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Air-breathing

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A mudskipper is an amphibious, air-breathing fish (photo credit: National Aquarium).

Guest post: Gretchen Stokes, University of Florida

Air-breathing fishes are those that respire using aerial gas exchange instead of, or in addition to, that of the water.  Air-breathing is an ancient vertebrate specialization that evolved from aquatic hypoxia (e.g., areas of seasonally flooded wetlands, under ice).  Air-breathing fishes are equipped with one or more modified respiratory structure to breathe, such as modified air bladders, lungs, skin or gills.  Some air-breathing fishes (facultative air-breathers) rely mainly on gills with supplemental gas exchange from aerial breathing when conditions are poor or it is too costly to obtain oxygen from water.  Other air-breathers (obligate) require continuous, regular surface breathing.  Yet another group of air-breathing fishes are amphibious, venturing to land either in an active or inactive state.

Air-breathing occurs in an estimated 450 species and 49 families of fishes, most of which are found in the tropics.  In North America, common air-breathing species include gar, tarpon and bowfin.  Aquarium enthusiasts may be familiar with the air-breathing habits of well-known betas.  Globally the list becomes expansive with many unique adaptations, such as lungfishes, mudskippers, snakeheads or air-breathing catfishes.  Whether a gulp of air at the water’s surface, crawling on land to feed, or maintaining a high tolerance to air exposure with specialized skin, air-breathing allows for unique adaptations and survival in extreme environments.

Aquaculture

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Pangasius catfish are a common aquaculture product from SE Asia.

Aquaculture is defined by the Food and Agriculture Organization of the United Nations as the farming of aquatic organisms (e.g., fish, mollusks, crustaceans, plants) and harvest by an owner who is responsible for the maintenance, protection, and enhancement of the cultivated stock.

Aquaculture is considered the fastest growing food production sector.  In terms of global production, farmed fish accounted for 44.1% of total production from capture fisheries and aquaculture in 2014 (FAO 2016).  While most farmed fish go directly to human consumption, not all wild harvest fisheries do.  Consequently, farmed fish provided more fish to the global food supply than capture fisheries for the first time in 2014 (FAO 2016).

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.

 

Anadromous

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Pacific salmons are well-known examples of anadromy. They live most of their lives at sea but spawn in fresh water

Pacific salmons are well-known examples of anadromy. They live most of their lives at sea but spawn in fresh water

An anadromous fish hatches and spawns in fresh water but spends most of its life in the salt water.  This dual life cycle allows the younger fish to grow and feed in the less perilous fresh water habitats (fewer predators in the smaller systems) and the older, larger fish to grow and feed in the marine systems, where there is a greater prey base.  Migrating between fresh and salt water requires complex osmoregulatory adaptations.  Some species, such as Pacific salmons, only transition between fresh and salt water twice, migrating to salt water early in life and then migrating back to freshwater to spawn once and die (a semelparous reproductive strategy).  Other species, such as Striped Bass (Morone saxatilis) will migrate between fresh and salt water every year to spawn (an iteroparous reproductive strategy).

For more information, check out the “CAN YOU SAY ANADROMOUS, CATADROMOUS, AMPHIDROMOUS, OCEANODROMOUS, OR POTAMODROMOUS?” post on The Fisheries Blog!

Striped bass are anadromous fish that migrate between fresh and salt water every year to spawn

Striped bass are anadromous fish that migrate between fresh and salt water every year to spawn

Anal fin

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Knifefish use their elongated anal fin for locomotion

Knifefish use their elongated anal fin for locomotion

The anal fin is found on the ventral side of fish, often, but not always, at the base of the anus.  Along with the dorsal fin, the main purpose of the anal fin is stabilize the fish and keep it from rolling in the water.  For knifefish (order Gymnotiformes), which have neither pelvic or dorsal fins, the anal fin has an additional purpose.  It is almost the entire length of their bodies.  Knifefish swim by rippling their anal fin while keeping the rest of their bodies rigid and straight.  As their primary form of locomotion, knifefish can swim backwards as easily as forward.

Snakeheads (family Channidae) also have an elongated anal fin.  Not native to the US, snakeheads were found in a pond in Maryland in 2002.  They are now permanently established in the Potomac River as an invasive species.  Snakeheads are food fish in their native range, so the introductions to the US were likely intentional.  Snakeheads are an ecological concern because they are top-level predators with no natural predators in US.  They also can survive on land for up to four days (as long as they are still wet) and can “walk” up to a quarter mile on wet land to find other habitable water bodies.

Snakeheads are invasive to the Potomac River

Snakeheads are invasive to the Potomac River

Adipose fin

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The adipose fin may serve as a "precaudal flow sensor"

The adipose fin may serve as a “precaudal flow sensor”

The adipose fin is a small fleshy fin found posterior to the dorsal fin and anterior of the caudal fin.  It is only found on few fish, including trout, salmon, and catfish.  When it was named, it was thought to hold fat, or adipose, tissue.  Once this hypothesis was disproved, fisheries biologists long considered this fin to be “non-functional.”  More recently, research suggests that the adipose fin may serve as a “precaudal flow sensor” to improve maneuverability in turbulent waters.  This new research may raise concerns because adipose fin clipping is commonly used to “tag” hatchery reared salmon to distinguish them from wild salmon for catch and release management purposes.

For more information on the adipose fin, please visit: http://thefisheriesblog.com/2013/05/28/the-adipose-fin-old-mysteries-with-new-answers/