Why must external fertilization in animals always occur in water?

External fertilisation of the female gamete by the male gamete takes place mostly in water; because water facilitates: extended period of gamete survival and easy movement of sperms towards egg.

External fertilization is where the eggs and sperm involved in unite outside the body of the organisms. In this process, there needs to be some way of getting the sperm to the egg. Since the morphology of sperm in most is adapted to mobility in watery liquids, the sperms of many animals take advantage of the water to reach the egg by swimming. Typically such sperms have one or more flagella and in some species they are streamlined. Hence external fertilization occurs in water. In animals like earthworm external fertilization takes place in cocoon. This doesn't need presence of water.