Biology of sperm and artificial reproduction in carp

Abstract

Although the common carp, Cyprinus carpio, has been cultivated for several thousand years and is produced in large quantities, research on reproduction has been very limited. Traditionally, spawning occurred naturally in situ in rearing ponds. In slightly improved methods large breeding ponds stocked with brood fish were devoted to reproduction with fry collection in autumn, or in small spawning ponds with collection of larvae a few days after hatching. Controlled reproduction in hatcheries started only in the 1950s. This paper reviews some of the basic work on carp sperm and describes the technologies of artificial reproduction. The sperm is of a primitive type with uncondensed chromatin and a small midpiece. As in most teleost fish, the sperm is immotile in the male genital tract and in the semen and is activated after release into fresh water. The initiation of motility is due to the decrease in osmotic pressure. The structure of the flagellum is rapidly disorganized in fresh water and sperm stop moving after 30 s. When dilution occurs in a 50 mM NaCl solution, the osmotic change is sufficient to initiate motility, but the flagellum is not disorganized and swimming lasts a few minutes. Motility depends mainly on endogenous ATP stores, about 12 nmol 10 8 spermatozoa, and stops when 50–80% of the ATP is exhausted by hydrolysis. The procedure of artificial insemination includes collection of gametes (from “hypophysised” males and females), mixing in an extender (45 mM NaCl, 5 mM KCl, Tris 2.5 mM, glycine 19 mM, pH 8) in a ratio 1 litre of eggs to 1 litre of diluent and 1 ml of semen. Sperm motility can be triggered during collection by contamination of the semen with urine; this could interfere with fertilization and, if present, urine should be discarded by pouring out the top part of the tube. The sticky layer of the egg is removed by adding a “dissolving solution” (20 g urea/1 + 4g NaCl/1 of water) or milk (diluted 1 5 in water) to the fertilized egg and stirring. One hour later the swollen eggs are transferred to incubators (usually Zug bottles, sometimes 200 litre circular jars).