Unusual catch of octopus benefits science
A specimen of the in Sweden unusual seven-arm octopus is now being examined by researchers at the Tjärnö Marine Laboratory. A fishing boat caught it at the end of October, but it had no commercial value.
Instead, Christin Appelqvist, a marine biologist at the University of Gothenburg, was contacted, and she saw an opportunity to increase knowledge about the animal.
"The other day, a man from Gothenburg Fish Auction called and asked if I wanted to bring an octopus to the lab that is quite unusual, a seven-arm octopus or Haliphron atlanticus, as it is called in Latin. I thought it sounded very exciting to take advantage of the opportunity and take samples," says Christin Appelqvist.
How unusual is it in Swedish waters?
"We don't really know. We rarely get any specimens to study, but it has been filmed on a few occasions using remote-controlled underwater vehicles. A researcher here at the Tjärnö Laboratory has seen it twice in Swedish waters."
The seven-arm octopus has eight arms just like other octopuses. But in males, the eighth arm is transformed into a mating organ that attaches to the female and then detaches. Hence it´s name. The male dies after mating. Females often die after laying eggs or releasing their young.
It is a pelagic species that mostly swims freely in the water and does not burrow into muddy bottoms or attach itself to rocks and stones. It normally lives in deep waters in the Atlantic, from a few hundred to several thousand metres deep, but it also occurs in other oceans.
The specimen that arrived at the Tjärnö Marine Laboratory weighed 25 kilograms and was 125 centimetres long. According to the information Christin Appelqvist received, the squid was a bycatch from a fishing boat trawling for shrimp at a depth of 200 metres in the western Skagerrak.
"Bycatch is fish that is not the species being targeted. This one slipped into the trawl and came on board. We don't know how often it happens that bycatches with no commercial value are thrown back. But we researchers are very grateful that the octopus was taken to the auction and that we were then able to obtain it from there. It can contribute to basic research," says Christin Appelqvist.
What does it mean for research to obtain a specimen like this?
"It means a great deal. If we went out looking for it ourselves, the chances of finding it would be very slim. Attempts are sometimes made to film it at greater depths, and test trawling is also carried out, but finding these animals is a matter of chance. So when a fine specimen like this turns up, we get the chance to study it and try to understand more about what is happening beneath the surface."
Christin Appelqvist has now examined the octopus, taken DNA samples and preserved one eye and the mouth apparatus. This particular part is characteristic of the species and can be used to compare with octopuses caught in other waters to see if they are the same species. Christin has also looked at the internal organs and examined what the octopus had in its stomach.
"This one had eaten shrimp, which may explain why it got caught in the trawl," she says.
What will happen to the octopus now?
"It's in our freezer. I'm now searching my network for researchers who want to do more research on it. I'm also going to write a short scientific report on the find so that it can be searched for in scientific databases."
Text: Mikael Andersson
- The largest specimen found was almost four metres long and weighed 75 kilograms, a female.
- The species is one of the two largest known octopus species, the other being Enteroctopus dofleini, which does not have an established Swedish name, but is sometimes called the giant Pacific octopus or eight-armed giant octopus.
- Males are much smaller, with a maximum size of about 30 centimetres. This is called sexual dimorphism – large differences in size between the sexes.
- Until 2025, it had only been sighted five times in Scandinavia.
- It is not classified as endangered in international species databases.