Meet the sea spider, a bizarre and fascinating arachnid that’s helping scientists unravel the evolution of eight-legged creatures. Despite its unusual appearance, this bottom-dwelling creature is a crucial anchor for understanding the genetics of chelicerates, a group that includes spiders, scorpions, ticks, and horseshoe crabs.
Sea spiders are characterized by their long, spindly legs and lack of abdomen. They breathe through their skin using peristalsis, a muscle contraction mechanism similar to how humans squeeze food down our throats. Males even cement fertilized eggs onto themselves, carrying them around until they hatch.
Researcher Prashant Sharma and his team have been studying sea spiders, particularly the knotty species Pycnogonum litorale, which has never experienced whole-genome duplication. This unique trait makes it an important reference point for tracing back the evolution of modern spiders and related species.
The study reveals that sea spiders are missing a gene called “Abdominal-A” from the Hox cluster group, which is responsible for organizing body parts. As a result, they’ve stuffed their reproductive organs and breathing systems into their legs. Interestingly, fossil sea spiders from millions of years ago do have an abdomen, suggesting that this structure may have been lost at some point in evolution.
Sharma’s team has published the first high-quality genome of a sea spider species, providing valuable insights into the evolution of chelicerates. By studying these creatures, scientists can gain a better understanding of how their bodies have developed over time and uncover new mechanisms for genetic variation.
The discovery of the knotty sea spider has shed light on the mysterious process of whole-genome duplication and its effects on the evolution of eight-legged creatures. As Sharma notes, “They are incredibly cool and understudied animals” that continue to fascinate scientists with their unique biology and intriguing evolutionary history.
Source: https://phys.org/news/2025-07-weird-sea-spider-evolution-legged.html