The eye, which is the organ that obtains optical information from the external environment, must be located on or near the surface of the body. Thus, eyeballs face a potential risk of damage from mechanical, chemical and biological hazards. Many terrestrial and aquatic Vertebrates prevent this risk by the coverage of eye surfaces with eyelids. For example, Carcharhinid and Sphyrnid Sharks have nictitating membranes or 'third eyelids', which cover their eyes completely during their feeding activities. The outer surface of this membrane is covered with dermal denticles, which likely increases its protective ability. In contrast, many other Elasmobranchs that are not equipped with nictitating membranes have to protect their eyes in different ways, such as retracting the eyeballs into the head (e.g. Electric Ray, Guitarfish), or rotating the eyeballs back into the orbit (e.g. White Shark). However, the eye-protection mechanism in many Elasmobranch species remains largely uninvestigated.
In a paper published in the journal PLoS One on 29 June 2020, Taketeru Tomita of the Okinawa Churashima Research Center and the Okinawa Churaumi Aquarium, Kiyomi Murakumo, also of the Okinawa Churaumi Aquarium, Shinya Komoto of the Imaging Section at the Okinawa Institute of Science and Technology Graduate University, Alistair Dove of the Georgia Aquarium, Masakatsu Kino, also of the Okinawa Churaumi Aquarium, Kei Miyamoto, again of the Okinawa Churashima Research Center and the Okinawa Churaumi Aquarium, and Minoru Toda, once again of the Okinawa Churaumi Aquarium, present the results of a study in which they examined the eye protection mechanism of the Whale Shark, Rhincodon typus (Elasmobranchii, Orectolobiformes, Rhincodontidae).
The Whale Shark is the largest Fish, reaching over 18 m in total length. Its eyes are located at the antero-lateral corner of the roughly square-shaped head, and are considerably projected from the orbit. These features are expected to increase the risk of injury to the eyes, during swimming through drifting/floating objects in water, for example. As far as Tomita et al. know, the only description about the eye protection mechanism of the Whale Shark was made in 2007 by Aiden Martin of the Fish Museum at the University of British Columbia and Banyan Tree Maldives Marine Lab, who reported as a personal observation, that the Whale Shark protects its eyes by rotating the entire eyeball back into the eye socket. However, no objective data was provided.
Eyes of the Whale Shark. (A) Anterior view of the whale shark, showing the locations of the eye (arrows). Note that Whale Shark eye is well projected from the orbit. Photo was taken in the sea near Saint Helena Island. (B) Close-up view of the left eye of a captive Whale Shark (Specimen A). Tomita et al. (2020).
Recent success in the care and maintenance of Whale Sharks in aquariums made it possible to study the eye protection mechanism in this species. The paucity of information on the eye protection mechanism of the Whale Shark may be attributed to the extremely low access to live specimens. In general, studies on large aquatic vertebrates are difficult because of their small population size, strong swimming ability, and pelagic habitat. Thus, most studies on these animals have been based on a small number of dead specimens. Tomita et al. aimed to describe, for the first time, the detailed kinematic and morphological features of Whale Shark eyes that are associated with eye protection. They did this by applying some recent techniques, such as underwater sonography and micro-computed tomography, to analyse both live and dead specimens, and to compare them with those of other Elasmobranchs.
Animal handing during ultrasound was done in strict accordance with the guidelines for Animal experiments of the Okinawa Churashima Foundation, with the same consideration for Animal care and welfare as that for 'higher' Vertebrates (Reptiles, Birds, and Mammals). However, as the guidelines stipulated, the approval from the Institutional Animal Care and Use Committee of Okinawa Churashima Foundation, required for higher Vertebrates, is waived for 'lower' Xertebrates including Fish.
Morphological observation was conducted on the formalin-preserved eyeball of a Whale Shark (OCF-F04248) in the collection at the Okinawa Churashima Research Center. This eyeball was extracted from a dead Whale Shark specimen during a dissection conducted by the aquarium staff at the Okinawa Churaumi Aquarium in 2017. This specimen was originally caught as a bycatch in a net set by local fishermen in Okinawa, Japan, and was donated to the Okinawa Churaumi Aquarium for scientific purposes. The anteroposterior diameter of the eyeball was 65.0 mm.
Computed tomography data were acquired from this specimen using a micro computed tomography scanner at an X-ray setting of 50 kV, at the Okinawa Institute of Science and Technology. Three-dimensional reconstructions were prepared using IMARIS software. The voxel size of the three-dimensional data was 18.2 μm for the observation of the entire eyeball morphology and 5.2 μm for the observation of the more detailed structure (i.e. the morphology of each denticle).
