What is the difference between external and internal fertilization?

External fertilization takes place in both invertebrates and vertebrates (bony fishes and amphibians) where both sperms and eggs are released in the watery environment: here gametes fuse by chance encounter.

Such external fertilization may be highly specialised as seen in earthworms where eggs are laid in a cocoon, then sperms are collected from another earthworm and later deposited inside the cocoon containing eggs.

Internal fertilization occurs by transfer of sperms in female body (insemination) and fertilization takes place in vivo. Internal fertilization takes place in insects, in nematode worm Wuchereria bancrofti, in cartilagenous fishes as well as in amniotes (reptiles, birds, mammals).

For internal fertilisation, opposite sexes must copulate. Chance of fertilization of gametes is very high when it takes place in vivo. Male animals often have copulatory organ to facilitate copulation.

External fertilization is a reproductive strategy in which eggs are fertilized outside of the organism’s body, typically in a water environment. Organisms that use external fertilization, such as many fish and amphibians, release eggs and sperm into the water, where fertilization occurs.

Internal fertilization, on the other hand, involves the direct deposition of sperm into the female’s reproductive tract, allowing fertilization to occur inside the body. This method is common in mammals, reptiles, birds, and some species of fish and insects. Internal fertilization often requires specific copulatory organs and behaviors for successful sperm transfer.

The main difference between the two is the location of fertilization: external occurs in the external environment, while internal occurs within the female’s body. This distinction leads to varying reproductive strategies and adaptations in organisms to protect their offspring and ensure successful reproduction.