BULLETIN OF MARINE SCIENCE, 70(3): 947–952, 2002
NOTES
A FURTHER OBSERVATION OF THE PREY-HANDLING BEHAVIOR OF THE GREAT HAMMERHEAD SHARK, SPHYRNA MOKARRAN: PREDATION UPON THE SPOTTED EAGLE RAY, AETOBATUS NARINARI
Demian D. Chapman and Samuel H. Gruber
Although observations of natural predation by large sharks are extremely rare, they can provide important insights into the feeding behavior, ecology and functional morphology of these apex predators (Klimley et al., 1992; Strong, 1991; Tricas and McCosker, 1984). In 1988, Strong et al. (1990) made an unprecedented underwater observation of the preda- tory behavior of a great hammerhead shark, Sphyrna mokarran, during the pursuit and capture of a southern stingray, Dasyatis americana. Amidst much speculation about the functional significance of the laterally expanded head (hereafter referred to as the ‘cephalofoil’) of the hammerhead sharks (Nakaya, 1995; Compagno, 1988; Johnsen and Teeter, 1985), this observation demonstrated that the cephalofoil can be directly useful in prey handling. The shark used its head to ‘pin’ the fleeing stingray against the bottom and then ‘pivoted’ its body into position to deliver an immobilizing bite to the ray’s pectoral disc. Because Strong et al.’s (1990) observation is the only one reported thus far, it re- mains unknown how frequently the great hammerhead employs such prey-handling be- havior. Herein we describe a new observation of batoid prey-handling by the great hammer- head, upon a spotted eagle ray, Aetobatus narinari. Unlike the observation by Strong et al. (1990) this attack took place close to the surface and involved a pelagic species of myliobatid ray. Despite this, there were some similarities in the two predatory events. Most notably, the hammerhead once again used its cephalofoil to pin the prey to the bottom, but in this case only during the post-capture manipulation and consumption of the immobilized eagle ray. To our knowledge, this is the only observation of natural batoid prey-handling by the great hammerhead since Strong et al. (1990) first documented this behavior.
OBSERVATION
The predatory episode took place on 2 August 1998, inside a 30 m wide pass between the islands of North and Mid Turtle Rocks (25°40.13'N, 79°18.34'W), the two northern- most islets of a chain of small cays running in the north-south direction, starting approxi- mately 5 km south of South Bimini, Bahamas. Both islands were fringed by a bank (<2 m wide) composed largely of fire coral, Millepora complamata, in less than 1 m of water. The bank dropped steeply to a uniform depth of approximately 3 m inside the pass. Several witnesses on a skiff made the initial observations. We were first alerted to the episode when we observed an approximately 1 m disc-width (DW) spotted eagle ray leap out of the water vertically, inside the pass, over the shallow bank fringing Mid-Turtle rock. Over the next 30 s the ray made two or three more vertical breaches and as we
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Figure 1. Spotted eagle ray, Aetobatus narinari, approximately 2 min after being bitten by a large great hammerhead, Sphyrna mokarran. approached it we observed a 3.5–4.0 m total length (TL), female great hammerhead shark rapidly closing in at the surface from a distance of less than 2 m. Almost perpendicular to the rays left side, the hammerhead appeared to be herding the ray into shallow water. For approximately 10 s the two animals quickly moved a short distance eastward along the shore, the ray making two more vertical leaps. At this point there was a vigorous splash as the shark’s tail broke the surface. We believe the shark grabbed the ray by the left pectoral fin and physically pulled it into deeper water. After about 3 s, the shark turned and swam towards the middle of the pass. The ray could not be seen. The shark then swam in a wide circle to the northern side of the pass, and turned back towards Mid Turtle rock. At that moment (< 2 min. after the initial observation), one of us (DDC) entered the water to make detailed observations. DDC immediately spotted the eagle ray on the bottom, ap- proximately 3 m north of the fringing bank of Mid Turtle rock. Its right pectoral fin had been entirely removed in what appeared to be a single bite (Fig. 1), and there was a more superficial laceration on its left pectoral fin. At this point the shark swam slowly to within 1 m of the observer, and then turned and continued to swim in a wide, counterclockwise arc around the immobilized ray. After about 20 s an approximately 1.5 m TL blacktip shark, Carcharhinus limbatus, appeared from the west (ocean) side of the pass and closely circled the ray in an apparently excited manner. The hammerhead almost immediately altered its course and closed from about 7 m to within 2 m of the blacktip, its mouth partially agape. At that point the blacktip darted away and was not seen again. NOTES 949
Figure 2. Great hammerhead executing ‘pin and pivot’ behavior on the immobilized eagle ray. (A) Diagrammatic representation. The shark initially approaches the ray (solid outline) and depresses its cephalofoil (arrow) onto the ray’s dorsal surface, pinning it to substrate. The shark then twists its body, pivoting atop the ray, until it reaches the position of the dashed outline where, in this case, it manipulates the ray’s head into its jaws. (B) Photograph of ‘pin and pivot’ behavior.