Two captive Whale Sharks (specimens A and B) were used for the external kinematical observations. Specimen A (male, 8.7 m in total length) and specimen B (female, 8.1 m in total length) have been maintained in an exhibition tank at the Okinawa Churaumi Aquarium since 1996 and 2012, respectively. Kiyomi Murakumo swam with these specimens, and eye movement was recorded from approximately 30 cm away from the eye surface using a GoPro HERO5 video camera.
Ultrasound experiments were also conducted on two captive Whale Sharks (specimens A–C). Specimens A and B were the same individuals that were used in the external kinematical observation described above, and specimen C (male, 5.54 m in total length) was maintained in an open water fish pen near the Okinawa Churaumi Aquarium. A portable ultrasound device with pressure- and water-proof housing was used to document eye retraction. The transducer of the ultrasound was held 0.5 cm away from the surface of the eyeball, and acquired the ultrasound footage during eye retraction. This is possible because water conducts the ultrasound signal well, and direct contact with the skin is not required as it is in terrestrial Animals.
Numerous dermal denticles are distributed on the eye surface around the iris According to the object-counting option in the IMARIS software, the total number of eye denticles was about 2900 in OCF-P04248.
Eye denticles of the Whale Shark. (A) Distal view of left eyeball (OCF-P04248). (B) Line drawing of OCF-P04248, showing the distribution of the eye denticles (gray area). (C) Three-dimensional image of eye denticle aggregation obtained from computed tomography data. Horizontal and vertical lines of no denticle area (arrowheads) are artifacts. (D) Posterior view of panel (C). Scale bar are 0.5 cm. Tomita et al. (2020).
The general morphology of each eye denticle is characterised by a central ridge running through the longitudinal axis of the denticle and several (usually 6–8) sub-ridges branching laterally from the central ridge. Due to these ornamentations, the eye denticle presents an oakleaf-like appearance from the apical view. The base of the eye denticle is oval-shaped in basal view and comprises multiple (generally more than 8) foramina of the neck canal in the lateral view. These features are in contrast to the denticles distributed over the rest of the body that have triple ridges arranged in parallel on the apical surface of the crown, rectangle- shaped base in basal view, and 4 foramina of neck canal in the lateral view.
3D reconstruction of eye denticle obtained from computed tomography data. Tomita et al. (2020).
Eye retraction behavior was observed in all Whale Sharks (specimens A–C) examined in this study. When an object approached (in this case the diver), the eyes were retracted into the orbit with a duration (from the onset of eye retraction to complete retraction) of less than 1 second. Ultrasound data showed that the maximum retraction distance was 3.3 and 2.8 cm (50.4 and 49.8% of the eyeball diameter) in specimens A and C, respectively. During retraction, the eyeball rotated ventrally, and white connective tissue from the retrobulbar space was displaced around the eyeball and partially filled the space where the eye used to be. In general, eye retraction occurs for a short duration only when an object approaches the eyes of a specimen. However, longerterm eye retraction behavior was observed once in a captive specimen (female, 5 m in total length at the time) at the Georgia Aquarium, which kept its eyes retracted for approximately 10 days in June, 2006, immediately following its transport to Atlanta from Taiwan.
Eye retraction of Specimen A. Tomita et al. (2020).
Tomita et al.'s study confirmed that the eyes of the whale shark are covered with eye denticles. In general, the superficial layer of the Elasmobranch eye, like in other vertebrates, consists of two specialised epithelial tissues, the cornea on the iris region, and the conjunctiva on the outer iris region. Considering that these tissues are exposed and that Whale Sharks lack eyelids; the eye surface is less protected from mechanical damage than other regions of the body that are covered with mineralised dermal denticles. Thus, the covering of the eye surface with denticles in the Whale Shark is probably useful in reducing the risk of mechanical damage to the eye surface. As far as we know, eye denticles have not been found in other Elasmobranchs, including species closely related to the Whale Shark, such as the Tawny Nurse Shark, Nebrius ferrugineus, and the Zebra Shark, Stegostoma fasciatum. It seems likely, therefore, that eye denticles are a characteristic unique to the Whale Shark. It should be noted that eye armor was previously described in some fossil Chondrichthyans. In 1894 Bashford Dean reported that the Devonian Shark Cladoselache has ring-like plates around the iris ('circum-orbital plates'), and assumed that this structure was derived from dermal denticles. However, this structure is now reinterpreted as a sclerotic ring, not a denticle-derived structure.
Ultrasound footage of eye retraction in Specimen A. Tomita et al. (2020).
