Later on, Kate and Sawyer were fishing for sea urchin while flirting. ("S.O.S.")
Jin offered some to Hurley, who refused ("Pilot, Part 2"). Hurley later believed that this act made Jin hate him. While fishing, Hurley was stung in the foot by a sea urchin and tried to make Jin pee on it to neutralize the poison. ("Hearts and Minds")
Sun used a spine from a sea urchin as a makeshift needle, in order that Jack could give some of his blood to a fatally wounded Boone. ("Do No Harm")
Libby was attacked by a sea urchin while picking mussels. ("Lockdown")