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
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