QUESTIONS AND ANSWERS: Sharks and Rays

Content Updated: 4th September 2008

QUESTIONS:

Are sharks primitive?
How do Whale and Basking sharks grow so big eating such small fodder?
How do sharks and rays control their buoyancy without a swim-bladder?
How do elasmobranchs crush hard prey, with a ‘soft’ skeleton?
Do sharks have any predators other than man?
When did the first sharks appear on Earth?

Q: Are sharks primitive?

A: People often refer to elasmobranchs (sharks and rays) as ‘primitive eating machines’, an inference that seems almost entirely based on their cartilaginous skeletons. Back in the late 1880s, German biologist Ernst Haeckel promulgated -- quite incorrectly, it has since been found -- that the stages of embryonic development and differentiation corresponded to the stages of evolutionary development of the species (“ontogeny recapitulates phylogeny”). In other words, because humans (the animal that many consider to be the most highly evolved on Earth) start life with cartilaginous skeletons, which later undergo a process of ossification (turning to bone), having a cartilaginous skeleton must be a primitive trait. This is a quaint notion, but inherently wrong. Leaving aside the recent discovery that Haeckel appears to have faked his results, one major problem with this idea is that evolution has no ultimate goal. Consequently, having features similar or dissimilar to another organism is not a measure of the ‘advanced’ or ‘primitive’ status of an organism.

People generally seem to regard elasmobranchs as more ‘primitive’ than bony fishes. Even if we fail to consider the inherent problems associated with comparing evolutionary trends across lineages (which is a big evolutionary no no!), bony fishes and neoselachians (modern day sharks) are the products of about 425 million years of independent evolution. This makes deciphering a ‘higher’ (i.e. more advanced) form profoundly difficult. Moreover, a paper published in the journal Genetics by a French research team led by Christiane Delarbre at the Institut Pasteur back in 1998, produced a phylogenetic analysis (i.e. he used genes to infer a taxonomic scheme) confirming that the Chondrichthyes (cartilaginous fishes) are the sister group of what were the Osteichthyes (or bony fishes - now Sarcopterygii and Actinopterygii) - this means that bony fishes and cartilaginous fishes are more closely related to each other than any other group. There is also evidence to suggest that bony fishes appeared in the fossil record before the Chondrichthyes, not the other way around as is frequently cited. Indeed, according to a posting made to the discussion list SHARK-L by Aidan Martin, bony fishes -- and bone in general, which was classically thought to be derived from the plate-like scales of the first fish from the Silurian (443 to 417 million years ago) -- predate the first cartilaginous fishes by some 75 to 50 million years. (Back to Menu)

Q: How do Whale and Basking Sharks grow so big eating such small fodder?

A: Through something known to biology students as the “ten-percent rule”. In most biology textbooks, you will come across the idea that each organism is linked to others in its ecosystem by what it eats – at its most basic, this concept is represented graphically as a ‘food chain’. The figure below shows a basic food chain. Before I go further, I should point out that the concept of a food ‘chain’ is misleading, because it suggests that each organism only feeds on a single prey item. Of course, this is not the case, and most animals feed on many different animals and plants – this more complex idea is depicted as a food ‘web’, showing the many and varied interconnecting feeding relationships of an ecosystem. For the sake of simplicity, however, I will stick to the food chain for this example.

As a general rule of thumb, about 90% of the potential energy contained in the food is lost at each level of the food chain (via respiration, excretion, etc.); only 10% of the energy from the level below is assimilated (i.e. turned into tissue mass) by the level above. Thus, as you move up the food chain, the amount of energy ‘available’ to the next level becomes less and less to the point that the secondary carnivores only assimilate 0.1% of the energy that was originally in the plants. To put it another way, as you move down the food chain, the amount of available energy increases. So, the closer you get to the bottom of the food chain, the more energy you have to put towards your growth and development. For the Basking (Cetorhinus maximus - left) and Whale sharks (Rhincodon typus), the closest they can get to the bottom of the food chain is to be herbivorous, and this allows them to make use of a much larger energy base than their non-planktivorous kin. These sharks aren’t truly herbivorous, because they don’t discriminate between zooplankton (minute floating animals) and phytoplankton (minute floating plants), and Whale sharks will also eat small fish and coral spawn. This “ten-percent” rule does, however, serve to illustrate why there are generally more herbivores than there are carnivores on the planet.

