Spawn & Sacs

In 7.3 only, any mobs that are purely aquatic (so, excluding the semi-aquatic spinosaurus and sarcosuchus) hatch from either spawn or egg sacs. They do not function like non-avian eggs because they don't have a model or a waiting period, and they also do not function like avian eggs because they can't be thrown. They will also always hatch when right-clicked into water of sufficient area for the creature.

All eggs follow the same creation process. DNA must be put in a culture vat to produce a sac or spawn, giving the player the achievement "What Species is this?". The sac or spawn will always match the species of the DNA cultivated. The cultivation process can sometimes fail, producing a failuresaurus and not an egg. When a male and female plesiosaurus, mosasaurus, or liopleurodon breed, they will drop an egg sac item that will not hatch unless used by a player. Fish and nautilus will occasionally drop spawn without mating (since they do not have genders), which also do not hatch unless used by a player.