Chondrichthyes (cartilaginous fish, including sharks, rays and chaemeras) >

Behaviour of cartilaginous fish

Behaviour is perhaps the least known aspect of the biology of cartilaginous fishes. The behaviour of sharks is obscured by numerous myths, legends, and misinterpretations, mostly emotional and related to shark attack and shark paranoia, while the behaviour of rays and chimaeras is a virtual cipher. Cartilaginous fishes, living in an environment alien to humankind, are of course more difficult to study than land predators, but the vast gap between the chondrichthyian mythology and behavioural reality is partially based on the lack of positive interest shown by the general public and the scientific community, in direct contrast to the enormous popularity of marine mammals.
Whales, dolphins, and seals are seen as intelligent, humanlike, and worthy of sympathy and intensive study, while sharks are supposedly primitive, simple, vicious and voracious brutes that can only be of morbid negative interest. Rays and chimaeras have little popular impact, pro or con, but the negative man-eating shark image is continually promoted by the popular news and entertainment media, which find it a potent symbol or architype to excite and influence the public. Many studies that touch upon shark behaviour have been negative and concentrated on the two major poles of research on cartilaginous fishes, shark attack and fisheries biology.

The advent of SCUBA diving and underwater photography opened a window on the behaviour of cartilaginous fishes, as did research on the senses of cartilaginous fishes during the last thirty years. The former allowed us to see cartilaginous fishes in their own environment, and revealed a quite different picture of their activities and role in nature. The latter helped to change our conception of their capabilities, and revealed that cartilaginous fishes have highly sophisticated senses and large brains, permitting them complex behaviour and allowing them to reign as supremely adapted predators in the marine environment, fine-tuned by 450 million years of evolution.

Cartilaginous fishes are as varied in their behaviour as in their body form. Many rays and sharks are relatively inactive bottom dwellers, minutely exploring the substrate and swimming just over it. Some houndsharks (Mustelus) swim at speed over the bottom, probably supported a slight distance above it by ground effect, and can make sudden transitions to vertical surfaces and can swim vertically or laterally on them. Many flattened sharks and rays live on soft bottoms, and ambush prey above or in front of them or root for it in sand or mud. Such bottom-dwellers may actively select the type of bottom they lie on and bury themselves in it. A Biscuit skate (Raja straeleni), placed in a tank with a rough bottom of coarse gravel and rocks, refused to lie on it and instead plastered itself vertically on the tank glass.

Many cartilaginous fishes are social, and are found in aggregations or true schools, sometimes including thousands of individuals. Even species thought to be primarily solitary, such as the Great white shark, form aggregations of the same individuals that may seasonally reappear at given sites each year. A few studies on the behaviour of captive and wild populations of sharks have shown complex behavioural displays between individuals in such groups, and a subtle social hierarchy based on size and possibly sex. Many cartilaginous fishes segregate into schools based on size and sex, and these may show spatial segregation, both in microhabitat and over great distances. Movements are often complex, seasonal, long-range, and based on size of individuals as well as seasonal movements of water masses of given temperature. Some large sharks show an ability to locate and keep to water masses of favorable temperature. Reef-dwelling sharks that are nocturnal and live in caves and crevices in reefs show remarkable site-specificity, with the same individuals returning to the same sites day after day; several individuals may continually use the same sites, while other, nearby sites that seem suitable may be unoccupied by any sharks.

Courtship between males and females is complex and protracted. Males often will bite and grip a female with their jaws prior to copulation; when the female is receptive, the male then inserts one or both of his claspers in the female's cloaca. Some slender small sharks may coil about each other while resting on the bottom, but larger sharks may swim in parallel when copulating. Male rays may copulate from above or below the female while swimming, or when resting on the bottom. Male chimaeras use their head and prepelvic claspers to hold the female during copulation.

Some sharks and rays move close inshore to drop their young, then depart these `pupping grounds' after giving birth. Young may stay in food-rich `nursery areas' for some time while growing, then migrate out of these areas. Some newborn sharks, faced with large predatory sharks and even cannibalistic adults of their own species, seek dense cover such as mangrove roots or eelgrass to hide and feed until they reach sufficient size to range open areas. Some dogfish sharks (genus Squalus) live near the bottom but their newborn young are pelagic on the outer continental shelf, and range the entire water column from surface to bottom. This may help them to avoid bottom predators, may reduce competition from the non-cannibalistic adults, and allows the young to feed on a wider variety of small prey.

