Physiology of Finfish and Shellfish (2+1)

3.1.28. A sexual Reproduction

Some branchiopods such as fairy shrimp and water fleas, ostracods such as mussel shrimp and isopods such as wood lice are parthenogenic, meaning they are asexual and therefore produce eggs that do not need any fertilization. They hatch on their own. Marmokrebs are a species of crayfish that are all female and reproduce asexually. The female creates a clutch of eggs attached to her abdomen with between 20 and 200 eggs that later hatch without sperm fertilization.

Penaeids are dioecious and the external structures of the genital system are the major dimorphic features. The male has two pairs of modified abdominal appendages (Pleopods) on the first and second abdominal segments (the petasma and appendix masculina) that deliver sperm to the female's external receptacle (the thelycum) located between the bases of the fifth walking legs (pereopods). The petasma, appendix masculine and thelycum are located on the ventral surface.

The petasma is formed by the endopodites of the first pair of pleopods which are modified as interlocking structures for spermatophore transfer. The appendix masculina are on the endopodites of the second pair of pleopods and serve to separate the petasma into two component halves. The thelycum may be "open" or "closed", depending on the species. "Closed" thelyca are those where the spermatophore is placed by a male in the groove below the plates while the female is in the soft exoskeleton stage following molting. The spermatophore is stored for some time before spawning. "Open" thelyca are not enclosed by plates, and the spermatophore must be placed on it by a male when the female's exoskeleton is hard; usually within hours of spawning. The presence of a spermatophore on the female is evidence that she has successfully mated. Open thelyca are found in some shrimp species endemic to the Western Hemisphere, such as P. stylirostris and P. vannamei; while closed thelyca are characteristic of most Asian species, such as P. monodon, P. chinensis, P. indicus and P. merguiensis.

Internal organs of the male reproductive system include a paired testes, vas deferens and terminal ampoules for spermatophore storage. The female reproductive system includes paired (but partially fused) ovaries that extend from the mid-thorax to the posterior end of the abdomen, and oviducts terminating adjacent to a single thelycum.

(Dioecious species are those whose individual members can usually produce only one type of gamete ; each individual organism belonging to a dioecious species is distinctly female or male (the word deriving from the Greek for "two households"). The majority of animal species (for example, all mammals and most reptiles ) are dioecious).

Crustaceans produce from eggs, which have been fertilized by sperm in much the same manner as other animals. The eggs are produced in the ovaries in the female and passed to the outside through oviducts. The sperms are produced in tubular testes in the male. After the eggs have been fertilized, they begin development and then hatch.

The eggs of most crustaceans are carried attached to the female until hatched. Some females develop brood pouches in which the young are retained for a time. A nutrient secretion which sustains the young until they are released is produced in some species having a brood chamber. Penaeid shrimp and a few of the lower crustacea deposit their eggs in the medium in which they live, in some cases attaching them to aquatic vegetation.

When the eggs hatch, this can take several days to several weeks depending on the species, the young larvae are detached. From this point, they are on their own and must feed, grow, swim and survive. After a series of transformations, the larva becomes a miniature adult.