#embryos

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)

"Two water fleas (Daphnia sp.) with embryos (left) and eggs (right)."

Marek Miś

Suwalki, Podlaskie, Poland

Lab Closure

For ethical reasons studying human embryo development is prohibited beyond a very early point, so much is now achieved using lab-grown organoids – models of embryogenesis derived from human pluripotent stem cells. Here, in such a system, researchers have elucidated molecules regulating neural tube closure (ZIC2, SOX11 and ZNF521 – shown normal left column and without each of these in the remaining columns) – the process necessary for development of the nervous system – providing insights for understanding and treating neural tube defects such as spina bifida

Read the published research article here

Image from work by Roya E Huang and Giridhar M Anand, and colleagues

Department of Stem Cell and Regenerative Biology, Harvard University, Cambridge, MA, USA

Image originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)

Published in eLife (reviewed preprint), April 2026

Embryonic stem cells that play a critical role in determining our facial features during development can be hindered from growing when placed under increased pressure. An international team of researchers took a look at the growth of mouse and frog embryos, as well as human embryoids (clusters of embryonic cells developed in the lab) to better understand how some cells tell others how to grow and differentiate. They noticed that when an increase in hydrostatic pressure was applied externally to the embryo or embryoid, important cell signaling pathways in neural crest cells were disrupted.