Some sharks are pack hunters, with several individuals bunching small prey for easier predation. A few species, including lantern sharks and some larger species, may use pack-hunting to subdue prey much larger than themselves. Some deepwater and oceanic sharks, including the well-known `cookie-cutter' sharks (Isistius), are semiparasitic, and cut plugs of flesh out of large bony fishes, other sharks, rays, chimaeras, and marine mammals; these can also catch and kill large bony fishes and cephalopods.

Some large sharks may use turbid water or bottom cover as a blind to approach their prey, then enter a sudden high-speed dash to catch and kill their victims. When presented with fish at feeding stations, grey sharks (Carcharhinidae) and hammerheads are seen to be precision predators, accurately striking and eating their food at high speed. Activity of individual sharks in such instances is apparently directly proportional to the number of sharks competing with them, with more sharks making for more frenetic activity. Smoothhound sharks (Triakidae: Mustelus) attack large crabs with precision, either grabbing their large claws and breaking them off, or gripping and crushing their carapaces away from reach of the claws. Reef whitetip sharks (Triaenodon obesus) are adept hunters of crevices and holes in coral reefs, and use specific hunting tactics to extract prey from such crevices.

Angel sharks (Squatiniformes: Squatina) are ambush predators which lie half-buried in sand, and use a specialized `snap-up' attack to capture prey overhead. Wobbegong sharks (family Orectolobidae) in Australia have camoflage color patterns and have been observed to creep up catlike to potential prey, which may be taken in a sudden lunge or snap. In California a Great white shark was observed to lunge partway out of the water to pull a sea lion off a rock. Both white sharks and Shortfin makos are known to jump into boats while chasing fishes that are being hauled in by anglers. Oceanic whitetip sharks (Carcharhinus longimanus), Spotted spiny dogfish (Squalus acanthias), and Leopard sharks (Triakis semifasciata - not found in southern Africa), have been observed swimming open-mouthed in dense schools of prey, and it has been suggested that in such circumstances the sharks merely close their mouths like a trap when prey swim into them!

Many rays and skates use their jaws like forceps to pull invertebrate prey from the substrate. Eagle rays (family Myliobatidae) use their flexible, muscular snouts to root out invertebrates.

Some bottom-dwelling electric rays may suddenly raise the front of their pectoral disks when prey animals approach, creating a partial vacuum to suck the victim into the cavity created, and use their electric organs to subdue the prey before eating it. More active, free swimming electric rays search out their fish prey, shock it, and envelop it with their pectoral fins before swallowing. The electric organs are powerful weapons of defense in electric rays. One electric ray in a tank shocked and drove off a large octopus that had approached it. A modest-sized Atlantic electric ray (Torpedo nobiliana) examined by us had cuts that indicated an abortive attack by a Sixgill shark (Hexanchus griseus), which apparently started to bite its disk but jerked away after being shocked. The Pelagic stingray (Pteroplatytrygon violacea) has been observed swimming on its back at the surface, and uses its pectoral fins like hands to manipulate prey to its mouth.

The Great white shark has been termed `slow' and `clumsy' when it is observed to leisurely take baits at feeding stations, and cruises about at a modest speed. This is belied by observations of its jumping out of the water, by observations and even videos of its lightning attacks on seals and sea lions, and by the impressively fast-swimming victims, including Blue sharks and mako sharks , dolphins, sea lions, and fur seals, that are found in its stomach. Large eagle rays fed in captivity can be extremely quick and agile in taking food and in keeping it from being stolen by other fishes.

Thresher sharks (family Alopiidae) swim around a school of fishes or squids to compress them into a close-packed group, then swim through or around them while flicking their tails to stun or kill prey. The sawsharks (Pristiophoriformes) and sawfish (Pristoidei) swing their saws rapidly from side to side in a group of prey, hit and disable prey with the saw-teeth, and suck them into their small, slotlike, small-toothed mouths. Sawsharks have long barbels on their snouts that may help to locate bottom prey, and sawfish can also use their saws in a poking, rooting action to dislodge invertebrates buried in the substrate.

The Basking shark (Cetorhinus maximus), Whale shark (Rhincodon typus), Megamouth shark (Megachasma pelagios), and devilrays (family Mobulidae) are filter feeders, and have different types of filter structures on their internal gill openings that prevent small prey from being lost through the gills, and different forms of feeding behaviour. The basking shark cruises with its huge mouth open and passively strains small crustaceans from the water, but the whale and megamouth sharks may suddenly open their mouths like bellows to ingest prey organisms. The sluggish Megamouth shark may lure its small shrimp prey close to its mouth by luminous lips. Devilrays use their mobile, prehensile head fins to direct small fishes and other prey into their slotlike mouths.

