The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled
organ that contributes to the ability of a fish to control its buoyancy, and thus
to stay at its current water depth without having to waste energy in
swimming. Also, the dorsal position of the swim bladder means the center of
mass is below the center of volume, allowing it to act as a stabilizing
agent. Additionally, the swim bladder functions as a resonating chamber, to
produce or receive sound. The swim bladder is evolutionarily
homologous to the lungs. Charles Darwin remarked upon this in On the Origin of
Species. Swim bladders are found in many bony
fish, but are not found in cartilaginous fish. In the embryonic stages some
species, such as redlip blenny, have lost the swim bladder again, mostly
bottom dwellers like the weather fish. Other fish like the Opah and the Pomfret
use their pectoral fins to swim and balance the weight of the head to keep a
horizontal position. The normally bottom dwelling sea robin can use their
pectoral fins to produce lift while swimming. The cartilaginous fish do not
have swim bladders. Some of them can control their depth only by swimming;
others store fats or oils with density less than that of seawater to produce a
neutral or near neutral buoyancy, which does not change with depth.
The gas/tissue interface at the swim bladder produces a strong reflection of
sound, which is used in sonar equipment to find fish.
Structure and function The swim bladder normally consists of
two gas-filled sacs located in the dorsal portion of the fish, although in
a few primitive species, there is only a single sac. It has flexible walls that
contract or expand according to the ambient pressure. The walls of the
bladder contain very few blood vessels and are lined with guanine crystals,
which make them impermeable to gases. By adjusting the gas pressurising organ
using the gas gland or oval window the fish can obtain neutral buoyancy and
ascend and descend to a large range of depths. Due to the dorsal position it
gives the fish lateral stability. In physostomous swim bladders, a
connection is retained between the swim bladder and the gut, the pneumatic duct,
allowing the fish to fill up the swim bladder by "gulping" air. Excess gas can
be removed in a similar manner. In more derived varieties of fish, the
physoclisti, the connection to the digestive tract is lost. In early life
stages, fish have to rise to the surface to fill up their swim bladders, however,
in later stages the connection disappears and the gas gland has to
introduce gas to the bladder to increase its volume and thus increase buoyancy.
In order to introduce gas into the bladder, the gas gland excretes lactic
acid and produces carbon dioxide. The resulting acidity causes the hemoglobin
of the blood to lose its oxygen which then diffuses partly into the swim
bladder. The blood flowing back to the body first enters a rete mirabile where
virtually all the excess carbon dioxide and oxygen produced in the gas gland
diffuses back to the arteries supplying the gas gland. Thus a very high gas
pressure of oxygen can be obtained, which can even account for the presence
of gas in the swim bladders of deep sea fish like the eel, requiring a pressure
of hundreds of bars. Elsewhere, at a similar structure known as the oval
window, the bladder is in contact with blood and the oxygen can diffuse back.
Together with oxygen other gases are salted out in the swim bladder which
accounts for the high pressures of other gases as well.
The combination of gases in the bladder varies. In shallow water fish, the
ratios closely approximate that of the atmosphere, while deep sea fish tend to
have higher percentages of oxygen. For instance, the eel Synaphobranchus has
been observed to have 75.1% oxygen, 20.5% nitrogen, 3.1% carbon dioxide, and
0.4% argon in its swim bladder. Physoclist swim bladders have one
important disadvantage: they prohibit fast rising, as the bladder would burst.
Physostomes can "burp" out gas, though this complicates the process of
re-submergence. In some fish, mainly freshwater species,
the swim bladder is connected to the labyrinth of the inner ear by the
Weberian apparatus, a bony structure derived from the vertebrae, which
provides a precise sense of water pressure, and improves hearing.
In red-bellied piranha, the swimbladder may play an important role in sound
production as a resonator. The sounds created by piranhas are generated
through rapid contractions of the sonic muscles and is associated with the
swimbladder. Evolution
Swim bladders are evolutionarily closely related to lungs. It is believed that
the first lungs, simple sacs connected to the gut that allowed the organism to
gulp air under oxygen-poor conditions, evolved into the lungs of today's
terrestrial vertebrates and some fish and into the swim bladders of the
ray-finned fish. In embryonal development, both lung and swim bladder
originate as an outpocketing from the gut; in the case of swim bladders, this
connection to the gut continues to exist as the pneumatic duct in the more
"primitive" ray-finned fish, and is lost in some of the more derived teleost
orders. There are no animals which have both lungs and a swim bladder.
The cartilaginous fish split from the other fishes about 420 million years ago
and lack both lungs and swim bladders, suggesting that these structures evolved
after that split. Correspondingly, these fish also have a heterocercal and
pectoral fins which provides the necessary lift needed due to the lack of
swim bladders. On the other hand, teleost fish with swim bladders have
neutral buoyancy and have no need for this lift.
Deep scattering layer Sonar operators, using the newly
developed sonar technology during World War II, were puzzled by what appeared to
be a false sea floor 300–500 metres deep at day, and less deep at night. This
turned out to be due to millions of marine organisms, most particularly
small mesopelagic fish, with swimbladders that reflected the sonar.
These organisms migrate up into shallower water at dusk to feed on
plankton. The layer is deeper when the moon is out, and can become shallower
when clouds pass over the moon. Most mesopelagic fish make daily
vertical migrations, moving at night into the epipelagic zone, often
following similar migrations of zooplankton, and returning to the depths
for safety during the day. These vertical migrations often occur over a
large vertical distances, and are undertaken with the assistance of a
swimbladder. The swimbladder is inflated when the fish wants to move up, and,
given the high pressures in the messoplegic zone, this requires
significant energy. As the fish ascends, the pressure in the swimbladder must
adjust to prevent it from bursting. When the fish wants to return to the depths,
the swimbladder is deflated. Some mesopelagic fishes make daily migrations
through the thermocline, where the temperature changes between 10 and 20
°C, thus displaying considerable tolerances for temperature change.
Sampling via deep trawling indicates that lanternfish account for as much as
65% of all deep sea fish biomass. Indeed, lanternfish are among the most
widely distributed, populous, and diverse of all vertebrates, playing an
important ecological role as prey for larger organisms. The estimated global
biomass of lanternfish is 550–660 million metric tonnes, several times the
entire world fisheries catch. Lanternfish also account for much of the
biomass responsible for the deep scattering layer of the world's oceans.
Sonar reflects off the millions of lanternfish swim bladders, giving the
appearance of a false bottom. Human uses
In some Asian cultures, the swim bladders of certain large fishes are
considered a food delicacy. In China they are known as fish maw, 花膠/鱼鳔, and
are served in soups or stews. Swim bladders are also used in the food
industry as a source of collagen. They can be made into a strong,
water-resistant glue, or used to make isinglass for the clarification of beer.
In earlier times they were used to make condoms.
Swim bladder disease Swim bladder disease is a common ailment
in aquarium fish. A fish with swim bladder disorder can float nose down
tail up, or can float to the top or sink to the bottom of the aquarium.
Risk of injury Fish with a swim bladder are at greater
risk of injury after exposure to loud impulsive sounds than those without.
Similar structures in other organisms Siphonophores have a special swim
bladder that allows the jellyfish-like colonies to float along the surface of
the water while their tentacles trail below. This organ is unrelated to the
one in fish. Gallery
References Further references
Bond, Carl E. Biology of Fishes, 2nd ed., Saunders, pp. 283–290.
Pelster, Bernd "Buoyancy at depth" In: WS Hoar, DJ Randall and AP Farrell
Deep-Sea Fishes, pages 195–237, Academic Press. ISBN 9780080585406.