The knotty sea spider has a fundamental physical difference from land-based spiders: It has no abdomen. Instead, it stashes its reproductive, digestive and respiratory organs in its legs. This peculiarity, common among all sea spiders, intrigues evolutionary biologists. How did this abdomen-free lifestyle get started?

In a paper published on Wednesday in the journal BMC Biology, researchers who have sequenced the whole genome of the knotty sea spider, which is found on both sides of the Atlantic Ocean, revealed possible answers to how these organisms diverged from other arthropods. The findings are a reminder that evolution, that great winnower of genes, sometimes moves in mysterious ways.

Biologists interested in reconstructing the family trees of spiders and their relatives have long sought a complete sea spider genome, said Prashant Sharma of the University of Wisconsin-Madison, who is an author of the new paper. Because sea spiders are members of a group that are siblings of arachnids on land, characteristics they share with modern land spiders could be traced to a common ancestor.

But sea spiders are difficult to grow in the lab, which is partly why previous work exploring their genes has been somewhat piecemeal. To sequence the whole genome of a sea spider species, Dr. Sharma turned to a thriving colony of Pycnogonum litorale, or knotty sea spiders, cultivated by Georg Brenneis of the University of Vienna. Dr. Sharma calls Dr. Brenneis “the mastermind of sea spider development.”

To collect the creatures, Dr. Brenneis and his colleagues head to an island in the North Sea at low tide and turn over rocks, exposing the stout, slow-moving spiders underneath. Once a sea spider has been ferried back to the lab, scientists present it with a buffet of sea anemones attached to mussel shells.

“It goes and grabs whatever it likes,” Dr. Brenneis said. Through long experience and observation, the scientists have determined the species of anemone the sea spider is most partial to.