The Amazing Sensory Talents
No question about it: the Great White Shark is among the most impressive and awe-inspiring animals with which we share this planet. While skates and rays - collectively termed batoids - may not seem as outwardly spectacular as the White Shark, biologically they are no less wonderful. Recent studies of batoid sensory biology attest to the validity of this.
A 1991 paper by opthalmologists Christopher Murphy and Howard Howland examined the functional significance of the unusual eyes of batoids. In contrast with humans - which have circular pupils - many skates and rays have crescent-shaped pupils, a feature also found in flatfishes (Pleuronectiformes), suckermouth armored catfishes (Loricariidae), some cetaceans - such as the Bottlenose Dolphin (Tursiops truncatus), and at least one terrestrial mammal, the rock hyrax (Procavia sp.). Murphy and Howland found that this deceptively simple pupil shape confers some significant advantages to batoids. For example, a crescent-shaped pupil preserves a small depth of field (only a narrow range of depths are in focus) while limiting light flux to the retina (a feature particularly important in an aquatic environment characterized by flickering light. Among other optical benefits, this pupil shape also decreases the effects of spherical aberration (distortion) due to the globular lens typical of batoids, provides a larger visual field (the better to spot potential predators and prey), affords a higher resolution limit (ability to distinguish fine details), and enhances contrast.
Many batoid eyes also have a complex flap of iris tissue hanging over and partially covering the pupil. Called a pupillary operculum, this flap is often fringed with finger-like extensions that change the behavior of light passing between them. Such a pupillary operculum effectively creates multiple pupillary apertures, a feature that is also found in some cetaceans, llamas, horses, and other artiodactyl mammals. Murphy and Howland deduced that objects in a batoid's environment that lie either in front or behind the plane of optical focus will form multiple images on the animal's retina. In addition, these workers suggest that - similar to crescent-shaped pupils - the pupillary operculum of batoids provides a larger visual field than a circular pupil of identical area and may diminish the effects of lens-induced spherical aberration. As such, a humble, mush-mouthed skate may be more sensitive to movements within a larger visual field than the superpredatory, saw-toothed Great White.
Like the Great White and other sharks, all batoids have a lateral line system. Many skates and rays feed on bottom-dwelling prey, including buried bivalves, worms, and crustaceans. Buried prey are excavated from the substrate through a combination of blowing jets of water from the mouth and creating a plunger-like suction with the pectoral disc. In intertidal areas, these prey excavation techniques leave distinctive pits that record intensity of batoid feeding. A recent study of feeding pits left by Short-Tailed Stingrays (Dasyatis brevicaudata) on tidal flats in New Zealand indicated that the rays prey primarily upon buried bivalves and concentrate their feeding efforts in areas of high prey density.
An intriguing 1997 paper by New Zealand marine biologists John Montgomery and Even Skipworth demonstrated that the Short-Tailed Stingray locates buried prey via the elaborate network of lateral line canals on its ventral surface. These lateral line canals apparently detect the weak vertical jets of water created by buried bivalves. During breathing and feeding, bivalves continually draw in water and particulate organic matter over the gills, then expel the processed water through a fleshy tube called an excurrent siphon. Montgomery and Skipworth imitated the excurrent siphons of bivalves with water forced through a scattering of buried plastic tubes, any of which could be selected to jet water at a rate of about 1.2 inches (3 centimetres) per second. Which tube was actively jetting water at a given time varied randomly. Using rewards of food over a period of four weeks, Montgomery and Skipworth taught the rays to associate the presence of an actively jetting tube with the overhead tank-lights.
When a tube was actively jetting, the tank lights were switched on and the food searching behavior of the rays was videotaped. By this method, Montgomery and Skipworth found that individual rays readily identified the active tube and oriented so that their mouth was directly over the jet; a food reward placed near the active jet was quickly hoovered up. Thus, Short-Tailed Stingrays probably use their ventral lateral lines to detect the weak jets of water expelled through the excurrent siphons of buried bivalves. This prey-location strategy is reminiscent of that employed by Walruses (Odobenus rosmarus), which use their highly touch-sensitive vibrissae (whiskers) to detect the excurrent jets of buried clams. This experiment also demostrates that - like Pavlov's dogs - these rays can be conditioned fairly quickly to associate a specific stimulus with the presence of food.
Perhaps the most fascinating shark sensory talent is electroreception. Not to be out-done, batoids also employ this mysterious sense in astonishingly complex and subtle ways. Perhaps the most fascinating of these was reported in a 1998 paper by elasmobranch biologists John Sisneros, Timothy Tricas, and Carl Luer. Sisneros and his co-workers studied how behavioral responses of Clearnose Skates (Raja eglanteria) to electical stimuli changes with age. They discovered that fetal Clearnose Skates respond most strongly to weak electric fields at a frequencies between 0.5 and 1 Hertz (cycles per second). This happens to be the same electrical frequency generated by the skates' natural fish predators, suggesting the intriguing possibility that fetal Clearnose Skates can detect potential predators through the wall of their egg cases. Sisneros and his co-workers went on to demonstrate that when presented with an artificial electric field of the appropriate frequency, fetal Clearnose Skates actually respond by 'freezing' within their egg cases, remaining motionless as though to avoid detection! In contrast, adult Clearnose Skates responded most strongly to weak electric fields with a frequency of 2 to 3 Hertz. This happens to correspond well to the electric field that sexually mature members of this species generate with specialized muscles in their tail stalks. This ability may have important implications for Clearnose Skate social behavior, including mating.
Tricas and Sisneros teamed with fellow elasmobranch biologist Scott Michael to explore just this possibility. A wild population of Round Stingrays (Urolophus halleri) mates during winter months (January to March) in clear, shallow waters of Bahia Kino, in the Sea of Cortez, providing exceptional opportunities to observe courtship and copulation among many individuals of this species. Tricas and his co-workers reported that solitary male Round Stingrays actively search this area for potential mates, using both visual and electrosensory cues. Careful measurements indicated that a reproductively mature female Round Stingray emits a localized positive electric field above her spiracles (paired respiratory organs, one of which is located behind each eye) that has the same frequency as her ventilatory movements: about 1 Hertz (cycle per second). To test whether male Round Stingrays could detect potential mates by electroreception alone, Tricas and his co-workers used an electric dipole to imitate the electrical signature of female Round Stingrays. They found that males of this species efficiently oriented themselves directly over the dipole as though it were a reproductively mature female, apparently following the concentric field lines of electric force. Female Round Stingrays also oriented to the dipole, but - unlike the males - buried themselves a short distance away (typically 4 to 40 inches [10 to 100 centimetres] or about 2/5 to 4 times their average disc width). This behavior may allow sexually receptive female Round Stingrays to cluster in specific mating arenas yet space themselves out to maximize mating opportunities. These results were published in a fascinating 1995 paper by Tricas et alii, and demonstrate for the first time that elasmobranch electroreception is used to co-ordinate social behaviors.
It is possible that White Sharks may also exchange electric cues during mating, but this remains to be demonstrated. However, there can be little doubt that the sensory talents of batoids are - in their own quiet way - every bit as impressive as the spectacular predatory prowess of the Great White.