Sea spiders live on the seafloor in oceans around the world, including along the coast of Norway. They range from a few millimetres in size to 70 centimetres. With long, thin, multi-jointed legs, they resemble oversized underwater crane flies.
Researchers from Austria, Norway, and the US have now mapped the genome of a sea spider in detail. This provides new answers about why the sea spider looks the way it does.
For the first time
Sea spiders are arthropods and belong to the chelicerate group, alongside horseshoe crabs and arachnids like spiders, mites, and scorpions.
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Bastian Fromm, a researcher at UiT The Arctic University of Norway, contributed to the new study.
"There are about 8,000 to 9,000 animal genomes that have been sequenced," says Fromm.
Most of these mapped genomes belong to mammals, fish, or animals we like, are related to, or have an economic interest in, he explains.
Researchers have mapped the genome of the species Pycnogonum litorale.(Photo: Georg Brenneis)
"Out of the more than a thousand species of sea spiders that exist in the world, none have had their genome mapped," he says.
This is what researchers have now addressed.
Several steps
The species they analysed is called Pycnogonum litorale and can be found along the Norwegian coast, among other places.
The gene analysis happened in several stages, beginning with sequencing – reading the order of the letters, or base pairs, in the DNA.
"After that, it has to be pieced together. Then you need to identify what kinds of genes are in the genome. There are protein-coding genes and non-coding DNA," says Fromm.
Fromm's group specialises in some of the non-coding parts of the DNA. This is DNA that does not code for proteins but can still have important functions. Fromm specifically works with micro-RNA, small RNA molecules that help regulate genes.
Bastian Fromm is a researcher at UiT.(Photo: UiT)
Missing a blueprint for the abdomen
Sea spiders lack a body part that spiders or scorpions have, says Fromm.
"It's just gone. So we asked: why? Can we see it in the genome?" he says.
The researchers discovered that sea spiders lack a specific Hox gene. Hox genes control body development in animals, ensuring that the right body part develops at the right time and in the right place.
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Mutations in these genes can cause body parts to form in the wrong location. For example, a mutation in the Antennapedia gene in fruit flies causes legs to grow on the head instead of antennae, according to the encyclopaedia Britannica.
"In arthropods, Hox genes play a central role in the correct specification of the different body segments; but also in many other animal groups they are essential 'master controllers' during body plan development," Andreas Wanninger, who also participated in the study, says in a press release.
The sea spider is missing a particular Hox gene called abdominal-A, which is responsible for developing the abdomen or the rear part of the body.
A female Pycnogonum litorale feeding on a sea anemone.(Photo: Georg Brenneis)
"What’s fascinating is that the micro-RNA that normally regulates this gene is also missing. It lines up perfectly," says Fromm.
This explains why sea spiders have virtually no abdomen.
"But we still don't know why they lost it, or why that loss wasn't harmful," says Fromm. "This might be the first clue to understanding why sea spiders have been so successful around the world."
Sea spiders are everywhere – they can be found just offshore, and in oceans across the globe, says Fromm. They are often small, just a couple of centimetres, but especially in the Arctic and Antarctic, they grow quite large. The reason for this size difference remains unclear.
"They feed on things like sea anemones. Sea spiders look rather creepy and alien-like, but they're not dangerous to humans," he says.
The researchers behind the study are interested in continuing to study sea spider development, Georg Brenneis at the University of Vienna mentions in the press release.
"From an evolutionary developmental perspective, sea spiders are very interesting: their mode of development may be ancestral for arthropods, but at the same time they boast multiple body plan innovations unique to themselves. Beyond this, they also possess remarkable regenerative abilities," he says.
