thickly bony).

More advanced fish (such as garpikes or the bowfin) have a smaller mouth, a less shark-like tail, and scales that are either ganoid or "cycloid" (disc-like and thinly bony).

Most modern fish have a relatively small mouth, a "homocercal" tail (with both top and bottom lobes similar), and scales that are either cycloid or "ctenoid" (rather disc-like and thinly bony but with tiny spines at the rear, which apparently lock the scales together).

FEEDING HABITS

Various ray-finned fishes are scavenging, herbivorous, and/or carnivorous. Some fish, such as remoras ("shark suckers"), are "commensalistic", feeding harmlessly on the leftovers of their hosts.

MOTION

The head-to-tail waves of muscular contractions on opposite sides of the backbone -- flexible because it is segmented into "vertebrae" -- produce the efficient squirming, swimming motion of fish, further aided by the typically paddle-like tail.

The fins of fish typically help provide stability and guidance, propulsion and lift during swimming.

Although many primitive fish keep from sinking by the overall "hydrofoil" design of their body (the upper surface of their body is more curved than the lower -- like the "airfoil" wing of a bird), many bony fish have an air-filled "swim bladder", evolved from the lung found in primitive fish (such as the Nile bichir, related to the sturgeon).

DIGESTION

Bones that supported the front-most gills in primitive fish (which, like modern lampreys, typically fed by sucking and rasping) evolved into the jaws of more modern fish. Jaws with teeth (evolved from bony scales) are more efficient in feeding, by biting and chewing.

The electric ray, the electric eel, and some other fish can generate hundreds of volts of electricity in the water, with which to stun prey (or would-be predators). Some deep-water fishes, like fireflies, biochemically produce light, which illuminates and perhaps attracts prey.

The digestive system of fish typically includes a mouth, pharynx, esophagus, stomach, intestine, and anus as well as accompanying digestive glands, such as a liver and gall bladder.

RESPIRATION

In modern fishes, gases are exchanged through internal gills, which evolved from "pharyngeal slits" (slits on the throat), like those present in all developing vertebrate embryos (including us) and like those used for filter-feeding by our evolutionary ancestors (akin to lancelets). Typically, a fish takes in a mouthful of water, which rushes out through the gill openings, on either side of the head.

The most primitive ray-finned fishes (such as the Nile bichir, often found in the mud) have a lung, evolved from an outpouching of the pharynx. See also lungfish.

CIRCULATION

Typically found below the gills, the heart of a modern fish has two chambers: The blood is pumped by the "ventricle" of the heart to the tiny capillaries in the gills (where carbon dioxide is released and oxygen is absorbed); then the blood travels through arteries and into capillaries embedded throughout the body, before returning through veins to the "atrium" chamber of the heart.

There is typically a spleen, towards the middle or rear of the body cavity, which filters the blood.

As lower vertebrates, fish are "ectothermic" (so-called "cold-blooded") -- their body temperature is controlled by the environment. Notably, the temperature of a large body of water stays fairly constant year-round, especially at its lower depths; and fish may migrate accordingly.

EXCRETION

In modern fishes, nitrogen-rich wastes are often secreted from the gills. The blood is also cleansed by kidneys, with the urine excreted through "Wolffian ducts", which merge with the ducts from the gonads, to form a "urogenital tract", emerging from the body as an opening just to the rear of the anus.

Excess nitrogen in fishes is typically excreted mostly as ammonia, which is highly toxic but readily water-soluble.

The excretory system is also used for salt- and water-balance -- saltwater species typically eliminating excess salt from their body through the gills, and freshwater species typically eliminating excess water from their body through the kidneys.

COORDINATION

As in other multi-cellular organisms, the growth, development, and activities of fishes are under genetic and hormonal control, influenced by the environment.

Fishes have evolved sense organs specialized for their watery world. For example, fish have an internal ear but no external ear -- sound waves travel through the body of the fish almost as well as through the surrounding water itself (Living tissues -- of all creatures -- are composed mostly of water). Along each side of many fishes is a "lateral line", sensitive to vibrations, even at a great distance. The "nares" (nostrils) of a fish function in the sense of smell (but not breathing, except perhaps in some lunged fish, with nostrils connecting into the mouth).

The instinctive schooling behavior of various, often small fishes helps protect individuals, especially by confusing would-be predators.

REPRODUCTION

The "gonoducts" (from the gonads, producing eggs or sperms) merge with the Wolffian ducts (from the kidneys) to form a urogenital tract, which opens just to the rear of the anus.

Fishes typically release swimming sperm directly into environmental water.

The thin-skinned, fertilized eggs of fishes are typically laid in watery environments. The larvae of fishes sometimes retain their yolk sacs and often look different from the adults.

The larvae of bottom-dwelling flatfish (such as halibut and flounder) are flattened from side to side; and during development, one eye grows over to the other side, which then has two eyes, both looking upwards.

Representatives

Vertebrates (Vertebrata)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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