The planet has an ocean, or a large body of water.Just nowhere near where these people are. When the eclipse happened, the water evaporated and the planet went cloudy - that's why no starlight, etc - and explains the rain. Yes, this raises other questions, BUT... it makes some really non-sensible things make a little bit more sense.
- Not that wild a guess, as the other crewman tells Fry that the surface they're about to crash-land on has gypsum, cinder, and evaporite deposits. The cinder would be volcanic, but evaporites (gypsum included) originate when seas dry up.
The Necromongers devastated the planet early in their reign.The idea that the Bioraptors decimated M6-117's surface ecosystems doesn't make sense, because apex predators don't do that sort of thing. Rather, the "whales" and other indigenous life above ground were wiped out by an early experimental version of the Conquest Icons, which the Necromongers were testing prior to deploying such weaponry on human-populated planets. The devices annihilated organisms on the surface, but subterranean life forms like the glow-slugs and Bioraptors were shielded beneath the ground. Prior to that catastrophe, above-ground predators had regularly crept down into the Bioraptors' tunnels to hunt the creatures, keeping their population in check; once there were no more surface-hunters to cull them, the Bioraptors bred out of control, accounting for their Super-Persistent Predator behavior and Monstrous Cannibalism by the time of Pitch Black.
The geologists who set up the base camp are to blame for the bioraptor attacks.When they arrived on M6-117, the giant creatures with the tree-like spinal growths scared the crap out of them, simply because they were so huge. So they wiped them out and left the carcasses to rot. Normally, those same animals would have been the Bioraptors' natural prey, but when they emerged during the eclipse 22 years ago, their food source had been reduced to dried-up bones. Famished, they devoured the geologists and every other animal species in the area, reducing the desert's sparse, but stable ecosystem to complete desolation: no animals left, and with no wildlife that can pollinate them or distribute their seeds, eventually no plants to speak of either. 22 years later, the desert has yet to recover, so the current batch of emergent Bioraptors have nothing to eat but the castaways and each other.
Riddick's personality change between this film and Chronicles is because of Fry's Death.Up until that moment nobody has sacrificed themselves (intentional or not) for Riddick, hence the whole "not for Me" he says upon the Bioraptor making off with her, one of Riddicks defining characteristics is he honours his promises or what he owes (he never promised to get the others to safety ever in the entire movie), Part of the reason he is so protective over Jack is because Fry was, while he does care about Jack, it began as him honouring a debt he owed to Fry to keep them alive, which again is another reason why he was so angry over Imam being killed by the Necromongers, by doing so The Necromongers caused part of his "debt" to Fry to be broken.