Scientists have found that elephantnose fish have excellent vision due to the unique structure of their retina.
Most vertebrates have both rod and cone photoreceptors in their retinae, which are used and located separately. The rods are extremely sensitive at low light levels, but become saturated by bright light and colour. Cones, on the other hand, require brighter light to become activated and allow the experience of colour vision.
In a study published in Science, Moritz Kreysing from the University of Cambridge and an international research team described the unique retinal structure of the elephantnose fish (Gnathonemus petersii).
They found that these fish are able to activate both photoreceptor types at the same time. A feature that is probably responsible for the species excellent vision, allowing them to navigate easily through murky and dim environments and also spot predators.
Elephantnose fish are native to the muddy rivers in West and Central Africa. Until recently, scientists actually thought these fish were virtually blind and relied on their ultra-sensitive trunk-like mouth to sense other fish and objects via electroreception.
In the current study the researchers looked at vertical cross-sections of the fish’s retina under microscopes and also conducted behavioural analyses using predator-mimicking visual stimuli.
They were very surprised to find that, contrary to popular believe, this species has extremely good vision due to their eyes being highly complex in structure and function.
“In the retina of the elephantnose fish, cone receptors are grouped together within reflecting, photonic crystal-lined cups”¦ But rod photoreceptors are positioned behind these reflectors,” explained the researchers. This is a peculiar structure, because in most vertebrates the rods and cones are located in completely separate positions in the retina.
Furthermore, they found that the elephantnose fish only has red-detecting cone receptors. This is because red is the only wavelength that reaches through the murky water where this fish primarily lives.
They also compared responses of the elephantnose fish to other species that do not have this special grouped retina structure — such as goldfish — and found that they had superior responses to a number of different visual stimuli.
The researchers suggested that the complex eye structure of the elephantnose fish developed as an evolutionary advantage to help detect large predators moving though murky environments.