Sea Spider Genome Reveals Secrets of Body Plan

Genetic study offers insights into evolution of chelicerates.

A newly completed genome of the sea spider *Pycnogonum litorale* is providing clues to the unusual body structure of these marine animals and the evolution of chelicerates. These creatures have reduced abdomens and legs so long that organs extend into them.

Key Development

Researchers from the University of Vienna and the University of Wisconsin-Madison successfully assembled the first chromosome-level genome of the sea spider. This breakthrough explains the species’ body structure and offers insights into the evolution of chelicerates, including spiders, scorpions, and horseshoe crabs.

The team combined long-read sequencing with data on DNA arrangement to build 57 pseudochromosomes, covering the genome with high precision. They also included data on gene activity across different developmental stages. The scientists focused on the Hox cluster, a gene family that defines body segments.

Missing Gene

Scientists discovered that *Pycnogonum litorale* lacks a key Hox gene called abdominal-A (Abd-A), responsible for shaping the rear end of arthropods. Its absence may explain why sea spiders have extremely reduced abdomens.

This supports an evolutionary pattern: when certain Hox genes disappear, the body parts they control often shrink or vanish. Sea spiders now join the list of species showing this gene-body connection. The research was published in BMC Biology.

Unlike spiders and scorpions, *P. litorale* shows no signs of ancient whole-genome duplications. Since sea spiders are considered the sister group to all other chelicerates, this suggests that genome duplications occurred later, in specific subgroups.

Implications

The completed genome of *P. litorale* gives scientists a powerful tool for comparing chelicerate species and studying the evolution of their body plans. According to a 2023 study, understanding the genetic basis of body plans can inform conservation efforts by highlighting unique evolutionary lineages (Nature Scientific Reports).

Sea spiders stand out because:

• Their development may reflect how early arthropods grew.
• They have unique body features not seen in other species.
• They can regenerate parts of their body.

With the genome and gene activity data now available, researchers like Georg Brenneis can explore these traits in detail at the molecular level.

The study is referenced in BMC Biology 23, 196 (2025). DOI: 10.1186/s12915-025-02276-x