If you think sea spiders are just odd marine critters, think again. A new study reveals that these strange and spindly bottom-dwellers may hold the key to understanding the evolution of all eight-legged creatures—ranging from spiders and scorpions to ticks and mites.
What Are Sea Spiders?
Despite their name, sea spiders (Pycnogonids) are not true spiders. With over 1,300 known species, these marine arthropods are found across oceans worldwide. Unlike other arachnids, sea spiders have no abdomen. Their body is essentially a network of tubes—housing vital organs inside their long, skinny legs.
They also breathe uniquely: through their skin, using peristalsis—wave-like muscle contractions—to circulate oxygen. When it comes to reproduction, male sea spiders take center stage by cementing fertilized eggs to their bodies and carrying them until they hatch.
The “Weirdness” of Sea Spiders and Why Scientists Love It
Dr. Prashant Sharma, a professor of integrative biology at the University of Wisconsin–Madison, leads a research team that studies the genetics and evolution of arachnids, collectively known as chelicerates. These include spiders, scorpions, mites, ticks, and horseshoe crabs.
“Sea spiders are incredibly cool and understudied animals,” says Sharma. His lab has previously worked with blind cave arachnids, acid-spraying vinegaroons, and other exotic species.
Sea spiders have become particularly valuable to scientists because they represent an early evolutionary branch of chelicerates. Their unique genome may help researchers understand how complex traits like venom, specialized limbs, and disease transmission mechanisms evolved across different species.
Genomic Milestone: First High-Quality Sea Spider Genome
In a groundbreaking study published in BMC Biology (2025), Sharma’s team, along with collaborators from the University of Vienna and Arctic University of Norway, unveiled the first high-quality genome of a sea spider species, Pycnogonum litorale. This species is commonly found along rocky seabeds of the North Atlantic Ocean and resembles a tiny, tangled root.
The researchers discovered something extraordinary: unlike many arachnids, P. litorale never underwent whole-genome duplication. In most chelicerates, genome duplication has led to evolutionary advancements by providing extra genetic material for new traits. The absence of this duplication places P. litorale close to the base of the chelicerate evolutionary tree, making it a critical reference point for comparative genomic studies.
The Mystery of the Missing Abdomen
Another surprising discovery was the absence of a key developmental gene known as Abdominal-A, part of the well-known Hox gene cluster. These genes play a vital role in determining body segment identity in arthropods. The absence of this gene might explain why sea spiders lack a conventional abdomen.
Instead, sea spiders have packed their internal organs—digestive systems, reproductive organs, and respiratory structures—into their legs. Even more intriguing, ancient fossil records reveal that early sea spiders did have an abdomen. When and how they lost it remains a mystery.
“We know they started out looking like more typical arthropods,” says Sharma. “Then, somewhere along the evolutionary path, they just got incredibly weird.”
Why This Research Matters
This study provides a genetic foundation for studying the evolutionary history of chelicerates, a group that includes some of the world’s most significant animals—agricultural pests like mites, disease vectors like ticks, and beneficial species like spiders. Understanding how these animals evolved can lead to insights in biodiversity, pest control, and even medicine.
Key Takeaways:
- Sea spiders are ancient, bizarre marine arthropods with over 1,300 species.
- They lack an abdomen and breathe through their skin using muscle contractions.
- New genomic research on Pycnogonum litorale helps trace the evolutionary roots of chelicerates.
- Sea spiders never experienced whole-genome duplication, unlike many arachnids.
- The absence of the Abdominal-A gene might explain their unusual body structure.
Further Reading:
Papadopoulos, N. et al. (2025). The genome of a sea spider corroborates a shared Hox cluster motif in arthropods with a reduced posterior tagma. BMC Biology. DOI: 10.1186/s12915-025-02276-x