The Great white shark has the unusual habit of striking or gashing other marine vertebrates, including penguins, gannets, other sharks, seals and sea lions, and sea otters, without eating them, and often uses only its upper teeth to do this. White sharks may inflict relatively light two-jaw bites on such victims and on human beings, not exerting the maximum force available with their powerful jaws. Penguins and humans have been grabbed by white sharks, held without the shark completing its bite, and released; and survived the experience and eventually healed up. These `inhibited' bites have been interpreted as prey sampling or `mistaken identity' attacks, but they may be the result of aggression or even `play'. White sharks may also grab inanimate objects, such as surfboards and floats, and bite and shake them.

Scratches on white sharks of both sexes have been interpreted as combat scars, perhaps inflicted in fights over dominance. These scars are very minor compared to the massive damage that the white shark is capable of inflicting with its jaws, and may be delivered by the tips of the upper teeth and by the tusk-like lower front teeth, which protrude horizontally from the shark's lower jaw. Sharks of other species have been seen with similar scars on both sexes, indicative of combat or, in at least one species, attempts at cannibalism. The Spotted raggedtooth shark may bite other, moderately large sharks in captivity, and occasionally bites divers and swimmers in the field, but the inhibited bites of this fisheating shark suggest that the bites may be delivered in threat rather than as attempts at predation.

Divers and the occupants of small submersibles approaching certain grey sharks (Charcharhinidae: Carcharhinus) have experienced a stereotyped `hunch' display in which the shark arches its back, thrusts its pectoral fins downwards and caudal fin upwards, and swims back and forth in a conspicuous manner. This is widely interpreted as a defensive threat display, a warning probably given to other sharks as well as humans, and can be succeeded by a quick `hit and run' bite if the intruder does not back off from the shark. Territorial defense has been suggested, but defense of a given area by any shark has yet to be proved. Near feeding stations the Great white shark performs a similar display, with jaws protruded and back arched, but churns the water with its body and tail. White sharks and Spotted ragged-tooth sharks, approaching divers, have given protruded-jaw displays like a yawn or gape. At least one local spearfishing diver, faced with such a gape from an oncoming white shark, surrendered his fish to the shark, which ate it and departed. Shortfin makos give similar jaw displays and may also swim in a figure-8 pattern when near a diver.

Many sharks and rays may approach divers underwater in apparent curiosity, unaggressively examining them and then departing. These can be as large as Basking sharks, Whale sharks, threshers, and manta rays. Great white sharks are known for their disconcerting habit of swimming right up to SCUBA and free divers without biting, and then enigmatically departing. Hammerheads and other sharks following a scent trail to a planted bait may briefly examine divers without aggression and then proceed to the bait. Large Scalloped hammerheads (Sphyrna lewini) congregating around a seamount in the Gulf of California were found to be singularly unaggressive to diver-scientists, who had to study them by free-diving because SCUBA gear disturbed them and made them flee the researchers.

Experiments in captivity and in the wild have shown that sharks and rays have a high learning capacity, which supplements the innate, instinctive behaviour that is programmed into the nervous system of a cartilaginous fish, and allows them a flexible, adaptive response to external stimuli. One group of researchers taught large Lemon sharks (Negaprion brevirostris - not found off southern Africa) to distinguish between stimuli presented at two targets, and another researcher taught California bat rays (Myliobatis californica -  not found off southern Africa) to retrieve small circular floats on their snouts for food rewards. Divers have noticed that tropical requiem sharks soon learn to associate auditory stimuli such as an anchor dropped on a reef or a speargun discharge with food, and quickly come to the source of the sound.

Sportsfishing boats in California have been harassed by large great white sharks, which have apparently learned to seek out these boats and patrol under them. When an angler catches a fish on hook and line, the white shark clips it off. Large requiem sharks are reported to use similar tactics off the east coast of southern Africa. Off Australia and California, individual white sharks have learned to come to boats to be hand-fed, and accept fishes with minimal fuss and no aggression. Off central California an angler in a small boat less than 4 m long had white sharks stealing his hooked fish, and decided to bring them fish and hand-feed them instead. A fisheries biologist, shown pictures of these sharks, was startled to see that there were at least two individuals that would regularly come to accept food from the angler, and which were larger than the boat and perfectly capable of sinking it. On the other hand, a party of inebriated anglers in a skiboat in False Bay who tried to ram one of two large white sharks swimming in tandem were rewarded with a swift attack, apparently in retaliation; the shark tore a hole in the bow and the chastened anglers had to ride bunched on its stern back to port to avoid its filling with water!

Text by Leonard J.V. Compagno, David A. Ebert and Malcolm J. Smale

  Iziko Museums of Cape Town, 2008

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