Baby Horseshoe Crabs: these eggs contain tiny horseshoe crab embryos; the hatchlings usually emerge after 2-4 weeks, but it takes another 10 years for them to mature into adults
Horseshoe crab eggs are initially opaque, with a greenish-gray, blue, or pink appearance, but they become increasingly translucent as the embryos mature, providing a glimpse of the tiny horseshoe crabs developing within.
The embryo's legs become visible about five days after fertilization, and it starts moving shortly thereafter, eventually flexing its legs and twirling its body around in the egg. It molts for the very first time after about a week; the embryo must shed its shell and grow a new one four times in total before it's finally ready to hatch.
The hatchlings usually emerge after 2-4 weeks. They measure less than 1cm long, and they look just like miniature versions of the adult horseshoe crabs, except that they don't have tails/telsons yet and their exoskeletons are still soft and translucent. The larvae are also known as "trilobite larvae."
A horseshoe crab can lay more than 80,000 eggs per year, but very few of those eggs actually survive to adulthood. Most of the eggs are eaten or destroyed before they can even hatch, and many of the remaining larvae perish at some point during the 10 years that it takes for them to reach full maturity (i.e. the age at which they begin to reproduce).
Some wild horseshoe crabs can live to be more than 20 years old, however.
Horseshoe crabs have existed for at least 445 million years, which makes them about 200 million years older than dinosaurs, and yet their basic physiology has changed very little since then. Modern horseshoe crabs are often described as "living fossils," because they still look strikingly similar to their fossilized ancestors.
It's worth noting that horseshoe crabs are not true crabs. In fact, they're not even crustaceans; they belong to a completely different group of arthropods known as chelicerates, and they're more closely related to spiders and scorpions than they are to crabs.
Sources & More Info:
Smithsonian National Zoo and Conservation Biology Institute: Horseshoe Crabs
PBS: Once a Spawn a Time: Horseshoe Crabs Mob the Beach (video)
Maryland Department of Natural Resources: Horseshoe Crab Life History
Current Zoology: Developmental Ecology of the American Horseshoe Crab, Limulus polyphemus
National Wildlife Federation: Horseshoe Crabs
U.S. Fish and Wildlife: The Horseshoe Crab (PDF)
iNaturalist: Atlantic Horseshoe Crab Eggs