Physiology of Finfish and Shellfish (2+1)

3.1.3. Types of reproduction

3 types of reproduction are possible

1. Bisexual reproduction
2. Hermaphroditic reproduction
3. Parthenogenetic reproduction

1. Fishes are generally bisexual and sperms and eggs develop in separate individuals having either the testes or the ovaries. Eg. Carps.

2. The eggs and the sperms develop in the same gonad and self fertilization takes place. There are 2 types of hermaphroditism
   1. Synchronous or simultaneous hermaphroditism refers to the condition in which both parts of the ovo-testis mature at the same time. This gives rise to a fish which is functionally both male and female. (eg. Coral reef fish Hamlet, Hypoplectrus unicolor- Atlantic ocean. Fish of this species usually form pair bonds with a single partner. Each individual spawns alternatively as male and female. Shortly prior to spawning the fish ready to assume the male role gives a courtship call and the fish assuming the female role responds and releases eggs. Following fertilization of the eggs the spawning pair breaks apart and takes cover on the reef. In the next spawning event, the roles of the 2 fish are reversed with the original male acting as the female and the original female taking over the role of the male.

   2. Successive hermaphroditism in which the fish undergoes sex reversal. The sex change varies among hermaphroditic species with protrandrous- males change sex to become females. Protogynous- females change sex to become males.
      Sequential or successive hermaphrodites are born with one sex and change sex sometime during the course of life. An example of protandry is found in the anemone fishes. The fishes live with anemones in a symbiotic relationship; the anemone provides the fish with shelter and protection from predation, and the fish supply the anemone with food. Groups of fishes will live with one anemone, and will not switch anemones. Only the two largest will mate; the largest female and the second largest, the male. With the female being the largest, she can produce the most eggs. When the female dies, the largest male will change sexes and become the female. The rest of the fish are immature males. Some sea basses (Serranidae) are protrandric hermaphrodites, being males at first, females later. Eg. Sparidae and Serranidae are true hermaphrodies. 
      A classic example of protogyny is found in the wrasses and parrotfishes. The males in these species form harems, with one large male sequestering and defending a group of smaller females. The male enjoys spectacular reproductive success, as it has many females to mate with. The females also enjoy a limited reproductive success, producing as many eggs as they can, all fertilized by the one male. The male has the advantage over the females; it has many females producing eggs for him to fertilize, whereas the females only have themselves.
      For example some damselfishes (family Pomacentridae) begin life as males and change into females. In some, females can revert back to males.
      Some seabasses (family Serranidae) change from female to male, and are capable of reverting back to female.
      Most wrasses (family Labridae) are born female, grow into sexually mature females, and have the potential to transform into functional males later in life. In many of the wrasses, sex change correlates with social hierarchy and social behavior: social structure includes a large dominant male and many smaller, subordinate females. Removing the male from the group triggers the largest female to begin transforming into a male.

3. Parthenogenesis is the development of young without fertilization. This is also called gynogenesis. Males of other species are required for sexual reproduction where the sperms only stimulate gynogenetic development without contributing their chromosomes (it does not take part in heredity). Thereby the resultant young are always female with no trace of paternal characters. Eg. Poecilia formosa (Guppies).
   (Some species are unisexual. In unisexual species, there is no fusion of sperm and egg. The sperm is necessary to trigger the egg cell to develop into an adult, but the sperm cell ultimately degenerates and does not take any part in heredity. The resulting young always are females, with no trace of paternal characteristics. Thus, unisexual species are entirely female. They mate with males of related species to produce female offspring. Poecilia formosa is an example of a unisexual species. Always female, P formosa mates with male P mexicana or P latipinna).