Changelings - Anne McCaffrey, Elizabeth Ann Scarborough

Changelings is the first book in the Twins of Petaybee series.

Petaybee is changing. It's growing and expanding its land. The twins of Yana and Sean are also growing. Their exuberance has outgrown their caution. They're almost caught in seal form by illegal poachers. To save them from the dangers on Petaybee Yana and Sean have sent them to spend time on the space station. It's important to do this while they're young enough to survive off of Petaybee. But were they really being sent to safety? Time will tell. Another great Petaybee adventure. Very highly recommended.