We used to think that sharks were just “mindless eating machines”. Indeed, type that search string into Google and the first page of results is about sharks. In part, this label meant that the killing of sharks appeared more justifiable because they didn't have the same public support as many mammals receive. As technology has improved and the study of this fascinating group of animals expanded, however, it because apparent that sharks weren't pre-programmed loners we initially took them for. Indeed, in 1995, I remember watching, enthralled, David Attenborough's wildlife special Great White Shark: The True Story of Jaws during which remote cameras attached to the back of their study individual gave a tantalising glimpse that these sharks may actually be a social species. Since then we have observed social and/or aggregative behaviour in many more species and, in a paper to Fish and Fisheries published in 2010, David Jacoby and colleagues list 29 species of shark and ray known to either exhibit social or aggregation tendencies.
Obviously, the study of animal behaviour is a complicated field and I know many people who have devoted their lives to the study of shark and ray behaviour. I concede that I could do their work and devotion justice, even if I were to devote my entire site to the topic. Instead, I intend to provide a very brief summary of some of the best-documented behaviours. For a more comprehensive description of various shark and batoid related behaviours, I would recommend a visit to the websites of ReefQuest and Ila France Porcher.
Probably one of the most frequently cited behavioural exhibitions is the agonistic display seen in certain reef sharks when pursued by a diver or submersible. This behaviour is perhaps most frequently observed in the grey reef shark (Carcharhinus amblyrhynchos). Carcharhinus amblyrhynchos is a reef dweller that often aggregates to form loose groups of up to 100 individuals during the day, disbanding at night to search for food.
If a grey reef shark is relentlessly pursued by a diver or submersible it begins a characteristic swimming display, during which it hunches its back, lowers its pectoral fins and raises its snout. This is accompanied by a more exaggerated swimming style in a wide, sinusoidal path. If whatever is pursuing the shark ceases and moves away, the exaggerated swimming stops and the shark returns to its normal cruising motion. Should the pursuer continue, the shark will ordinarily attack. Some interesting experiments carried out by the late Donald Nelson on grey reef sharks at Enewetak Atoll in the tropical Indo-Pacific Ocean, have helped understand the situations that may elicit an agonistic display in these species. In one 1981 paper, Nelson summarizes that which was known about this behaviour.
Nelson noted that partially cornered sharks and lone individuals were more prone to agonistic displays than those in open water or in groups. The swimming motion is highly exaggerated and very obvious, suggesting that it has no relation to feeding, and the observation that the sharks don't exclude other sharks from their vicinity made the author question whether the display had much to do with territoriality. Interestingly, Nelson also observed a case where a lone reef shark approached a diver and displayed a “mild or moderate posture” without any provocation on the diver's behalf. Similarly, there are reports that this shark only displays exaggerated swimming in certain areas, being docile and almost timid in other areas—an observation that does lend some credence to the theory of territoriality, although territoriality has yet to be scientifically demonstrated in any shark species.
The reason for this display is still something of an enigma and, as Nelson pointed out, under such a variety of conditions, there is no single motivation that can adequately account for all scenarios. My personal belief is that, this swimming pattern—variations of which have also been reported in the great white, silky (Carcharhinus falciformis), shortfin mako, blacknose (Carcharhinus acronatus), silvertip (Carcharhinus albimarginatus) and bonnethead sharks (Sphyrna tiburo)—is a threat display with “meanings” that vary according to the specific conditions. In most cases, it seems to indicate an aggravated shark and is a warning to back off. It is interesting that the other reef sharks that Nelson studied failed to exhibit this swimming behaviour, except for a silvertip shark that displayed a “mild threat display” reported by Nelson et al. in a subsequent paper for the Bulletin of Marine Science in 1986, opting instead to flee from the pursuer.
Gaping, slapping & breaching
It is really only in the last decade or so that one of the most iconic sharks has come under the spotlight of rigorous behavioural study. In a series of papers presented in the volume Great white Sharks: The biology of Carcharodon carcharias published in 1996, various biologists described some of the behaviours observed in the world's largest extant predatory fish. One paper in particular, by Wesley Strong of the Cousteau Society in Virginia, described a thwart-induced behaviour he termed “Repetitive Aerial Gaping” or RAG. Strong observed RAG in six white sharks, three of each sex, during his study off south Australia.
The display was exhibited when the bait was pulled away immediately prior to contact and consisted of the shark raising its head out of the water, rolling on its side and opening and closing its mouth in a sequence of slow yet rhythmic partial gapes; the display sequences averaged 10 seconds. Analysing each instance separately, Strong proposed a number of hypotheses including thwarting, aggression reduction, extended feeding attempt, and predator-prey communication. He concluded that RAG is probably a:
“manifestation of frustration incurred during a series of thwarted feeding attempts and may serve to reduce intra-specific [between individuals of the same species] aggression”.
