Scientists have discovered of a bristle worm with eyes so big – and so sharp-seeing – that its vision rivals that of mammals and octopuses.
The eyes of Vanadis bristle worms – tiny, transparent marine worms found around the world – are about 20 times as heavy as the rest of the creature's head. If our eyes were proportionally as big as theirs, they would weigh 100kg.
It's fair to say this marine creature is pretty, um, eye-catching. So, why do Vanadis bristle worms (or polychaetes) have such gigantic eyes?
Anders Garm, a neuro and marine biologist from the University of Copenhagen's Department of Biology, along with his colleague Michael Bok from Lund University were as eager as any to find out. Their study, published in Current Biology, reveals that it may have something to do with a secret language related to mating.
"Together, we set out to unravel the mystery of why a nearly invisible, transparent worm that feeds in the dead of night has evolved to acquire enormous eyes," says Bok.
"As such, the first aim was to answer whether large eyes endow the worm with good vision."
Garm says "it's really interesting because an ability like this is typically reserved for us vertebrates, along with arthropods (insects, spiders, etc.) and cephalopods (octopus, squid).
"This is the first time that such an advanced and detailed view has been demonstrated beyond these groups. In fact, our research has shown that the worm has outstanding vision. Its eyesight is on a par with that of mice or rats, despite being a relatively simple organism with a minuscule brain."
"Its eyesight is on a par with that of mice or rats, despite being a relatively simple organism with a minuscule brain."
The challenge for the researchers is to find out what caused such a simple organism to develop such good eyesight.
"What we do know is that its most important activities, like finding food and mating, occur at night. So, it is likely that this is when its eyes are important," says Garm.
"We have a theory that the worms themselves are bioluminescent and communicate with each other via light. If you use normal blue or green light as bioluminescence, you also risk attracting predators. But, if instead, the worm uses UV light, it will remain invisible to animals other than those of its own species. Therefore, our hypothesis is that they've developed sharp UV vision so as to have a secret language related to mating."
"Our hypothesis is that they've developed sharp UV vision so as to have a secret language related to mating."
"It may also be that they are on the lookout look for UV bioluminescent prey. But regardless, it makes things truly exciting as UV bioluminescence has yet to be witnessed in any other animal."
Garm and his colleagues say that "further studies will be required to fully explain the visual ecology of the alciopids (a family of polychaetes) and why they, apparently alone amongst the annelids, have evolved the capacity for object vision."
