The specialised retinas of some nocturnal coral fish result in faster vision and greater sensitivity to dim and bright light. The same adaptation may enable deep-sea animals to see in darkness.
Fish living in dim conditions can have as many as 28 layers of light-sensitive rod cells at the back of their eyes, known as a multibank retina. But exactly how this helps them to see in low light was unclear until now.
at the Queensland Brain Institute in Australia and her colleagues have uncovered how these specialised retinas are advantageous when light is scarce. 鈥淥ur findings highlight that not all vertebrates see the same way or even have the same basic structures to see the world,鈥� says Fogg.
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The team carried out experiments on sammara squirrelfish (Neoniphon sammara) and lattice soldierfish (Myripristis violacea), two nocturnal species with multibank retinas, as well as the ochre-striped cardinalfish (Ostorhinchus compressus), which has only one layer of rod cells. They placed electrodes on the eyes of the fish to investigate how well the animals could differentiate between evenly spaced light pulses.
The fish with multibank retinas were able to distinguish between much faster pulses of light. 鈥淚t implies that species with multiple rod layers can make out moving objects and especially the details of a moving object, such as a predator or a fleeing prey item, way quicker than a species with a single rod layer,鈥� says Fogg. This ecological benefit is key to survival, she says.
The squirrelfish and soldierfish also had enhanced sensitivity to dim light and bright light at night. Fogg says some fish living in low-light environments might still need to see well in bright conditions, like when feeding on the surface of the water under a full moon, and multibank retinas may help with this ability.
In daylight conditions, sammara squirrelfish had higher sensitivity to bright light than the other two species did. Like mammals, fish have two types of light-sensitive cells: rods and cones. Sammara squirrelfish have a higher density of cones, which are active during the day, than lattice soldierfish, which have a mostly dim-light visual system. The soldierfish like to hide deep inside the coral matrix or inside caves 鈥� emulating the conditions faced by deep-sea fish.
at the University of Sussex in the UK is hopeful that the principles of this study can be applied to deep-sea animals. 鈥淭here is no obvious reason that would suggest a deep-sea multibank would work very different from a shallow-sea one,鈥� he says.
However, working on deep-sea animals is notoriously difficult, says Baden. 鈥淭hese will be some of the very last visual systems to be truly understood, if ever.鈥�
Reference: bioRxiv, DOI:
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