Bigmouth Galaxy is unlocked by feeding a Hungry Luma 800 Star Bits, who can be found outside of the Bedroom dome. The Hungry Luma will appear when you’ve completed Soaring on the Desert Winds in Dusty Dune Galaxy.
There is only one level in this galaxy, therefore there is only one star up for grabs.
Bigmouth’s Gold Bait
The goal of this level is to find the Golden Shell that can smash open the Golden Chest.
Big Eels - The Giant Eels will not chase you, but if you’re caught in their path they will either bite you or slap you with their tails.
Koopa Troopa - There is one Koopa Troopa in this level. You can just on top of it to get rid of him and then you may use its shell to throw at other enemies.
Jellyfish - Jellyfish will shock you if you go near them. They can be defeated using koopa shells but there isn’t much point in attacking it as it isn’t really hostile.
Boos - The Boos will move towards you when you aren’t facing them. You can’t spin attack or jump on them to defeat them - and if you try, you’ll get damaged. You can defeat them when you’re holding the golden shell as it acts like a spotlight.
Jammyfish - Jammyfish are often found swimming in groups of four. They are underwater creatures that will damage you if you touch them, but otherwise they aren’t hostile towards Mario.
Crabbers - The crabbers can be temporarily stunned by jumping on them, but they cannot be defeated permanently this way. To defeat them, you’ll need to throw a shell at them.
Whirlpools - The whirl pools in the large cavern are spinning water tornados that will suck you in and cause you to spin uncontrollably and take damage.