Sea urchin spines do not dissolve naturally in the skin. The spines are composed of a hard, calcified material called calcium carbonate, which does not readily dissolve in body tissues. If sea urchin spines become embedded in the skin, they may need to be manually removed by a healthcare professional. Leaving the spines in the skin can increase the risk of infection and other complications. It's important to seek medical attention for proper evaluation and treatment to ensure the prompt and safe removal of sea urchin spines.