Big Whale Sharks Have Lots of of Tiny Tooth-Like Constructions Round Their Eyes

Whale sharks, the majestic creatures that additionally occur to be the world’s largest fish, are way more mild than the title suggests. They swim lazily round tropical waters, filter-feeding on plankton.

Now, biologists have found the enigmatic giants have a curious trick up their sleeve – teeth-like scales adorning their eyeballs. It seems to be the whale sharks’ model of eyelids, and one we’ve not seen in every other vertebrate.

“Eyeballs face a possible threat of harm from mechanical, chemical and organic hazards,” the researchers write in a brand new paper describing the discover.

“This report elaborates on diversifications of the eyes of the whale shark (Rhincodon typus), together with the invention that they’re coated with dermal denticles, which is a novel mechanism of eye safety in vertebrates.”

Dermal denticles, often known as placoid scales, are the tiny powerful scales that cowl sharks and rays. However though they’re scales as we consider them, they’re additionally structurally similar to tooth, and embrace an interior core of pulp, a center layer of dentine or bony tissue, and a tough enamel-like coating on prime.

We already knew that many sharks – together with whale sharks – are coated with these denticles. Nonetheless, discovering them on the eyeballs of those fish is kind of the shock.

Eye denticles of the whale shark. (Tomita et al., PLOS One, 2020)

That is partially as a result of it is lengthy been assumed whale sharks haven’t got to make use of their sense of sight a lot: their eyes are extremely small in comparison with the remainder of their physique, and so they haven’t got plenty of midbrain, the a part of the mind that processes imaginative and prescient.

“Nonetheless, the extremely protected options of the whale shark eye, in distinction to the normal view, appears to counsel the significance of imaginative and prescient on this species,” the crew argues of their examine.

Researchers took a CT scanner to a preserved whale shark eyeball, in addition to taking ultrasounds of two captive stay whale sharks on the Okinawa Churaumi Aquarium in Japan.

3D rendering of the morphology of every eye denticle. (Tomita et al., PLOS One, 2020)

In addition to discovering the attention denticles, the researchers additionally elaborated on the mechanisms whale sharks use to retract their eyes again into their head. More often than not this may be for only a quick time frame, nevertheless not at all times. As soon as, a very unlucky shark ended up with its eyes retracted for 10 days after it was transferred to a brand new aquarium from Taiwan to Atlanta, Georgia.

“It’s doubtless that whale sharks preserve their imaginative and prescient throughout eye retraction as a result of the pupils of the whale sharks on this examine weren’t utterly coated with surrounding white tissues when their eyes had retracted, although their visible area could be way more restricted than when their eyes are positioned usually,” clarify the researchers.

“In truth, the animal that stored its eyes retracted for about 10 days on the Georgia Aquarium appeared to haven’t any drawback navigating the exhibit area, till its eyes returned to their regular positions abruptly and, apparently, spontaneously.”

So, whereas whale sharks aren’t precisely a fearsome predator of the ocean, these two eye-protecting options are a fearsome competitor to… eyelids.

The analysis has been printed in PLOS One.