Main Walkthrough

Location: Martha's Rest
Petitioner: Clarke, Fishmonger
Unlocked: Complete Homecoming
Reward: 1,000 gil, 10 Wyrrite, 10 Magicked Ash

In order to trigger this side quest you first need to complete the main story quest Homecoming. This involves completing two additional quests, Building Bridges and A Bearer's Lot. All of them take place in and around Martha's Rest, and you won't need to go far from the region to complete Building Bridges and A Bearer's Lot. Once you've started Holding On three side quests will appear, including Crystalline Lifeline.

Look south of the Martha's Rest Obelisk, near the bridge down to Three Reeds, and you'll see a man peering at a bundle of barrels and buckets filled with fish. His name is Clarke, and he dropped a crystal that he uses to chill fish. He wants Clive to go get it for him. Accept and Clarke will suggest that a monster might have eaten the thing.

Climb down the ladder near Clarke and head into Three Reeds. Down by the water you'll find a group of Bog Crabs, which are scuttling melee enemies that can spew out a spread of bubbles. They're nothing special, and you can swiftly smack them down with an AOE attack or two.

Defeat the Bog Crabs you'll receive a Foul Crystal. Return to Martha's Rest - the Obelisk is a bit faster than climbing - and speak to Clarke to receive your reward.