Morphology, composition and fertilization of carp eggs: a review

This paper is a review of the literature on eggs of carp (Cyprinus carpio L.) and some related species and includes data taken from the former USSR and eastern European countries. Reported data relate to fecundity, egg morphology and composition, and fertilization. Information is also available on the ovarian fluid composition with characterization of proteins and lipids. The fecundity of common carp is very high, ranging from 100 000 to 300 000 eggs · kg−1 body weight per oogenetic cycle, with one cycle/year in the case of females reared in an outside natural pond environment. The diameter and weight of the eggs are in the range of 1.24–1.42 mm and 0.86–1.41 mg, respectively. The ooplasm includes large amounts of yolk and various organelles (endoplasmic reticulum, Golgi apparatus, mitochondria), cortical granules and alveolae. The main characteristics of cyprinid ovarian fluid are an osmotic pressure of about 300 mOsm, a Mg2+ concentration of 2.58 mM and a high ovarian fluid pH of 9. Energy in the form of ATP which is necessary for egg metabolism originates from glycolytic and oxidative reactions. The egg has a relatively thick vitelline envelope (VE) or zona radiata which is reorganized into the fertilization envelope (FE) after fertilization. The micropyle located at the animal pole is a funnel-shaped structure leading spermatozoa to the ooplasmic surface on which a fertilization cone develops after fertilization. A site of sperm attachment is identified on the plasma membrane at the level of the internal aperture of the micropylar canal. Major differences are observed between the two outermost VE and FE layers, as revealed by electron microscopy, enzyme or carbohydrate cytochemistry, and imunohistochemistry. FE extracts have strong bactericidal and fungicidal effects.