Swim Bladders and Lungs

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Swim Bladders and the

Origin of LungsSeth Andrew J. Salih


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Swim Bladders and Lungs

Every osteichthyan from fish to humans developsan unpaired evagination from the foregut that

becomes pneumatic sacs The pneumatic sacs become either swim bladders or

lungs

These sacs fill with gases derived directly orindirectly from the environment


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Pneumatic sacs do not imply a function Swim bladders and lungs suggest functions for

buoyancy and respiration

Air sacs of fishes called swim bladder and affectbuoyancy

Air sacs of tetrapods referred to as lungs


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Withoutpneumatic(air) sacs

Latimeria

A clade oftailed

amphibians

Bottom

dwellers

Marine

Teleosts


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Swim Bladders

After the budlike anlage (foundation) for a swimbladder evaginates, the resulting duct may retain

the connection to the foregut or may close duringlater development

Fishes with ducts that remain open arephysostomous

Examples are: Chondrosteans,basal neopterygians, the3 living dipnoans and some teleosts


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Swim Bladders

Fishes with ducts that close arephysoclistous Most teleosts arephysoclistous.


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Swim Bladders

Physoclistous Physostomous


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Swim Bladders

Swim bladders lie close to kidneys and areretroperitoneal in position

As the embryonic anlagen grow longer, they pushtheir way caudad in the roof of the coelom betweenthe embryonic parietal peritoneum and body wall

However, adult swim bladders may bulge into theroof of the coelom


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Swim Bladders


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Swim Bladders

Swim bladders serve as hydrostatic organs,respiratory organs, organs participating in sounddetection, and organs of communication

Their capacity for gases can be increased/decreaseddue to the elastic and smooth muscles in the bladderwalls


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Swim Bladders

In teleosts, they function mainly as hydrostaticorgans that maintain an appropriate depth or hoverover a specific location

To do this, a fish must attain a body density equal tothat of the displaced water at that depth

This is done by controlling the volume of gas in thebladder. A difference in density will affect thebuoyancy, causing the fish to rise/sink


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Swim Bladders


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Swim Bladders

However, a fish must use its fins to compensate forthe disturbances in the water

Some fishes make daily changes in depth and bodydensity as a correlate of the tome of day

The gas in the bladder comes from the blood and isactively transported to the lumen of the bladderfrom the red gland, a localized rete (network) ofsmall arteries in the bladder lining


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Swim Bladders

The gas is eventually resorbed into the bloodstreamof the bladder near its caudal end in a pocketlikearea of modified epithelium

The pocket can be closed off from the main cavity bya sphincter during passage of gas into the lumen ofthe bladder and relaxed when resorption of gas is

taking place

In physostomes, the gas may be bubbled to theexterior through the mouth


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Swim Bladders

The gases in the bladder differ among fishes Some contain 99% nitrogen Some, up to 87% oxygen All contain traces of carbon dioxide and argon

In deepwater fish, nitrogen may be transportedfrom the blood into the lumen of the bladder


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Swim Bladders

In physostomous fishes, swim bladders function aslungs to one degree

Air is gulped at the surface of the water and anoropharyngeal pump forces it into the swim bladder

Air depleted of oxygen is expelled by the vacuumcreated by lowering the floor of the oropharyngealcavity

Expulsion is facilitated by the elasticity of thebladder wall, its smooth muscle musculature andthe pressure of the surrounding water

Air is then bubbled through the mouth into thewater


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Swim Bladders

2 of 3 genera of lungfishes, Propterus and Lepidosiren,require continual gulping of air for survival becausetheir gills are incapable of producing sufficient O

2

They drown if held underwater for a certain lengthof time

Their swim bladders act as lungs during tropicalsummers, when swamps dry up, and allow them tosurvive

Additionally, these fish also absorb O2 from the skin


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Swim Bladders

The swim bladder of the other dipnoan(Neoceratodus), 2 genera of ray-finnedAfrican

freshwaterfishes, Polypterus, Calamoichthys, and relictbasal neopterygians (Amia andgars) function as alung only when the O2 content of water is so lowthat gills cannot produce the respiratory quota ofthe organism


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Swim Bladders

Polypterusand

Calamoichthys

Truelungfishes

Smooth liningSupplied by arteries

from the 6th embryonic

aortic arch (along withAmia)

Lined with low septaand exhibit thousandsof air sacs

Supplied by arteriesfrom the 6th embryonicaortic arch (along with

Amia)


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Swim Bladders

Like tetrapods, the venous return in dipnoans isdirectly to the left atrium of the heart

Some teleosts use their swim bladders for sounddetection In Cypriniformes, small bones, weberian ossicles,

connect the anterior end of the bladder with the

sinus impar, an extension of the perilymphatic spaceof the inner ear.

In Clupeiformes, a thin-walled anterior extension ofthe bladder makes direct contact with the inner ear.


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Swim Bladders

Bottom feeders have degenerate swim bladders Eliminating this hydrostatic organ optimized their

body density, enabling them to hover close to foodwith little energy expenditure

An analogous condition is in salamanders who livein swift mountain streams and whose lungs are onlya few mm or absent

Optimal body density and the saving of energy arethe benefits


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Swim Bladders

However, no member of the plethodont family haslungs. These urodeles breath through their skin


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Origin of Lungs

The similarities between swim bladders and lungssuggest that they may be the same organs

During the Devonian, the swamps were warm adstagnant, causing them to be low in O2. Thiscondition may have caused the development ofaerial respiration among organisms

At the time, most rhipidistians had pneumatic sacs


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Origin of Lungs

A pneumatic sac was functioning in aerialrespiration before craniates ventured onto land

The closure of the pneumatic duct in physoclistousfishes is probably derived from a more primitiveopen-duct condition

Swim Bladders and Lungs

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