Brisinga endecacnemos entered the fjord from the vast Norwegian Sea, where it lives at depths of 3,000 to 4,000 metres.
And this is no ordinary starfish.
A sensation in 1853
In 1853, the folklorist and amateur zoologist Peter Chr. Asbjørnsen became the first person to haul up a nearly 70-centimetre-wide Brisinga with 11 arms from the depths of the Hardangerfjord.
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Asbjørnsen’s discovery was a sensation.
Before 1853, few people believed that anything at all could live hundreds of metres below the ocean's surface.
Asbjørnsen named the starfish after Brísingamen, the necklace of the Norse goddess Freyja. According to legend, it was a piece of jewellery that the god Loki threw into the sea.
The eye – or, more accurately, the eyespot – is located at the very tip of each arm.
Each eye consists of numerous tiny light-sensitive units called ommatidia. They function somewhat like the eyes of insects, but in a much simpler way.
For an animal that has no head, placing the eyes at the tip of each arm is an excellent solution.
In a study published in 2014, researchers Anders Garm and Dan-Eric Nilsson showed that starfish may have much better vision than previously believed.
What's the point of producing light if you're blind?
Researchers believe that all starfish once had eyes at the tip of each arm.
But when Anders Garm, Henrik Glenner, and their colleagues examined 21 deep-sea species of starfish, they found eyes in fewer than half of them. In the depths of the ocean, many of these species appear to have lost the ability to see.
One of the species that completely lacked eyes was Brisinga endecacnemos from the Sognefjord.
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So what on earth is the point of lighting up in the darkness?
"It may seem paradoxical that an animal produces light without being able to see it itself," says Anders Garm. He is a professor of marine sensory biology at the University of Copenhagen.
"That's exactly why this starfish from the Sognefjord is so fascinating," Henrik Glenner adds. He is a professor of marine biology at the University of Bergen.
In this video, you can join researchers Henrik Glenner and Anders Garm on their search for Brisinga in the Sognefjord.
The obvious theory was not confirmed
Brisinga has evolved the ability to produce bioluminescence.
Many fish, jellyfish, and other deep-sea animals have the same ability.
One fairly obvious hypothesis for Garm and Glenner was that the deep-sea starfish glows to attract small prey.
"We have made several trips to the bottom of the Sognefjord using a remotely operated underwater vehicle (ROV). But we were never able to confirm that this hypothesis was correct," says Glenner.
Brisinga does not lie on the floor of the Sognefjord glowing continuously. It only lights up sometimes.(Photo: MAREANO / Institute of Marine Research / CC BY-SA)
Researchers still don't know why
Because the starfish living on the bottom of the Sognefjord has no eyes, the researchers assume its bioluminescence must be intended for other organisms rather than itself.
What that function is, however, remains unknown. But researchers are eager to find out.
What Garm, Glenner, and their colleagues have discovered is that Brisinga's bioluminscence fits an increasingly familiar pattern in deep-sea biology:
Even in total darkness, many animals invest considerable resources in both producing and responding to light.
Why they do so is still, in many cases, a matter of educated guesswork.
Eggs that seem to be asking to be eaten
Henrik Glenner tells Science Norway about another strange feature of the Brisinga starfish.
This is another mystery that researchers have not yet been able to explain.
"It releases eggs that are bioluminescent too! When we placed the eggs in an aquarium in complete darkness, we could see them glowing," says Glenner.
Henrik Glenner is a professor of marine biology at the University of Bergen.(Photo: University of Bergen)
The eggs drift slowly upwards through the water in the fjord. But why do they continue glowing throughout the journey?
"They're practically shouting: Come and eat me!" he says.
Researchers now suspect the answer may involve a small fish that is abundant in the Sognefjord: the lanternfish.
One hypothesis developed by Garm and Glenner is that the starfish actually wants the lanternfish to eat its eggs.
The same strategy used by blueberries
On land, many plants rely on animals to eat their seeds.
Blueberries, apples, and raspberries all send the same message: Come and eat me!
Perhaps the starfish in the Sognefjord has simply evolved the same ingenious strategy.
If fish eat its glowing eggs, they may transport them away before eventually excreting them in completely new areas.
For now, this is only a hypothesis.
Researchers still do not know whether lanternfish actually eat Brisinga eggs.
"Our problem is that, so far, it has proven impossible to keep lanternfish alive in our aquariums long enough to answer that question," says Glenner.
Want to solve the mysteries
Henrik Glenner, Anders Garm, and their colleagues continue to investigate several unanswered questions surrounding the Brisinga starfish deep down in the Sognefjord.
"That's where our research stands today. But we are very keen to find answers," says Glenner.
