Jan,
As an aircraft mechanic with many 10+ years in fuselage structres and general flight operation of aircraft the cargo pit of any domestic or international flight is pressurized and will not be subjected to the cold enviroment at 30k plus feet, so basically it will not be -30 in the cargo area. To make a long story short if you pack the coral in tightly sealed double bagged you will be fine and they will not freeze. The coldest it gets in the cargo pit is about 70. And unless you have a dog or cat going into the cargo pit area (in a kenel) they most likely want the animal to be check by the vet for approval to fly in the event the animal dies (which happend the other day to one unlucky family) on a United flight in bound from Rome.