An international collaboration featuring the University of Vienna and the University of Wisconsin-Madison (USA) has led to the first-ever chromosome-level genome assembly of a sea spider (Pycnogonum litorale). The genome informs about the development of the characteristic sea spider body plan and constitutes a landmark for revealing the evolutionary history of chelicerates in general. The study was recently published in BMC Biology.
Sea spiders (Pycnogonida) are marine arthropods with highly unusual anatomy: their trunk is very narrow and short, many of their internal organ systems extend into their long legs, and their abdomen is extremely reduced, to the point of being almost unrecognisable. Together with much better known animals such as spiders, scorpions, mites, or horseshoe crabs, sea spiders belong to the group called chelicerates, named after their claw-like mouthparts, the chelicerae. The bizarre body plan of these "no-bodies" raises fascinating questions: what genetic factors underlie its formation? And what can this tell us about the evolutionary history of chelicerates? The answers lie in their genome.
A high-resolution genome
To produce the genome assembly, researchers combined complementary sequencing technologies. First, the genetic material of a single P. litorale individual was obtained using so-called "long-read sequencing", a technology that is able to capture very long stretches of DNA. This facilitates the correct assembly of otherwise challenging repeated or complex genomic regions. Then, the spatial organisation of the genome was assayed in a second P. litorale individual, revealing which DNA pieces lie close to each other in the cell nucleus. By leveraging the distance information, the correct order of the sequenced DNA stretches can be ascertained. This combination of data sources led to the assembly of 57 pseudochromosomes, representing almost the entirety of the sea spider genome in unprecedented resolution. This was additionally supplemented by novel datasets of gene activity in various developmental stages of P. litorale. "The genomes of many non-canonical laboratory organisms are challenging to assemble, and Pycnogonum is no exception. Only the combination of modern high-throughput data sources made a high-quality genome possible", says the study's first author Nikolaos Papadopoulos from the Department of Evolutionary Biology at the University of Vienna. "This can now serve as a stepping stone for further research.
Original publication:
Nikolaos Papadopoulos ;Siddharth S. Kulkarni; Christian Baranyi; Bastian Fromm; Emily V.W., Setton; Prashant P. Sharma; Andreas Wanninger; and Georg Brenneis. The genome of a sea spider corroborates a shared Hox cluster motif in arthropods with a reduced posterior tagma. In BMC Biology (2025).
DOI: https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-025-02276-x