Denticles on the eye are morphologically distinct from denticles distributed over the rest of the body. The apical surface of the eye denticle is ornamented with laterally branched ridges, resulting in an oakleaf-like appearance. Interestingly, a similar morphology is found in the body denticles of the Horn Sharks, Heterodontus spp. A previous study classified the morphology of Shark denticles into four groups based on presumed function: defense, protection from abrasion, bioluminescence, and hydrodynamic drag reduction. Following this classification system, the denticle of the Horn Sharks is a typical example of a protection-from-abrasion-type denticle. Morphological similarity between eye denticles of the Whale Shark and body denticles of the Horn Shark supports our hypothesis that the main function of the eye denticle is mechanical protection. The denticles on the rest of the Whale Shark’s body, such as the head, trunk and fins, are characterised by parallel, triple ridges on the upper surface, presenting a drag-reduction type morphology. Interestingly, the denticles that cover the nictitating membranes of Carcharhinid and Sphyrnid S|harks have also been thought to play a role in eye protection. Though the overall morphologies of the denticles in the nictitating membranes of Carcharhinid and Sphyrnid Sharks are different from those of Whale Sharks, they both have especially thick denticle crowns, which suggests that they have a similar function of mechanical protection.
Morphology of each eye denticle of the Whale Shark. (A) Close-up of aggregated eye denticles. (B)–(D). 3D reconstruction of eye denticles obtained from computed tomography data, showing the morphological variations. (E) Dermal denticle from the skin just above of the eyeball. In (B)–(E) apical, basal, lateral, and posterior views from left to right. Scale bars 500 μm in (A) and 100 μm in (B)–(E). Tomita et al. (2020).
Tomita et al.'s study also revealed that the eye protection mechanism of the Whale Shark involves active eye retraction with partial ventral rotation. Maximum retraction distances were calculated to be approximately half (49.8% and 50.4%) of the eyeball diameter. These values are comparable to Vertebrates with highly retractable eyes, such as the Northern Leopard Frog, Rana pipiens (50%, calculated by X-ray), and Bottlenose Dolphin ,Tursiops sp. (40%–60%), The Batoid Giant Guitarfish, Rhynchobatus djiddensis, demonstrates the highest eye retraction of any Vertebrate, at 101%. Among Elasmobranchs, eye retraction ability has been well described in Batoids, but not in Sharks. Among Elasmobranchs, eye retraction ability has been well described in Batoids, but not in Sharks. As far as Tomita et al. know, the only published record of eye retraction behavior in Sharks is for the Sixgill Shark, Hexanchus griseus, which retracts the eyeball during biting behavior. However, eye retraction distance could not be calculated from this footage. It is likely that Whale Sharks maintain their vision during eye retraction because the pupils of the Whale Sharks in this study were not completely covered with surrounding white tissues when their eyes had retracted, though their visual field would be much more restricted than when their eyes are positioned normally. In fact, the Animal that kept its eyes retracted for approximately 10 days at the Georgia Aquarium appeared to have no problem navigating the exhibit space, until its eyes returned to their normal positions suddenly and, apparently spontaneously.
Eye retraction of the whale shark (Specimen A). Relaxed (A) and retracted (B) phases of the Whale Shark eye. The lower left of each panel is a schematic diagram showing the location and rotation of the eyeball. Tomita et al. (2020).
Tomita et al.'s data also revealed that eye retraction in the whale shark is accompanied by eye rotation. Eye rotation ability of the Whale Shark was previously documented although the rotating direction is different between the previous and Tomita et al.'s observations (rotating posteriorly in the previous study and ventrally, in Tomita et al.'s study). Eye retraction and rotation together was also observed in the Sixgill Shark.
Eye retraction of the Whale Shark in ultrasound data. (A) Left lateral view of the Whale Shark head, showing the location where the ultrasound data was obtained in this study. Ultrasound data are shown in the cross section passing through the maximum height of the eyeball (a–a’). (B) Relaxed (left) and retracted (right) phase of the Whale Shark eye (specimen C). Scale bar is 1 cm. (C) Schematic illustration of panel (B). Dotted line represent the distalproximal axis of the eyeball. Tomita et al. (2020).
The present study provides novel information on the visual capacity of the Whale Shark. Because of the relatively small eyes (eye diameter less than 1% of total length), it has been assumed that the Whale Shark depends little on vision compared with other senses such as olfaction. This notion was supported by the small proportional size of the midbrain (mesencephalon), which is a division of the brain that is responsible for processing the visual information. However, the highly protected features of the Whale Shark eye, in contrast to the traditional view, seems to suggest the importance of vision in this species. Interestingly the previous study showed that Whale Shark eyes actively track divers swimming 3–5 m away from the animal, suggesting that vision of the Whale Shark plays an important role in short-range perception. Such active visual tracking is also seen in the captive Whale Sharks at Okinawa Churaumi Aquarium and at the Georgia Aquarium. Future research should focus on the optical sensory capacity (e.g., visual field, acuity, colour range, and sensitivity) of the Whale Shark vision system.
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