Slimeballs and eyeballs

The hagfish are commonly grouped together with another extant species, the lampreys, in the cyclostome division (class Agnatha or jawless fish). The jawed fish, or gnathostomes, are a later division in our line. As the researchers write, controversy has long surrounded the interrelationship between hagfish, lampreys and jawed vertebrates. There are two modes of thought: one is that hagfish originated at the same time as the lampreys but diverged and then degenerated, losing their ability to form images. The other school is that hagfish diverged before the lampreys and as such must be considered a sister group.

Shaun Collin knows quite a bit about lampreys and hagfish, their eyes and proto-eyes. One of Australia's few marine neurobiologists, his work involves studying the evolutionary aspects of colour vision and of dim light vision, mostly working on primitive vertebrates.

Trevor Lamb is an expert on photoreceptors and the phenomenon of dark adaptation, the recovery of the visual system after exposure to very intense light. He has been working with psychologist and neuroscientist Ed Pugh for 25 years, collaborating on a unified theory on the photoreceptor's response to light and the cellular and molecular basis of dark adaptation.

Collin and Lamb have known each other for about 10 years and came together on this project when Lamb and Pugh were looking at the similarities and differences between rod and cone receptors within and across different species. Lamb and Pugh were probing the past to discover when rods and cones separated, and began delving even further back. They got to the lampreys, which have very similar eyes to us, but hit a stumbling block. Lamb asked Collin if he knew of something with a simpler retina or simpler photoreceptors and he mentioned hagfish. This is where the collaboration began.

Most people, including the fishermen who despair of catching them in their nets, think hagfish - or slime eels, as they are also known - are right ugly little beasts, Collin says. "They grow up to over one metre in length and we have been studying the New Zealand hag, which is a particularly large species. They've basically got a tough skin and the head region is quite strange.

"It doesn't have a jaw, so they are part of the jawless fishes group, and use a rasping mechanism to scrape flesh off their prey. It has a very strange way of feeding - it can actually attach to dead fish that have drifted down from above and proceeds to tie itself in a knot. The knot starts at the tail end and moves up towards the head, thereby enabling it to have something to hold onto while pulling flesh off."

Hagfish are a deep-water species, some extending down to a thousand metres or more, and they are called slime eels for a good reason, he says. They have slime glands in their skin which, when excited, produce large amounts of a combination of mucins and protein threads that turn the surrounding water into viscous mass of jelly.

"I examined these at the Scripps Institution of Oceanography some years ago and can still remember a colleague of mine putting a hagfish into a bucket, about half or three-quarters full of water, and disturbed the animal just so it swam around in circles," Collin says. "Within a few seconds, he could turn the bucket upside down and no water came out, no animal came out - it was like he'd put a jelly mould into the fridge and it has just gone solid. This is one of the mechanisms this animal uses to immobilise their prey."

While not a particularly pleasant creature, they have provided the perfect model to study the evolution of the eye. Hagfish don't have what we might consider an eye as such, merely photoreceptive tissue embedded in fat underneath a whitish patch of skin, he says.

"They have photoreceptors at the back of the eye which pick up light and dark and we suspect even may respond to different intensities of light, which we haven't proven yet. But there are definitely photoreceptors present, although they look a little bit different to ours."

Lampreys, on the other hand, have an eye that looks quite like ours, he says. "Effectively, it's like a human retina - the lens shape is a bit different because of the optical problems associated with living in water, but the retina looks very much like a rabbit or a squirrel or a human. Therefore, there has been a big jump between early animals and the lampreys. This is the contention of the paper, that the hagfish might be that link between those early eyes that are only able to see light and dark to something that can form an image with some resolution."