Utilizing a low tropic level is only part of the reason for the Basking shark’s success. Several studies by David Sims at the Marine Biological Association in Plymouth have revealed some fascinating insights into the feeding behaviour and energetics of the world’s second largest fish species. Not only was Dr Sims able to trounce the dogmatic notion that Basking sharks hibernate in winter, he was also able to show that basking sharks have much lower energy requirements than originally thought. Several lines of circumstantial evidence pointed to Basking sharks entering a period of hibernation (or perhaps more accurately torpor) during the winter months; one of these was that the sharks seemed to shed their gill rakers (feeding apparatus) in the autumn. The shedding of rakers in winter was, however, observed in only three animals. With the aid of satellite tagging, Sims was able to show that, although the sharks do shed their rakers in the autumn, they do so gradually. Thus, it seems that they are active year around, with specimens found in winter having both gill rakers and food in their stomachs. Furthermore, Sims and a colleague found that Basking sharks target specific types of zooplankton -- those that grow to a nice, juicy size -- and are very adept at doing so while cruising along water fronts (areas where two bodies of water, often of different temperatures, meet). Sims has also demonstrated that Basking sharks typically require only about one-third of the energy (or food) that they were originally perceived to need. (Back to Menu)

Q: How do sharks and rays control their buoyancy without a swim-bladder?

A: Many bony fishes have a swim-bladder, the presence or absence of which is related to the animal's life habits (and not, incidentally, its taxonomic status). The swim-bladder is normally an oval sac lying in the abdomen just below the vertebral column and is filled either by gulping air -- in fishes that have a connection between bladder and oesophagus (a physostome bladder) -- or from diffusion of gas from the blood into the bladder (a physoclist bladder). Air is less dense than water and so provides a source of lift to the fish. Elasmobranchs don’t have a swim-bladder, and they must find other ways to regulate their buoyancy; this is achieved via several methods.

The primary aspect that gives sharks and rays buoyancy is a large liver filled with low-density oil. The main component of elasmobranch liver oil is squalene, giving the oil a density of 870 to 880 grams per litre (at room temperature). Squalene is formed partway along the chain to cholesterol, and its low density makes it well suited to providing a source of static lift. According to a 1972 paper by H. David Baldrige Jr., liver oil is accumulated at an almost constant weight to tissue ratio in the liver of larger sharks – although the amount present in any given shark at a set time is related not only to species but also body condition. Indeed, in his 1960 paper on the natural history of the Sandbar shark (Carcharhinus plumbeus), the eminent late shark biologist Stewart Springer wrote that fatty livers are an indication of metabolic well-being in sharks, with small livers containing little oil frequently associated with sharks having severe injuries, individuals in obviously poor condition, or males at the end of the mating season. Squalene and other lipids accumulate in large fat vacuoles (a fluid-filled cavity in the cytoplasm of a cell) in the liver cells, constituting as much as 80% of the liver volume – in some of the pelagic (open ocean) sharks, squalene may constitute as much as 90% of the liver oil, giving almost neutral buoyancy. It is believed that many sharks can go long periods without feeding by metabolising their liver oil stores. Indeed, in a 1964 paper H.A.F. Gohar and M.F Mazhar report on a pregnant Whitetip Reef shark (Triaenodon obesus), which survived for six weeks without food in their vivarium. The shark’s liver weight decreased by just under 50%, suggesting that she was metabolising her liver oils. (Photo: The large, bilobed liver of a female Blacktip Reef shark, Carcharhinus melanopterus.)

