2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Ultrasound imaging of the retractable whale shark eye
Scientists have found that the eyes of whale sharks are covered with thousands of placoid scales in the form of oak leaves - probably, they protect the eyes from mechanical damage. In addition, as objects approach, the eyes of whale sharks are drawn into the skull. The authors of the work, published in the journal PLoS ONE, suggest that such eye protection indicates that vision plays a large role in the life of sharks.
The eyes must be on the surface of the body, and because of this, they are especially vulnerable to chemical and mechanical damage. Animals protect their eyes in different ways, for example, many terrestrial and aquatic animals have three eyelids: in addition to the usual two folds of skin, they have a nictitating membrane that closes the eye longitudinally. In gray sharks, the blinking membrane is additionally protected by dermal denticles, and some cartilaginous fish, which do not have a third eyelid, pull their eyes into the skull or screw them into the eye sockets.
Scientists from the United States and Japan, led by Taketeru Tomita of the Turashima Research Center in Okinawa, studied the eye protection mechanism of the whale shark Rhincodon typus. The eyes of these animals are located on the sides of the head and protrude beyond the sockets, which makes them especially vulnerable. Researchers performed a CT scan of an eyeball removed from a dead shark, and two live sharks from the Churaumi Aquarium were filmed and recorded with an ultrasonic sensor.
On the surface of the eye around the iris of the shark there were about three thousand placoid serrated scales, similar to oak leaves. On the surface of the body of whale sharks, scales of a different shape (with triple ridges), and dissected denticles are characteristic of multi-toothed sharks - the former serve mainly to reduce water resistance, and the latter for mechanical protection. Probably, the scales replace the eyelids of whale sharks and protect the eyes from mechanical damage. Such eye protection has not yet been found in other cartilaginous fish, including species close to whale sharks.
Scales on the eye of a whale shark
Scales on the eye of a whale shark
When a person with a camera approached the sharks, their eyes were drawn into the skull in a second, and the resulting hole was partially covered by a fold of white connective tissue. With the help of ultrasound, scientists have determined that the eyeball goes deep into half its diameter - about three centimeters. Usually the eyes were retracted for a short period (to protect against damage), but one of the sharks had an eye hidden for ten days after being transported to Taiwan from Atlanta, where the animal was caught.
Whale shark retracts the eye
Previously, it was assumed that whale sharks rely little on vision - their eyes occupy less than a percent of the body length, and the size of the midbrain is also small. However, the discovered double eye protection, the authors of the work believe, may be evidence of a greater role of vision in the life of sharks than previously thought.
Whale auls mainly feed on plankton, filtering water, and because of this, they have to migrate long distances. The record for the distance of migration among all sharks belongs to the whale shark named Anna - she swam more than 20 thousand kilometers, from the coast of Panama to the Mariana Trench.
