The knotty sea spider, Pycnogonum litorale, though not a true spider, plays a significant role in understanding the evolution of eight-legged creatures. This species represents an early branch in the genetic lineage that includes spiders, scorpions, ticks, and horseshoe crabs. According to Prashant Sharma from the University of Wisconsin–Madison, this makes it “an important reference for the evolution of all these species.”
Sea spiders are unique creatures with over 1,300 known species. Unlike their arachnid relatives such as scorpions and mites, they breathe through their skin using muscle contractions similar to peristalsis. Male sea spiders carry fertilized eggs on their bodies until they hatch. They lack an abdomen, leading them to appear like plumbing schematics.
Sharma’s research lab at UW–Madison focuses on these “incredibly cool and understudied animals.” He notes that sea spiders are crucial for understanding chelicerates’ genetics—a group including many-legged animals like spiders and ticks.
In a study published in BMC Biology, Sharma and his team presented the first high-quality genome of a sea spider species. Collaborating with researchers from the Arctic University of Norway and the University of Vienna—particularly Georg Brenneis—the study revealed that Pycnogonum litorale has never undergone whole-genome duplication.
Whole-genome duplication allows species to develop new traits by providing extra genetic material. However, P. litorale’s lack of this process places it near the base of the chelicerate family tree. This helps scientists trace evolutionary variations across modern arachnids.
Sharma’s research also uncovered why sea spiders have no abdomen—they lack a gene called “Abdominal-A,” part of the Hox cluster responsible for body organization. Consequently, their legs house organs typically found in an abdomen.
Interestingly, fossil records show ancient sea spiders did possess abdomens. Sharma remarks on this evolutionary shift: “We know they started out looking more like modern arthropods… And then at some point they just went totally bizarre.”
This research received support from the National Science Foundation (IOS-2016141), highlighting its significance in evolutionary biology.