Another paper in the same volume documented novel “tail slap and breach” behaviours in white sharks at the South Farallon Islands off Central California. The authors filmed 129 feeding incidents over the three years between 1988 and 1991, 83 of which involved some variant of the Tail Slap (TS). TS consists of the shark rolling onto its side, lifting its tail out of the water and slamming it down onto the water's surface with considerable force. The resultant splash is usually directed at the other shark.
The authors also documented Breaching behaviour (BR) that consisted of the shark raising two-thirds of its body out of the water at a 30 to 60 degree angle and landing flat. These behaviours were interpreted as a social signal between sharks vying for food, based on the observation that the shark whom the TS was directed at either returned the TS or withdrew to allow the “tail-slapper” to feed. The BR behaviour, as it displaces more water than TS, was considered to be a more intense form of the TS message. BR has also been observed in other species, including the basking shark, with parasite removal having a proposed reason. The aptly-named spinner shark (Carcharhinus brevipinna) gets its name from the rapid spinning jumps it makes out of the water. In this case, however, the “breaches” are a side-effect of the shark's feeding behaviour. Spinner sharks rush at fish shoals rotating and biting at any fish in reach—these “feeding runs” are often so powerful that the shark leaps clear of the water.
Many years ago I had volunteered at a local Blue Reef Aquarium and part of my job was to give talks to members of the public about the sharks and rays on display. One of the most common behaviours exhibited by rays and to a lesser extent the sharks was an apparent “treading water” during which the fish would turn from its normal horizontal path and swim vertically surface-ward. Upon reaching the surface, it will raise about half of its body out of the water and continue in an apparent bid to “swim into the air”. This behaviour usually occurred when people approached the open-top tank. The precise motivation for this behaviour is something that the aquarium staff are frequently asked to interpret.
I consider this behaviour probably had several purposes, dependent on whereabouts in the tank it was performed. For example, this behaviour was most frequently elicited at the periphery of the tank and in these cases, it probably represented the fish attempting to transverse the barrier it had encountered (i.e., the glass tank walls) by swimming over it. Some visitors considered the behaviour to be investigatory it is not an unwarranted consideration that these fish sought contact from members of the public at the tank side. Prior to Blue Reef taking over the aquarium, it used to be under the management of the SeaLife Centre chain and the pool was a “touch pool”, where customers could (under supervision) touch and stroke the inhabitants. The protocol under Blue Reef was such that patrons were not permitted to touch the fish. Several observations from aquariums across the globe indicate that the fish may actually benefit from human contact, however, laying more eggs than those kept off show. Finally, and realistically the most probable, is that the sharks and rays approached the visitors looking for food. The elasmobranchs in this specific tank are fed by hand from the surface and may thus consider any visitor to the tank a potential food-bearer.
Head out of water – spy-hopping
In a similar vein to the treading water behaviour exhibited by some species in captivity, several species of shark have been known to hang almost vertically in the water with their head above the surface—a behaviour known as spy-hopping. This behaviour is well-known among sharks as they approach boats and has been interpreted as them looking for food. The difference in refractive index between the air and water is such that it's unlikely the sharks can make out much detail, but they will almost certainly be able to make out shapes.
Two very specific cases of spy-hopping have been documented; one in the great white and the other in the oceanic whitetip (Carcharhinus longimanus). In the white shark, this behaviour is often exhibited around boats, as if trying to get a look at the activity on deck. It has also been recorded around seal colonies, specifically while seals or sea lions are hauled out on rocks. It has been suggested that spy-hopping around hauled-out pinnipeds may be a tactic to scare them off the rocks into the water where they're vulnerable to attack. Equally, however, it may be the shark attempting to examine the rocks for potential food. Indeed, in his 2003 Field Guide to the Great White Shark, the late Aidan Martin described how, when a shark is spy-hopping:
“Sometimes a small quantity of clear fluid (probably seawater) can be seen dribbling or squirting from around the eyeball. This may be due to the shark using its massive ocular muscles to flatten the back of its eyeball against the inside of the eye socket.”
In other words, seawater dribbles out at the shark tries to change the shape it its lens to account for air being 750-times less dense than water.
