swim bladder

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.

phylogeny

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Phylogeny of fishes (diagram credit: evolution.berkeley.edu).

Phylogeny is the representation of evolutionary relationships, often displayed as a “tree” (i.e., branching diagram) where the most closely related organisms are connected by a node and more distantly related organisms have multiple steps back in the tree before they are connected.  Nodes are often characterized by biogeographic events and evolutionary innovations.

In fishes, jawless fishes (such as hagfish and lampreys) are the most primitive, cartilaginous, and without paired fins; sharks and rays are cartilaginous with jaws and placoid denticles; and bony fish are the most diverse and derived fishes, with scales (see cycloid and ctenoid), swim bladders, and bony skeletons.

 

 

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.

Weberian apparatus

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The Weberian apparatus is amplifies hearing in Ostariophysian fish.

The Weberian apparatus is amplifies hearing in Ostariophysian fish.

This series of small bones links the swim bladder to the inner ear in Ostariophysian fish, including minnows, carp, catfishes, and characins.  When a sound wave hits the swim bladder, the vibration is transferred through the Weberian apparatus to the auditory region of the inner ear.  This action helps amplify sound and, as a result, these fish have very sensitive hearing and can hear sounds that many other fish are in capable of hearing.

Swim bladder

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Herring inflate their swim bladders by "gulping" air and deflate them by "burping" or "farting." (gma.org)

Herring inflate their swim bladders by “gulping” air and deflate them by “burping” or “farting.” (gma.org)

The swim bladder is an air filled organ used by some fish to maintain buoyancy at a desired depth and produce or hear sound.

  • Physostomous swim bladders are directly connected to the gastrointestinal tract so that fish with these swim bladders, such as herrings, must “gulp” air to inflate their swim bladder and “burb” or “fart” air to deflate them.
  • Physoclistous swim bladders are not connected to the digestive tract so that fish with these swim bladders must diffuse gas from the blood to fill and collapse them.

For many fish, the swim bladder has the additional role of transferring sound waves to the auditory system.  And in some fish, such as drums and croakers, the swim bladder is used to make sounds and communicate with one another and other fish.

Fish use swim bladders for buoyancy, hearing, and producing sounds.

Fish use swim bladders for buoyancy, hearing, and producing sounds.