#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

Jessica Valenti at Abortion, Every Day:

The Trump administration is allocating millions of federal dollars to an Embryo Adoption Program—classifying frozen embryos as “children who already exist” and funding services that treat adopting a frozen embryo exactly like adopting a child. It’s a significant step toward enshrining fetal personhood—and a brazen move from an administration that’s been trying to avoid public abortion battles ahead of the midterms. After all, it wasn’t so long ago that Republicans faced a massive national backlash when Alabama’s Supreme Court declared that frozen embryos were “extrauterine children.” The fallout was so bad for the GOP that Donald Trump released a statement distancing himself from the ruling and coming out in support of IVF. I can only assume the White House thinks American voters won’t notice this total about-face because it’s tucked away in a call for grants from the Department of Health and Human Services (HHS). But the swooning response from anti-abortion organizations and conservative media will make it difficult for the administration to lay low—especially since those activists and writers are loudly celebrating what the White House is hiding from: enshrining fetal personhood and giving embryos full constitutional rights.

The document uses the word “child” or “children” nearly 40 times in reference to frozen embryos, calling the program “a response to the needs of children who already exist and are in need of a family,” and asking applicants to center “the wellbeing and best interests of the child.” The administration also refers to the “rights” of the embryo—a deliberate gift to the conservative legal groups eager to make that precise claim in court.

Now, the embryo adoption program itself isn’t new: it was established in 2002 by President George W. Bush in response to anti-abortion outcry over stem cell research. It started as a “public awareness” grant, meant to let Americans know that embryo adoption exists. (I’m sure readers my age and older remember the whole “Snowflake baby” thing.) Bush would later go on to host children born from IVF at the White House, and say that “each of these human embryos is a unique human life.” What he did not say, however, was that these embryos were “children.” That’s what makes Trump’s grant change so significant: the administration isn’t just saying embryos are people—they’re writing that lie into federal policy.

[...]

The administration also notes they want to “close the gap” between embryo adoption and traditional adoption—in part by requiring grantees to offer everything from home studies and background checks to health evaluations.

[“Unlike traditional adoption, which is designed with the child’s best interest in mind and requires rigorous screening of prospective parents, embryo adoption currently lacks these safeguards, subjecting children to increased risk.”]

By “closing the gap” and pretending that frozen embryos are children who need to be protected, the White House is doing something very pointed: they’re giving conservative legal organizations like Alliance Defending Freedom the foundation they need to argue for fetal personhood in the courts. As Borchelt says, “Defining frozen embryos as children gives anti-abortion extremists so-called ‘evidence’ that they could use to convince an extremist judge that embryos and fetuses deserve the same legal rights as people.” This isn’t the first time that the White House has moved to codify fetal personhood: one of Trump’s first moves after inauguration was issuing an anti-trans executive order that defined human life as beginning “at conception.” And as I reported last year, the administration now classifies certain forms of contraception as “abortifacient birth control”—a false claim rooted in the same logic. This embryo adoption program is their latest attempt.

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