Boreas
Boreas is Jove's fourth and most distant moon, and he consequently receives the least tidal heating. This has made him the coldest and most desolate of the Seven Earths.
Most all of his surface is covered in sea ice and permafrost, blocking out the sun and strangling whatever plants might want to live there. The only marine life is huddled around hydrothermal vents in the deep sea.
Things are a bit better in and around the tropics. Here, the ice - sheets are replaced by slush, icebergs, and sometimes open sea. Light streams down into the water, feeding all sorts of plankton and algae, which in turn sustain a diverse ecosystem. Convergent evolution has worked her magic, and the penguin, the walrus, and the seal all have their rough analogues here.
But the most notable animals are the Boread Sea Dragons, who resemble the extinct Plesiosaurs of Tellus. There are about a dozen species; most hunt prey with sharp teeth, but a few are filter - feeders. All are endotherms with thick layers of blubber, but some also have fur. They're always social creatures, but their social structures vary with species and circumstance. Some, like the Whitehead Sea Dragon, live in nuclear families with a single mated pair. Others, like the Southern Pygmy Sea Dragon, live in large pods which may contain thousands of individuals. Whatever the case, all Sea Dragons are very intelligent, some moreso than human beings. But, as they live in the sea and lack apposable thumbs, they have never been able to produce much technology.
But, the Sea Dragons have complex languages. Some sound like rhythmic clicking or tapping to the human ear, others are hauntingly beautiful. They have no writing, but their oral traditions stretch back generations, some recall a time before human beings set food on the Earths. A few sea dragons, especially of smaller and lighter species, have made their way into the wider solar system, especially the broader Jovian system and the Asteroid Belts. They have sharper memories than humans, and they're better at spacial reasoning, allowing them to excel at some professions.
There are no humans native to Boreas, and he has little of economic value. Most of his scientists are fishermen or research personnel, usually from Cels or Laran.