Thursday 14 August 2014

Bioluminescent Sharks Strengthens its Survival with Newly Evolved Eyes

 Image credit: Dr. J. Mallefet (FNRS/UCL)
According to a recent study promulgated in the journal ‘PLUS ONE’, it has been observed that the luminescent sharks that used to dwell in the ‘Twilight Zone’ have gone through a certain evolution that allows them to see through the dark. These animals thrive at a depth of about 650 to 3,300 feet (200 to 1,000 meters), a bleak region known as the mesopelagic twilight zone, where hardly any sunlight can penetrate through.

The eye with the Superpower

Bioluminescence appears as brief as a flash and the ability of the sharks to see and tell whether it is a potential mate, lunch or a predator is an ability possessed by these species. These creatures possess a greater density of light sensitive cells in their retinas or to be more precise, higher amount of rod densities inside their eyes, while some have even acquired ocular adaptations allowing them to see the glimmering lights they use for signalling each other.

To predate they can camouflage themselves in the realms where hardly any light can perforate through. Nevertheless, this light also helps the sharks to find partners both for predating and mating purposes, one of the best examples, being the glowing lantern shark that possesses evolved sexual organs with light-producing capabilities that help attract its opposite sex, even in the dark.

Result from further studies...

Now, there always arises a question when it is about an optical system, be it an eye or a camera, whether there is a compromise in either the light sensitivity or the resolution, as most deep-sea animals are visual modality for light sensitivity and not resolution.

So, Claes and his confrères examined the shape of the eye, the structure and mapping of the retinene cells of the five known species of deep-sea bioluminescent sharks namely four lantern sharks (E. splendidus, E. spinax, Etmopteruslucifer andTrigonognathuskabeyai)and one dalatiid or kitefin shark (Squaliolusaliae) have been observed to have undergone through these evolutions, using a light microscope and other ophthalmic instruments.

The research workers compared the eyes of other non-bioluminescent sharks that account for the 45 of the 50 known shark species, and discovered that the luminescent ones have a higher density of photo-sensitive cells in their eyes, cognized as rods than the others, which gives the luminescent ones improved temporal resolution, or to be more precise “faster vision”. (For example, Persons possessing slower vision, when they would watch a tiger chasing its prey, they would only see disjointed frames of the chase compared to a smooth movement of the chase.)

Thus having more immobile vision allows them to quickly see the modifying light patterns, they usually use to interact with each other.

A further study has helped the scientists find a layer of tissue, just behind the retina which helps reflect and increase the light available to the photoreceptors. Gaps between the lens and iris have also been found that allows extra light to reach the retina, along with light absorbing rods at the back of their eyes.

A Few Words for Thought

Thus, it is well evident that with evolution taking place in almost every form of living, the ocular system of the bioluminescent sharks have also co-evolved with their photoluminescence, thus allowing its species to endurefrom getting endangered or extinct.

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