On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Symbiosis. This is one of the several side missions that you can unlock during the exploration of the Southern Shelf location.
2) Passage to Southern Shelf Bay
Southern Shelf Bay
2) Passage to Southern Shelf
4) Bullymong and Midget
After completing Bad Hair Day and Shielded Favors, you will be able to take on the last mission from Hammerlock  in this region. Head to the passage to Southern Shelf Bay . After getting there, head towards the bridge  and use the lever shown in the above screenshot to activate the bridge. You can also use the ladder to get across the water to the other side.
On the other side of the bridge you will come across another lever which will open a further passage.
Head through the Bandits camp to the south-east, eliminating all resistance you come across. Eventually you will reach a staircase shown in the above screenshot. Climb up and keep going north-west.
Eventually you will reach another staircase. Climb them up and you should see the above door . Approach them. They should open and you will see your goal.
Kill the Bullymong and the Midget riding on it, take the items they drop and return to Hammerlock  to end the missions.