Horseshoe crabs have a unique reproductive strategy.
After laying the eggs she pulls the male over the nest when he then fertilizes the eggs.
It’s a good thing that she lays so many eggs because they then leave the nest unattended and the eggs are then open for predators to eat, mainly shorebirds.
Typically it is estimated that only 1/10 of 1 percent of the eggs will end up surviving to adult hood. With this dismal chance of survival it’s a good strategy that they lay so many eggs.
In all but the warmer climates horseshoe crabs will only mate once a year. This being in late spring to early summer.
Down here in Florida they mate pretty much year round except in mid winter when they bury themselves in the sand for a couple of months till things warm up a bit.