We only see horseshoe crabs when they come ashore to spawn

A female horseshoe crab lays as many as 20 batches of up to 4,000 eggs on her annual visit to the beach. When the eggs

hatch,the juvenile horseshoe crabs often stay near shore, periodically shedding their shells as they grow. Once they leave

these shallow waters, they don't return until they reach sexual maturity ten years later. Despite our best efforts, we don't

know where they spend those years. Though we've spotted the occasional horseshoe crab as deep as 200 meters below the ocean's

surface, we only see large groups of adults when they come ashore to spawn.