Baby Horseshoe Crabs: these eggs contain tiny horseshoe crab embryos, and the hatchlings typically emerge after 2-4 weeks, but it takes another ten years for them to reach adulthood
These photos show the embryonic form of Limulus polyphemus, commonly known as the Atlantic horseshoe crab. The eggs of this species are initially opaque, with a grayish-blue, green, or pink coloration, but they become increasingly translucent as the embryos mature, providing a glimpse of the tiny horseshoe crabs developing within.
Above: several embryos twirling around in their eggs
The legs become visible roughly five days after fertilization, and the embryos begin to move shortly thereafter, eventually flexing their legs and twirling their bodies. They molt for the very first time just a few days later. Each embryo will shed its skin and grow a new one four times in total before it even hatches from the egg.
The color variations are likely related to the growth rate of each individual embryo, which can differ slightly based on the temperature, salinity, moisture, and oxygen levels around each egg. Certain colors can also arise from bacterial or fungal growth in the egg membrane.
Above: Limulus polyphemus embryos
The hatchlings finally emerge after 2-4 weeks. The freshly-hatched larvae measure less than 1cm long, and they look just like miniature versions of their adult form, except that they do not yet have tails (which are actually known as telsons) and their exoskeletons are still soft and translucent. These young horseshoe crabs are often described as "trilobite larvae."
Above: a young horseshoe crab discarding its egg
Atlantic horseshoe crabs generally spawn in May and June, with hundreds of thousands of individuals gathering along the coast on the night of the full moon and new moon. Each female lays up to 100,000 eggs per season, but very few of those offspring 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).
Above: the freshly-hatched larvae
Wild horseshoe crabs can live to be more than 20 years old, and they can measure up to 60 centimeters (2 feet) long. They have 10 eyes in total, including two compound eyes that are specifically adapted for the purpose of finding a mate:
The most obvious eyes are the two lateral compound eyes. These are used for finding mates during the spawning season. Each compound eye has about 1,000 receptors or ommatidia. The cones and rods of the lateral eyes have a similar structure to those found in human eyes, but are around 100 times larger in size. At night, the lateral eyes are chemically stimulated to greatly increase the sensitivity of each receptor to light. This allows the horseshoe crab to identify other horseshoe crabs in the darkness.
Above: a close-up of a horseshoe crab's compound eye, which is covered in tiny hatchlings for some reason
Horseshoe crabs have been around for at least 445 million years, which means that these creatures are about 200 million years older than the dinosaurs and at least 50 million years older than trees, and yet their morphology has changed very little in that time. In fact, modern horseshoe crabs are frequently described as "living fossils," because they still look strikingly similar to their fossilized ancestors.
Above: the juvenile form of Tachypleus tridentatus, commonly known as the Chinese horseshoe crab
It's important to note 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 are more closely related to spiders and scorpions than they are to crabs.
Above: Tachypleus tridentatus and Limulus polyphemus
This is a revised/updated version of a post that I published about two years ago, with much more information, photos, and sources.
iNaturalist: Horseshoe Crab Eggs
Maryland Department of Natural Resources: Horseshoe Crab Life History
Current Zoology: Developmental Ecology of the American Horseshoe Crab
PBS: Once a Spawn a Time: Horseshoe Crabs Mob the Beach
Smithsonian National Zoo and Conservation Biology Institute: Limulus polyphemus
National Wildlife Federation: Horseshoe Crabs
Maryland Department of Natural Resources: Horseshoe Crab Anatomy
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