In a fascinating paper to the Journal of Ichthyology in 1995, Russian anatomists Sergey Savel'ev and Valery Chernikov suggested that the oceanic whitetip shark may lift its head out of the water to smell for food. Savel'ev and Chernikov designed an “aerohydrodynamic apparatus” to study the shark olfactory organ. Put simply, they build a mechanical shark head, attached it to a pivot and placed it into a tank—the head could be angled upwards to it broke the water's surface or downwards to it was completely submerged. Additionally, they dissected the olfactory organs of some oceanic whitetips and found closely-packed epithelial cells and plenty of collagen fibres in the lamellae, suggesting they're particularly good at trapping air and analysing scent—almost directly opposite to the structure observed in smaller demersal coastal sharks such as the spiny dogfish (Squalus acanthias).
Using their model, Savel'ev and Chernikov observed that shark only needs to raise its head a couple of centimetres (less than an inch) out of the water for air bubbles to be collected on the olfactory lamellae, albeit that it needs to be moving at over 1.2 metres per second. Once captured, the shark can slow down and the air is held in the nose for processing. Overall, the biologists conclude that the combination of specialised lamellae arrangement and the construction of the nares and olfactory chamber to funnel water very specifically and break down incoming air into small bubbles makes the oceanic whitetip well adapted to sampling airborne odours. That scents disperse more quickly in air than in water means sharks with the ability to sample them can locate food from greater distances than those species not adapted to such “air scenting”.
The four-legged animals, or tetrapods, that we see walking around today are thought to have evolved from a group of fishes known as sarcopterygians during the Devonian period, between about 420 and 360 million years ago. Most scientists consider that early aquatic vertebrates must've used their fins/limbs underwater before the first began exploring the land and we see examples of how they probably did this in some fish around today, including lungfish, mudskippers, and a small number of elasmobranch species.
The curious “walking” behaviour of the epaulette shark (Hemiscyllium ocellatum) was featured in the third episode of The Great Barrier Reef with David Attenborough, aired by the BBC in January 2016, but was first described almost more than two decades earlier. In a paper to the rather obscure journal Zoology: Analysis of Complex Systems, published in 1995, Australian National University post-doctorial student Peter Pridmore described “submerged walking” by this small reef-dwelling shark. In his paper, Pridmore suggested that bending of the body and a substantial rotation of the pectoral girdle and pelvic fins allowed this shark to cross the reef as a “walking-trot”. A subsequent study of epaulette fin anatomy by Tomoaki Goto and colleagues, published in Ichthyological Research in 1999, suggested, however, that it was actually modifications to the attachment of the pectoral and pelvic fins to the body that make this walking possible, because the girdle is composed of a single bar that's tightly set on the lower and side regions of the body, as it is in other species.
Knowing how the sharks physically walk themselves around is one thing, but this raised the question of how they survive in shallow rock pools and even out of the water for long periods without suffocating. We now know that this species is highly resistant to very low oxygen concentrations. Graham Wise and his colleagues found that epaulette sharks remained active even down to oxygen concentrations of 0.39 milligrams of oxygen per litre of seawater (mg O2/L), which is about 6% the normal oxygen content of sea water and well below the threshold most other species would've gone into torpor or suffocated. The researchers found that the sharks only suffered “righting response” impairment (i.e. they kept rolling over) after four hours of exposure to this severe hypoxia and that the animals recovered within 30 minutes of re-oxygenation. A subsequent study by Veronica Söderström and her colleagues revealed that these sharks can decrease their blood pressure by half to maintain blood flow into their brain when oxygen levels drop away and activation of an adenosine receptor allows them to maintain brain ATP levels.
In 2002, Luis Lucifora and Aldo Vassallo published a paper in Biological Journal of the Linnean Society describing the anatomy of walking behaviour in five species of skate; four had well differentiated anterior and posterior lobes to their pelvic fins, while the fifth, the smallnose fan skate Sympterygia bonapartii, was less well differentiated. Based on analysis of video of the skates “walking” in a tank and dissection of frozen specimens, Lucifora and Vassallo concluded that, although the development of the musculature and appendages differed in the elasmobranchs compared with bony fishes or tetrapods, their waling behaviour resembled the “sprawling locomotion” seen in many salamanders and lizards today, rather than the walking-trot observed in sharks. The authors suggest:
“Our observations indicate that most movements of skates along the bottom are made by walking. Walking may be energetically less expensive than swimming by undulating the massive pectoral fins, because of the smaller muscular mass employed. Also, water displacement could be minimized precluding detection by potential prey and predators.”
There seems little doubt that the ability to walk and, in the case of some carpet sharks, survive protracted periods with low oxygen, provides a competitive advantage for these elasmobranchs—allowing them to find food in places most other sharks and rays could not exploit. The way epaulette sharks manage their brain response to hypoxia may also help us treat human stroke patients.