After about 30 s of swimming in very tight circles around the immobilized ray, the hammerhead performed the first of a behavior, which we call ‘pin and pivot’. This behav- ior can be described as follows: the shark forcibly pressed the ray against the substrate with the ventral surface of its cephalofoil, and then, with a twisting motion of its body, pivoted, its head remaining atop the ray (Fig. 2A,B). In this case it pivoted about 90° and then engulfed the ray’s head in its jaws. The shark picked the ray up off the bottom and swam towards the middle of the pass for several seconds (Fig. 3). It then dropped the ray without removing any tissue. After slowly circling the ray for 1–2 min, the shark per- 950 BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 3, 2002
Figure 3. Great hammerhead carrying the disabled eagle ray by the head, after executing ‘pin and pivot’ behavior. formed its second ‘pin and pivot’, again grasping the ray by the head and lifting it off the substrate, this time swimming westward. Once again it dropped the ray without removing any tissue, this time near the mouth of the pass. The shark circled once and performed its third ‘pin and pivot’, this time pivoting 180° to the intact left pectoral fin. It picked the ray up by the tip of this fin and swam out of the pass into deeper water. Over the next 30 s the shark swallowed the remains of the ray carcass whole from the left wingtip to the right side of the body, while slowly swimming away into deep water. The entire episode lasted less than 10 min.
DISCUSSION
Because of the dominance of demersal fishes in their diet, it is generally believed that benthic feeding is typical of the great hammerhead (Cliff, 1995; Stevens and Lyle, 1989; Dodrill, 1977). Supporting this, Strong et al. (1990) described previously unknown prey- handling behavior, whereby the great hammerhead used its cephalofoil to capture a stin- gray by pinning it to the bottom. In contrast, we observed a large great hammerhead pursue and capture a spotted eagle ray in mid-water, where such prey- handling behavior was not possible. Although benthic feeding may be more typical of the great hammer- head, our observation highlights that they are also capable of capturing and disabling pelagic batoid prey in the water column. NOTES 951
Although ‘pin and pivot’ behavior was not observed during the capture of the eagle ray, the new observation still generally supports of the hypothesis of Strong et al. (1990) that the cephalofoil of the great hammerhead shark can be useful in prey-handling. In this case, the great hammerhead was observed three times to use its cephalofoil to pin the already immobilized eagle ray against the bottom, then pivoting into position to grasp part of the ray in its mouth. In the attack observed by Strong et al. (1990) this type of behavior was an integral part of the pursuit and capture of a southern stingray near the bottom. The present observation suggests that the ‘pin and pivot’ behavior can also func- tion in the post capture manipulation and consumption of an already immobilized prey item. Contact between the ventral surface of the cephalofoil and the prey may allow the hammerhead to use its tactile sense to selectively grasp or avoid a specific part of the prey (e.g., the head or venomous caudal spines, respectively), or assess the condition or size of the prey before deciding when or how to consume it. It should also be noted that Nakaya (1995) demonstrated that hammerhead sharks have a much greater ability to move their head up and down on a vertical plane than otherwise similar carcharhinid sharks, which would facilitate ‘pin and pivot’ behavior in the great hammerhead. Another important parallel between theses two predatory episodes was the placement of the initial bite on the pectoral disc. In both cases the ray was first immobilized by a massive bite which removed most of one of its pectoral fins. This supports the suggestion by Strong et al. (1990) that great hammerheads deliberately attempt to disable large batoids with the initial bite by targeting the fins used for propulsion. Other species of shark have been observed to employ a similar strategy when handling large, potentially difficult prey. The initial bite of a white shark, Carcharodon carcharias, upon a phocid seal or cetacean is often placed towards the propulsive hind-flippers or fluke (Long and Jones, 1996; Klimley et al., 1996; Tricas and McCosker, 1984), while otariid seals and sea-lions are often bitten mid-body near their propulsive fore-flippers (Klimley et al., 1996). In both the present observation and that of Strong et al. (1990), blacktip sharks, C. limbatus, were attracted to the site of the hammerhead’s kill. Both great hammerheads were observed to actively exclude these smaller sharks as they attempted to scavenge the prey, yet neither hammerhead exhibited any aggression towards human observers in the immediate vicinity. Typically characterized as a predator of demersal fishes, this observation highlights that the great hammerhead is also a capable mid-water predator. It also indicates that the cephalofoil can play a direct and perhaps more general role in the capture and handling of both dasyatid and myliobatid rays.
ACKNOWLEDGMENTS
The authors would like to thank D. Abercrombie, S. Clermont, M. Corcoran, A. Petersen and the Earthwatch volunteers who participated in this observation. We would also like to thank T. C. Tricas, M. S. Shivji and three anonymous reviewers for their invaluable comments on the manuscript. This work was supported by the Florida Department of Education, the Earthwatch Institute, PADI Project AWARE, The Greentweed Foundation, Tom and Shelly Daniels and Todashi and Toshi Fujino. We thank Davey Marine (M. Aiello, President) for use of their 22' Aquasport, Carolina Skiffs Inc. for donation of six flat skiffs and Proline Boats. We are grateful to Mercury Motors (Brunswick Corp.) for 952 BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 3, 2002 kindly providing outboard motors and Nimrod for their fine underwater products. Finally, we thank M. Braynan, Director, Department of Fisheries of the Commonwealth of the Bahamas for issuing permits to allow us to work in their controlled waters.
LITERATURE CITED
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DATE SUBMITTED: December 11, 2000. DATE ACCEPTED: July 9, 2001.
ADDRESSES: (D.D.C.) Guy Harvey Research Institute, Oceanographic Center, Nova Southeastern University, 8000 N. Ocean Drive, Dania Beach, Florida 33004. E-mail: