From Creatures Wikia
Rainbow Sharkling (Odontaspis spectrumian)
The beautiful but deadly Rainbow Sharkling is one of the most vicious predators of the Marine Terrarium - it has even been known to attack wandering creatures! In addition, its coloration will often trick its prey into thinking it is a harmless animal. In time the Rainbow Sharkling can reproduce and create a whole rainbow of different colored sharks.
At the beginning of the game, several sharklings with randomly picked (although usually similar) coloration appear in the left side of the Marine Terrarium. They are also present with Aquanornia.
If your sharklings have died off, you can use the Shark Agent to replenish your stocks.
The sharklings' variations in color begin to decrease significantly after a short period of game time. This is because of a small programming glitch; the Sharkling was coded so that its offspring all have a tint somewhere between that of its two parents. It would have been better if it was programmed so that the offspring could also randomly mutate their colors dramatically every few generations. (This is perhaps something an enterprising developer can fix?)