Orders and Families of Fish

The approximately 25,000 species of fish are broadly placed into two main groups: elasmobranch (sharks, rays, and dogfish), which have cartilaginous skeletons; and teleost, or the bony fish. They are further subdivided into over 4,200 genera, 60 orders, and at least 485 families. The following is a list of several orders containing particularly unique or interesting fish. Hopefully this tiny peek at the marvelous diversity of this fascinating group of animals will inspire you to learn all that you can about them.

Jawless Fish (Orders Myxiniformes and Petromyzontiformes)

Truly testing the limits of what might be considered a “fish,” the forty-three species of hagfish and forty species of lamprey are evolutionary throwbacks to a group of fish known as the Agnatha. These odd creatures arose in the Ordovician Period, over 500 million years ago, and modern surviving representatives have changed little since then.

Included among their unfishlike characteristics are a rasping, tonguelike structure known as the piston, lack of scales, and absence of paired fins. Both hagfish and lampreys possess eel-like bodies and are found in cold and temperate marine-and freshwaters throughout the world.

Lampreys pass through a distinct larval stage, which is spent buried in the substrate and may be up to three years. After a three-month metamorphosis, the adults take up life as parasites of other fish species. Most types of lamprey are anadromous, which means they reproduce in freshwater but spend the majority of their lives at sea.

Taxonomy, the science of classification, is a constantly evolving discipline. Today, advanced techniques in the science of genetics are enabling researchers to more accurately pinpoint the evolutionary relationships of fish. This, combined with improved collecting techniques, has resulted in identifying more species and families of fish.

Lampreys attach themselves to the bodies of other fish by way of a diskshaped sucker and then use their tooth-lined tongue, or piston, to scrape through the body wall of the unfortunate host. Depending upon the species, lampreys may feed upon blood, muscle tissue, or both. Large, robust host fish may survive an attack, but a great many are killed.

Hagfish are bottom-dwelling scavengers of the deep sea. Unlike the rasping piston of the lamprey, that of the hagfish is extended into the flesh of the meal and then retracted, pulling off a chunk of meat in the process. Hagfish possess cartilaginous notochords as opposed to true backbones. These flexible structures allow the hagfish to literally tie themselves into knots. As the knot travels down the fish's body, it creates pressure that helps to tear flesh from the carcass upon which the hagfish is feeding. This unusual adaptation, combined with the production of copious amounts of mucus, can also be used as a means of escaping predators. Very little is known of the breeding habits of most hagfish species.

In a startling modification to their normal life history, a population of sea lampreys has become landlocked in the Great Lakes of the United States. This population has somehow been able to forgo the seagoing phase of its existence and now survives as a completely freshwater species. The adjustment has been so successful that the number of lampreys in the Great Lakes has skyrocketed, leading to the demise of several recreational and commercial fisheries.

Lizardfish and Relatives (Order Aulopiformes)

The over 200 mostly marine species of lizardfish and their relatives, ranging in size from 3 inches to 7 feet exhibit a unique combination of advanced and primitive features.

Many of the deep-sea forms are able to self-fertilize, perhaps an adaptation to a vast, featureless environment that renders finding a mate a highly unlikely prospect. The twenty-five or so species of true lizardfish are all elongated in body structure and possess large, toothy mouths to assist in capturing the fish and other marine life upon which they feed. The head truly is lizardlike in appearance, an impression furthered by the fish's habit of propping themselves up on their ventral fins as they scan the surrounding area for prey. Lizardfish are efficient, “sit and wait” predators, generally cryptically colored and able to bury themselves within the substrate, leaving their eyes protruding above.

The benthic-dwelling lancet fish (Alepisaurus ferox), the largest member of the group, has a high dorsal fin that runs from the head to the tail and a huge mouth filled with daggerlike teeth (a related species is commonly referred to as the “dagger tooth”). At 7 feet in length, lancet fish are among the longest of the deep-sea predators.

The closely related pearleyes are also fish of the ocean's abyss. They possess upward-facing eyes and binocular vision. Unique to these unusual animals is a white spot on the eye, known as the pearl gland, and also the existence of a second retina. It is speculated that the pearl gland functions to pick up what little light is available in this creature's deep-sea habitat and guide it to the eye's lens.

Flying Fish and Relatives (Order Beloniformes)

Flying fish are, in general, marine fish that dwell at the surface of many of the world's warmer waters. Nearly 200 species have been identified. True flying fish are equipped with pectoral fins that are uniquely modified into winglike structures. In several species, the pelvic fins also function in this manner, making them, in a sense, “four-winged” fish. All are surfacedwelling animals of the open seas. When preparing to glide above the surface of the water, these fish swim about rapidly to build up speed. The largest and most accomplished of the flying fish can attain a speed of nearly 40 miles per hour before leaving the water, and may soar up to 650 feet before returning to the sea.

Flying fish of all types are a prized food item in many countries, as is their roe. They are, however, difficult to catch on hooks, because most feed on plankton. Therefore, unique strategies have been devised to capture them, including taking advantage of the fish's attraction to light and also luring mating aggregations to floating rafts of vegetation (flying fish generally lay their eggs below floating debris or macroalgae).

Needlefish are closely related to flying fish and are instantly recognizable by their long, thin bodies and elongated, toothy jaws. Strangely, these seemingly benign animals have been implicated in several human fatalities. Attracted to the lights of small fishing boats operating at night, large needlefish leaping from the ocean have apparently impaled hapless fishermen in the eyes.

Oarfish and Relatives (Order Lampridiformes)

The oarfish (Regalecus glesne) is the longest of the bony fish and is exceeded in length only by the cartilaginous whale shark and basking shark.

Measured at a length of 26 feet, estimates of oarfish observed but not captured indicate that they may possibly reach lengths of up to 56 feet, which would make them the longest of all fish. Sightings of these unusual creatures have, no doubt, given rise to many myths concerning sea serpents.

The larval phases of several members of the order to which oarfish belong are very unusual in appearance, so much so that they and their corresponding adult phases were once classified as separate species.

Almost all members of this order have unique body structures and shapes, such as long caudal filaments. Because these fish are not often encountered and are nearly impossible to keep in captivity, people know very little concerning the uses of these appendages or of the fish's natural histories in general.

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