We may call them “crabs,” but horseshoe crabs are more closely related to spiders and scorpions than to the crustaceans they share their coastal homes with. The four species in existence today can be found along the east coasts of North America and South Asia, descendants of creatures found in the fossil record as far back as 445 million years. Sometimes called “living fossils,” the horseshoe crabs we see today have changed little in the last 250 million years. After all, when it comes to evolution, if it ain’t broke, don’t fix it.
In 1967, Haldan Keffer Hartline won the Nobel Prize in Physiology or Medicine for research on the eyes of horseshoe crabs, but in the decades that followed, the research on these animals’ unique biology slowed down considerably.
This has long puzzled 2024 Grass Fellow Guilherme Gainett, a postdoctoral fellow at Boston Children’s Hospital and Harvard University. As a development biologist, he is fascinated by these ancient creatures and what their eyes can teach us about the evolution of sensory systems. Because while spiders, scorpions and other related arthropods have eyes with a single lens (similar to our human eyes) their ancestors had eyes with multiple lenses, something that you see today in most crustaceans and insects. Today’s horseshoe crabs have two lateral compound eyes and two median single-lens eyes, as well as additional photoreceptors, such as those on their telson (tail).
“Horseshoe crabs are the only animals in this group that includes spiders that retain this ancestral compound eyes,” said Gainett. “So they’re really a key lineage for understanding this transition, and I’m interested in understanding these changes in types of sensory systems.”
Gainett set out to the Marine Biological Laboratory to develop new tools for genetic studies in the Atlantic horseshoe crab L. polyphemus, to help demystify the molecular mechanisms that control their eye development. First he looked at the gene expression across the eyes in embryonic horseshoe crabs, noting that some important genes that regulate eye development are expressed in the compound lateral eyes, but not in the single-lens median eyes, suggesting that different upstream regulators initiate their development.
He also used CRISPR-Cas9 to target the tryptophan 2,3-dioxygenase (TDO) gene, which encodes an enzyme required for pigment formation, resulting in the first genome-edited horseshoe crabs, in this case with clear, whitish eyes.
“Research-wise it was really amazing and transformative for me, describing this very efficient protocol for doing genome-editing in horseshoe crabs,” said Gainett. “It felt for me as a moment of refreshing my science.”
The results from Gainett’s summer at the Marine Biological Laboratory open new doors into our understanding of the molecular basis of compound eye development with implications for the evolution of different types of eyes. And equipped with the first-ever protocol for genome-editing in horseshoe crabs, researchers can unlock more secrets hidden in the genomes of these unique creatures.
Gainett credits his productive summer at MBL to his supportive cohort of 2024 Grass Fellows, the exceptional facilities, mentors such as horseshoe crab expert Dan Gibson, and the enthusiastic surrounding community of researchers.
“There’s definitely some magic that happens there at the MBL,” he said.