Question

MANY FISH SPECIES ARE PROTOGYNOUS MEANING THEY START LIFE AS FEMALES AND SWITCH TO BEING MALES LATER IN LIFE. THERE ARE MANY COLORFUL SPECIES OF PARROT FISHES AND WRASSES WITH THIS LIFE HISTORY PATTERN. WHEN PROTOGYNOUS FISH CHANGE SEXES, WHAT DO WE CALL THIS TYPE OF BEHAVIOR? ALSO, EXPLAIN HOW PROTOGENY MAY HAVE EVOLVED THROUGH NATURAL SELECTION FROM AN ANCESTOR WHERE THE FISH REMAIN ONE SEX THEIR ENTIRE LIVES.

Answer 1

Ans- when progynous fishes changes their sex the behaviour is known as sequential hermaphroditism.

Sequential hermaphroditism has two forms protoandry (change of sex from a male to female) and protogyny (change of sex from a female to male).

Among the several hypothesis proposed, for the evolution of protogyny, in different species of fishes- size/age advantage model has been largely accepted.

According to this model male has higher fecundity (gamete production) rates than female and also due to larger body sizes of adult males in later ages have higher chances of mating with females and to establish their territory.

Hence due to this reason these fishes reproduce as females and produces eggs in earlier stages of life when their sizes are small as well as their fecundity is high. When the female grows older and larger in size then they change their sex to males so that their reproductive success and gamete production rates will remain unaffected and higher number of offsprings are produced throughout their lifespan.

This is the reason why sequential hermaphroditism is favoured in due course of evolution but it is still rare in nature because the cost of energy paid by the organism for sex change is very high and hence only in adverse conditions and competition payment of this cost is advantageous.