'The eyes have it'—Photoreceptors in marineplankton form a depth gauge to aid survival27 June 2018

Scanning electron microscopy image of a 3-day-oldplanktonic larva of the marine ragworm Platynereisdumerilii. Credit: Jürgen Berger

The eyes of some marine-dwelling creatures haveevolved to act like a "depth gauge", allowing thesecreatures to swim in the open ocean at a certaindepth .

Pioneering new research, carried out by a team ofinternational scientists including Professor GasparJekely from the University of Exeter, has shed newlight on how sea-living planktonic animals use theirsimple eyes to measure depth in the ocean.

All eyes detect light using specialized cells calledphotoreceptors, of which there are two main kinds:ciliary and rhabdomeric. While crustaceans andinsects have rhabdomeric photoreceptors, animalswith backbones—including humans—have ciliaryphotoreceptors.

However, there are also several groups of animals,mostly sea-dwellers, which inherited both types ofphotoreceptors from their ancestors that livedmillions of years ago.

The new research, carried out by experts fromExeter's Living Systems Institute and collaboratorsat the University of Vienna and Emory University,has given a greater understanding of how the twokinds of photoreceptors interact in such a seadweller, shedding new light on the evolution of eyesand photoreceptors.

The researchers studied the larvae of the marineragworm, Platynereis dumerilii. The larvae ofPlatynereis are free-swimming plankton. Each hasa transparent brain and six small, pigmented eyeswhich contain rhabdomeric photoreceptors . Theseenable the larvae to detect and swim towards lightsources. Yet the larval brain also contains ciliaryphotoreceptors, the role of which was previouslyunknown.

The new research has revealed that ultraviolet lightactivates these ciliary photoreceptors, whereascyan, or blue-green light inhibits them. Shiningultraviolet light onto Platynereis larvae makes thelarvae swim downwards. By contrast, cyan lightactivates the rhabdomeric pigmented eyes andmakes the larvae swim upwards.

In the ocean, ultraviolet light is most intense nearthe surface, while cyan light reaches greaterdepths. Ciliary photoreceptors are therefore shownto help Platynereis avoid harmful ultravioletradiation near the surface. Though if the larvaeswim too deep, cyan light inhibits the ciliaryphotoreceptors and activates the rhabdomericpigmented eyes. This makes the larvae swimupwards again.

The research team also used high-poweredelectron microscopy to show that the neural circuitscontaining ciliary photoreceptors exchange

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messages with circuits containing rhabdomericphotoreceptors—suggesting the two work togetherto form a 'depth gauge'.

By enabling the larvae to swim at a preferred depth,the depth gauge influences where the worms endup as adults.

Professor Gaspar Jekely, from Exeter's LivingSystems Institute said: "The idea that marineanimals could use light to estimate their depth hasalready been proposed by theoretician, but to ourknowledge this is the first time that such amechanism has been experimentally studied."

Csaba Verasztó, one of the first authors of thestudy added: "Detecting different types of light withdifferent photoreceptor cells in marine planktonmay have been the ancestral framework for lightdetection in animals."

The depth gauge in Platynereis larvae representsan important new mechanism to influence thedistribution of marine animals. Its discovery shouldalso stimulate new ideas about the evolution ofeyes and photoreceptors.

Ciliary and rhabdomeric photoreceptor-cell circuitsform a spectral depth gauge in marine zooplanktonis published in the journal eLife.

More information: Csaba Verasztó et al, Ciliaryand rhabdomeric photoreceptor-cell circuits form aspectral depth gauge in marine zooplankton, eLife(2018). DOI: 10.7554/eLife.36440

Provided by University of ExeterAPA citation: 'The eyes have it'—Photoreceptors in marine plankton form a depth gauge to aid survival(2018, June 27) retrieved 2 February 2019 from https://phys.org/news/2018-06-eyes-itphotoreceptors-marine-plankton-depth.html

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