Fishes undergo external fertilisation,and the zygote is formed in the water. How is the zygote safe?

Answer 1

Some fish have internal, some have external! You are obviously just talking about "oviparous fishes." (There are some fishes with "gonopodia" which are penis-like fins that penetrate the side of the female and are used as an intromittent organ. Look up molley and platty MATING-PARING sexual behaviors.) These fish mate (not just breed with a spawning) and are ovoviviparous and give "birth." (not true birth like placental mammals.)

The zygote of oviparous anamniotes, however, is generally NOT that safe in the water! The very high number of eggs-sperm usually released-spawned and fertilized, however, helps the odds of some of the eggs surviving thru development. The mucus nature of anamniotic eggs helps prtect some, but not that much. The eggs are vulnerable to fungus and bacterial infection and to predation by other organisms.

There is a huge array of behaviors that many different species of fishes use to help protect their broods. Some are mouth brooders, some lay them clinging to rocks, into the sand or into crevices. Some lay them and stay nearby fanning them and guarding them. Eggs of seahorses are laid into the pouch of the male and he gives "birth." If you go thru ichthyology books, websites and investigate parental care of broods, you will come up with a HUGE range of behaviors exhibited by the various species of fishes. Even some species of sharks lay eggs, but their eggs are often camoflauged in the seaweed or along the ocean bottom.

Answer 2

Sometimes they are clumped, which is statistical safer. Some times a male will pouch them for protection. Sometimes females carry the zygotes in their mouths until further maturity. In some cases, " luck of the draw ".