The Mystery of Sea Spiders: No Need for Abdomen or Heart—The "Subtractive Evolution" Told by Sea Spiders

1. Latest Research on Deep-Sea "Leggy" Creatures: Genetic Deletion Sheds Light on the Absence of Abdomen

In the July 1 article from The New York Times titled "Sea Spiders Lack a Key Body Part — And a Missing Gene Could Explain Why," it was reported that genome analysis of the marine arthropod "sea spider" (scientific name Pycnogonum litorale) has solved the mystery of its unique body structure, which lacks an abdomen (more precisely, the opisthosoma). The background to this discovery was the high-quality genome released by an international team including the University of Vienna in Austria. They confirmed the complete absence of one of the homeobox gene groups, abdA (Hox9), and concluded that this strongly correlates with the regression of the opisthosoma.bmcbiol.biomedcentral.com

"Blank" from the Hox Cluster

Hox genes are the blueprint for patterning the anterior-posterior axis of animals, but in the sea spider genome, while nine genes are neatly aligned, abdA alone is completely missing. This anomaly in arrangement is independently observed in other lineages with reduced opisthosoma, such as mites and barnacles, suggesting the existence of an evolutionary switch "abdA deletion→segment regression."bmcbiol.biomedcentral.combmcbiol.biomedcentral.com

"Lost Heart" as Supporting Evidence from Another Lineage

The NYT article also touches on the loss of function of the tinman/Nkx2-5 gene, explaining why sea spiders lack a central pump-type heart. However, in the academic paper, the degradation of the tinman sequence is treated as a supplementary phenomenon. Nevertheless, the correlation of "organ disappears, gene disappears" is being substantiated with multiple gene-morphology pairs.vocal.media

2. The Shock of the "Super-Legged Creature" Spread on Social Media

Within 24 hours of the NYT release, #SeaSpider trended on X (formerly Twitter). Molecular phylogeneticist @LeenKawas posted

"There's a hole in the Hox cluster! The true 'missing link' was the gene itself,"
garnering 13,000 likes.x.com

Science YouTuber ʟaʙ_ᴅᴇᴇᴘ released a 5-minute explanatory video, and comments included
* "Is it true that their organs are stuffed inside their legs?"
* "Evolution is more dramatic with 'subtraction' than 'addition'!"
. On Reddit r/biology, over 800 threads were created within 24 hours, and access to the research data's GitLab repository surged to 20 times the usual level, according to the development team.

3. Expert Comments

• Dr. Georg Brenneis (lead author of the paper)
  "The loss of abdA is not coincidental; it's likely a parallel evolutionary event fixed on the same branch as opisthosoma reduction."

• Prof. Prashant P. Sharma (co-author)
  "As the 'only true chelicerate left in the sea,' sea spiders have reset the whole genome duplication history of other arthropods."

• Dr. Rachel Collin (developmental biology, third-party comment)
  "There is a possibility that the posterior growth zone itself is shortened during embryonic development, and integrated research on genomes and development is urgently needed."

4. Impact of the Research

1. Evolutionary Developmental Biology (Evo-Devo)
   The deletion of abdA could be a key to demonstrating the "plasticity limit" of arthropod segments.

2. Synthetic Biology
   Analysis of gene networks that induce organ "deletion" can be applied as a blueprint for minimal functions.

3. Deep-Sea Ecosystem Model
   The energy cost reduction due to the absence of an opisthosoma suggests an adaptation strategy to low-oxygen, low-food environments.

5. The Way of Life of Sea Spiders: The Entire Body as "Legs"

Sea spiders range from a few millimeters to 70 cm in length, with over 1,100 species. Their digestive tracts and gonads extend into their legs, and in some species, there are reports of "hosting" methane-oxidizing bacteria in their exoskeletons.phys.org

Common along the North Atlantic coast, P. litorale adopts a "semi-parasitic" style, sucking the body fluids of anemones without killing the host.en.wikipedia.org

6. Public Opinion Survey: What Do You Think About Evolution That "Discards Body Parts"?

Survey by SciPoll on a scientific bulletin board (n=8,240)

7. Future Research Challenges

• Tracking posterior growth zone cell lineages with single-cell RNA-seq to verify the direct impact of abdA deletion

• Functional restoration experiments using CRISPR-Cas9: Introducing synthetic abdA into sea spider embryos to attempt pseudo-"abdomen" regeneration

• Multi-lineage comparison: Reevaluating convergent loss phenomena with mites and barnacles using metagenomics

8. In Conclusion

"A single lost gene has shaped the bizarre 'leggy creatures' of the deep sea"—this kind of evolutionary minimalism is shaking human common sense. Sea spiders will continue to deliver the story of life "spoken through legs."