From Creatures Wikia
The Natatory Grendels are a genetic breed createdby KnyteTrypper to commemorate the CCSF 2006 release of the Aquatilis Pod and Aquatilis Caverna v.2. The Natatory Grendels are amphibious and long-lived. With just a little bit of instruction they quickly learn how to survive in an aquatic world. They are not prolific breeders, so they won't fill up your ship with grendels. Once impregnated, the females seem to experience a brief but normal pregnancy, and mutations are rare. They're a reasonably peaceful species for grendels, and seldom harm their fellow creatures.
Coloration of the Natatory Grendels varies by gender and by life stage. They change color at every life stage. At adulthood the males are green, the females are almost entirely black. They use a mixture of C3 grendel and aquanorn sprites, but players without C3 who have Pyrite Grendels or Dusky Grendels should still be able to see them correctly. The Watereggs Agent and the aquanorn Swimming Agent are necessary for them to function correctly in their environment.