To see – and survive – at night, some coral fish have developed visualadaptations that are similar to those of their cousins living in the ocean’sdarkest depths, new research shows.

University of Queensland researcher Dr Fanny de Busserolles said the reef-dwelling squirrelfish and soldierfish had well-adapted visual systems fornight-time activity.

“Like many nocturnal animals, their retinas have mainly rod cells,” Dr deBusserolles said.

“These are highly sensitive to the tiniest amount of light and are thereforeused at night.

“However, our study found the squirrelfish and soldierfish also have anexceptionally high number of these sensitive cells stacked in layers, formingwhat is called a multibank retina.

“This type of visual adaptation is rarely found in the animal kingdom, exceptin deep-sea fishes.

“We were surprised to find it in shallow-living reef fishes.”

Fish with multibank retinas often have between two and six rod banks, but theQueensland Brain Institute researchers found squirrelfish and soldierfish havemany more – between six and 17.

Dr de Busserolles said human and other vertebrates’ retinas had two types ofphotoreceptor cells: cones and rods.

“Cones are used during the day and they help us determine colours,” she said.

“Nocturnal animals like owls and cats have sacrificed cone cells in favour ofrod-dominated eyes with a single layer of rod photoreceptors.”

She said the advantage of a multibank retina was not clear, but scientistsspeculated it might increase sensitivity or enable fish to see colour in thedark.

Co-author Dr Fabio Cortesi said squirrelfish and soldierfish wereHolocentridae, “a really interesting” family.

“Our results suggest they have the potential to discriminate colours in a widerange of light settings – during the day with their cones, and in dimmerconditions with their multibank retinas,” Dr Cortesi said.

“It is exciting that we in Queensland have an entire family of coral reef fishthat can easily be studied in the aquarium,” Dr de Busserolles said.

“This study, along with additional experiments being run by the team, opensnew avenues of research on the function of multibank retinas and the abilityto see colours in the dark.”

Dr de Busserolles said the reef fishes’ retinas also gave indications ofactivity during the day.

“When the sun rises, holocentrids usually retreat to the reef’s dark corners,however we showed that the family may also have good daytime vision, havingthe potential to see colour through their green and blue-sensitive conecells,” she said.

“Reef fishes that are active during the day grab plenty of attention becausepeople are amazed by their brilliant colours.

“But this is an exciting discovery from some of their less colourful,nocturnal counterparts.”

The researchers work in the lab of Professor Justin Marshall.

The research is published in the Journal of Experimental Biology.

Image above: Squirrelfish are part of a family of coral fish that takeadvantage of a ‘multibank retina’ to survive on the reef at night. Image:Justin Marshall

Image above left: the sammara squirrelfish Neoniphon sammara, in a QueenslandBrain Institute aquarium. Image: Valerio Tettamanti.

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