Question

Why does hydra shows cross fertilization instead of self fertilization in some cases

Answer 1

Cross-fertilisation occurs as a rule in the different species of Hydra. To avoid self-fertilisation, even in monoecious species, the testes mature first, i.e., protandrous condition exists. However, fertilisation takes place when mature spermatozoa released from testes approach randomly to the naked ovum surrounded in gelatinous sheath.

Many sperms may penetrate the gelatinous sheath to reach the ovum but only one of them reaches to the ovum and fuses with it completely to form the zygote which becomes diploid with 30 chromosomes. The process of fertilisation takes place effectively only when the sperm reaches the ovum within its viable condition; that usually remains for two hours from its being exposed to naked otherwise it perishes.