How did fish lose their bladders?

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Nearly 20% of fish families include representatives which have lost their swimbladder through evolution, but the mechanisms responsible for the loss have remained a mystery.

However, a new study of Zebra danios, Danio rerio, may provide the answer.

Although Zebra danios haven't lost their swimbladders, the wild-form of the fish does have a surprisingly high number of genetic phenotypes which can lose their swimbladder entirely, or lose its functionality by preventing it from inflating with gas.

By studying these phenotypes, two scientists from Cornell University in the USA were able to study how the evolution of swimbladder loss might have occured in other fishes.

McCune and Carlson found that 22 of 27 recessive lethal mutations in wild Zebra danios involved loss or noninflation of the swimbladder. In nine cases, these mutations were the only apparent sign that something was wrong with the individual.

Their research claims that at least 19 of the 22 mutations are genetically distinct, and that there is a strong possibility that the remaining three mutations will also follow the same trend.

McCune and Carlson think that their discovery might have important implications for current thinking on the evolution of swimbladder loss: "Although adaptive explanations for gas bladder loss are convincing, a developmental bias toward bladderless phenotypes may also have contributed to the widespread convergence of this trait among teleosts.

"If gas bladder development in teleosts is as vulnerable to genetic perturbation as it is in zebrafish, then perhaps a supply of bladderless phenotypes has been readily available to natural selection under conditions for which it is advantageous not to have a gas bladder. In this way, developmental bias and selection can work together to produce widespread convergence."

For more details see: McCune AR, Carlson RL. (2004) - Twenty ways to lose your bladder: common natural mutants in zebrafish and widespread convergence of swim bladder loss among teleost fishes. Evol Dev. 2004 Jul-Aug;6(4):246-59.