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.
Once the sea spiders are settled in wire cages, they eventually reproduce. The researchers provide cnidarian larvae and other delights to the offspring, and study their development and growth.
It was these creatures’ genes that were examined in the new paper.
Within the larger genome, the researchers zeroed in on what are known as Hox genes. These bits of DNA control the development and organization of animal bodies, and they exist in some form in creatures as diverse as jellyfish and pandas. The genes help inform growing cells of what each segment of the body should turn into. Destroying a certain Hox gene in a fruit fly, for instance, can make it grow legs where its antennae should be.
Earlier work had suggested that sea spiders might have an aberrant Hox gene controlling abdomen development. But Dr. Sharma and his colleagues discovered that the abdomen-commanding Hox gene is just plain gone.
“This is the first genome assembly that’s put together a whole Hox cluster for a sea spider,” Dr. Sharma said. “And it’s missing from the cluster.”
That raises an interesting question, said Antonis Rokas, an evolutionary biologist at Vanderbilt University who was not involved in the study. Did the abdomen shrink in these creatures because the gene controlling its patterning was destroyed? Or did having an abdomen get less and less useful because of some circumstance in which a knotty sea spider ancestor lived, so that when the gene finally disappeared, nothing much changed?
“We don’t know what caused what,” he said.
The findings also highlight that while it’s common to imagine new traits arising from new versions of genes, Dr. Rokas continued, it has become clear that losing genes is important, too.
One way to explore the consequences of the gene’s loss, and potentially shed light on what happened millions of years ago on Earth, would be to try to destroy it in another species of arachnid, Dr. Sharma said.
Perhaps the abdomen segment would disappear, or perhaps it would take another form.
“That’s something,” he pondered, “you could test.”
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