Holomictic

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Dimictic lakes commonly have turnover in spring and fall with stratification in summer and winter (Nature Times).

Dimictic lakes commonly have turnover in spring and fall with stratification in summer and winter (Nature Times).

Holomictic references the most common type of lake which turns over at least once per year (as opposed to meromictic lakes which are constantly stratified).  This mixing is an important process for maintaining fish and aquatic communities by distributing nutrients and oxygen throughout the lake before stratification occurs again.  There are four categories of holomictic lakes:

  • Oligomictic: mixing is irregular,
  • Monomictic: turnover occurs once a year (most common in polar areas)
  • Dimictic: turnover occurs twice a year (most common in temperate areas), and
  • Polymictic: frequent turnover (most common in tropical areas).

Meromictic

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The Black Sea is the largest meromictic lake (NASA).

The Black Sea is the largest meromictic lake (NASA).

Meromictic references a type of lake which is constantly stratified.  The surface and bottom waters do not ever mix.  In most cases, the bottom layer has very low oxygen levels, where few fish and other organisms can live, restricting them to the surface layer.  Meromictic lakes are uncommon (most lakes are holomictic and turnover at least once per year) and may be formed because they are:

  1. small and unusually deep, or
  2. the lower layer is denser and more saline than the surface layer.

Homocercal

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The rose fish has an even-lobed homocercal tail.

The Rose Fish has an even-lobed homocercal tail.

A homocercal tail is a caudal fin composed of two lobes of equal proportion.  Homocercal tails are the most common caudal fin type in fish but can come in many symmetrical shapes.  A homocercal tail is contrasted with a heterocercal tail which has unequal lobes.

Heterocercal

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The heterocercal tail of a shark is a caudal fin with unequal lobes.

The heterocercal tail of a shark is a caudal fin with unequal lobes.

A heterocercal tail is a caudal fin composed of two asymmetrical lobes.  Often, such as the case in many sharks, the vertebral column passes through the upper lobe, making it the larger of the two lobes.  A heterocercal tail is contrasted with a homocercal tail which has equal lobes.

Lotic

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Darters have evolved to survive in fast moving lotic systems.

Darters have evolved to survive in fast moving lotic systems.

Lotic refers to freshwater ecosystems involving flowing water, such as a river, stream, brook or creek.  Certain fish species are lotic specialists and have evolved to live in higher flow water, such as darters which prefer to live in swift-moving riffles.

Lentic

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Cichlids have radiated in the lotic environment of Lake Victoria.

Cichlids have radiated in the lentic environment of Lake Victoria.

Lentic describes freshwater ecosystems characterized by still water and low flow, such as a lake or pond.  Certain fish species have evolved to become lentic specialists, such as the high diversity of cichlid species found in Lake Victoria.

Scute

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Sturgeon have multiple rows of protective bony scutes.

Sturgeon have multiple rows of protective scutes.

A scute is an external bony plate on the surface of a fish.  Scutes serve a protective function, acting as a body armor for fish against environmental abrasions and even predation. In some fishes, such as Alewife (Alosa pseudoharengus), scutes are a row of scales modified into sharp, protective plates.  In other fishes, like Shovelnose Sturgeon (Scaphirhynchus platorynchus), scutes serve a homologous function but are derived from ossified deposits in the dermis (essentially bone over skin).

Ampullae of Lorenzini

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The ampullae of Lorenzini on this chimera are the two rows of dots on its snout (NOAA)

The ampullae of Lorenzini on this chimaera are the two rows of dots on its snout (NOAA)

Ampullae of Lorenzini are a network of electroreceptors, sensory organs that detect electric fields in water, found in chondrichthyes (sharks, rays, and chimaeras).  The ampullae are a series of symmetrical pores, concentrated around the snout and nose, connected by gel-filled canals.  They can conduct electrical impulses so small, that chondrichthyes are likely to be more sensitive to electric fields than any other group of animals.  Because all muscle contractions produce a weak electrical field, these electroreceptors make sharks, rays, and chimaeras highly capable of detecting other organisms, such as prey, nearby in water.

Ampullae of Lorenzini, shown in red, are a network of electroreceptors which can detect electrical impulses from other organisms in water.

Ampullae of Lorenzini, shown in red, are a electroreceptors that can detect electrical impulses from other organisms in water.

 

Elasmobranch

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Spotted Eagle Rays are a member of the catilaginous subclass Elasmobranchii along with other rays, sharks, and sawfishes (NOAA).

Spotted Eagle Rays are a member of the cartilaginous subclass Elasmobranchii along with other rays, sharks, and sawfishes (NOAA).

Elasmobranchs, including sharks, rays, and sawfishes, belong to the taxonomic subclass of cartilaginous fish Elasmobranchii.  Like most chondrichthyes, they have exposed gills, no swim bladder, internal fertilization, and placoid denticles.  They differ from the other subclass, chimaera (subclass: Holocephali), in that they have rigid dorsal fins, placoid denticles cover most of their bodies, and they usually have spiracles (modified gill slits directly behind the eye).

Osteichthyes

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The Ocean Sunfish is considered to be the largest member of Osteichthyes, reaching over 2,200 lbs.

The Ocean Sunfish is considered to be the largest member of Osteichthyes, reaching over 2,200 lbs.

Osteichthyes are a taxonomic grouping of bony fishes.  This group includes ray-finned fishes (class: Actinopterygii) and lobe-finned fishes (class: Sarcopterygii).  This highly diverse group of fishes, which contains almost all fish species, is the most diverse group of vertebrates today.  Osteichthyes differ from chondrichthyes by (in most cases) possessing a bony skeleton, a swim bladder, scales (ctenoid, cycloid, or ganoid scales), and external fertilization.