Today, horseshoe crabs range from Maine south to the Yucatan. LAL is now used to test the sterility of injected medicines and such devices as heart valves and kidney dialyzers. Unlike human blood, horseshoe crab blood turns bluish when exposed to air because its oxygen-carrying molecule contains copper. Horseshoe crabs achieve sexual maturity at 9 to 12 years of their 19-year lifespan. They spawn from late April through mid-August, most intensely on nights with a full or new moon.
High tides provide additional protection to the females and eggs.
Horseshoe crabs move from deep water to protected bays and inlets. At high tide the animals mate and the female deposits between 2 and 30 thousand eggs. The male fertilizes the eggs, which are covered by sand before ebb tide, when the pair returns to deeper water.