Precisely how sharks regulate their buoyancy is still something of an enigma. A series of impressively lucid experiments by Quentin Bone -- currently at the Marine Biological Laboratory in Plymouth, UK -- suggested that squaloid (dogfish) sharks may regulate their buoyancy not by altering the amount of squalene in the liver oil, but by varying less abundant components. By hanging weights on the dogfish, Prof. Bone found that the sharks responded by increasing the amount low-density alkoxydiglycerides (specialized fats that nutritionally support the production of various immune cells) at the expense of more dense triglycerides (the chemical form in which most fats exist in food and in the body). This has, however, yet to be demonstrated in any other species.

It is not only liver oil that gives elasmobranchs buoyancy – several factors contribute to over-all lift. On his ReefQuest site (see Links) Aidan Martin notes that as much as 30% of a shark’s hydrodynamic lift (i.e. that caused by moving through the water) is a result of their flattened snouts and ventrum (bellies). Indeed, several studies have recently altered our classical perceptions of how sharks actually use their hydrodynamics to achieve lift. In a 1986 paper published in the Journal of Fish Biology, it was proposed that negatively buoyant fishes (i.e. those that would sink without some buoyancy aid) may adopt a positive body tilt (i.e. nose up, tail down) during steady swimming to increase total lift. Indeed, subsequent studies on a small north Pacific houndshark (the Leopard shark, Triakis semifasciata) have shown that they appear to actively alter their body tilt as required in order to moderate the amount of lift generated by the body profile. The experiments by Cheryl Wilga, at the University of California at Irvine, and George Lauder, at Harvard University, also found evidence to disprove the ‘classical theory’ of shark hydrodynamics (i.e. that the pectoral fins serve to generate lift to balance the lift generated by the tail) in Triakis. Instead, it seems that the pectoral fins produce negligible lift during normal horizontal swimming; Wilga and Lauder proposed five different components that interact to negate the lift generated by the tail during swimming.

Some authors suggest that the cartilaginous skeleton may serve to aid buoyancy; cartilage is about half the density of bone and a frame composed of cartilage would be considerably lighter than the same one composed of bone. Perhaps the most intriguing suggestion for a buoyancy-aid, however, comes from a 1994 paper in the Journal of Experimental Biology. In this paper, a team of Australian researchers suggested that urea and trimethylamine oxide -- a special chemical retained in the shark’s blood to counteract the destabilizing effects of urea on proteins; shortened to TMAO -- have a substantial effect on the buoyancy of marine elasmobranchs, contributing as much as five to six grams per litre. Moreover, it seems that TMAO contributes more to this positive buoyancy than urea.

Finally, some sharks employ air gulping as a way of controlling their buoyancy. There are several species in which air gulping is well known; most of these are the aptly-named Swellsharks (members of the Catshark family). There are 16 species of swellshark and, in most instances, they use this method to wedge themselves in rock crevices so that predators cannot dig them out. One shark that uses air gulping to an entirely different end is the Sandtiger shark (Carcharias taurus). Sandtigers gulp air at the surface, holding it in their stomachs and ‘farting’ it out gradually until it achieves its desired depth. This retention of air allows the shark to hover almost motionless at a depth of its choosing.

Consequently, without a swim bladder, elasmobranchs rely on several factors (especially their large oily livers) to maintain their desired position in the water column. Ultimately, liver oil is only part of a suite of adaptations that elasmobranchs have to help prevent them sinking, whilst allowing them the spectacular manoeuvrability and grace that anyone who has ever watched one cannot help but be in awe of. (Back to Menu)

Q: How do elasmobranchs crush hard prey, with a ‘soft’ skeleton?

A: Until very recently biologists had wondered how an animal with a cartilaginous skeleton could crush the hard shells of marine molluscs such as bivalves (e.g. mussels, clams, oysters, cockles, etc.). Studies by Adam Summers at the University of California at Irvine’s Ecology and Evolutionary Biology Department have, however, shed some light on this conundrum. Dr Summers’ studies on the feeding mechanisms of the Spotted Eagle ray (Aetobatis narinari) -- a large tropical ray that feeds on clams, oysters and snails amongst other things -- have revealed that these elasmobranchs have evolved calcified struts that run right through the jaws to support the crushing plates. Interestingly, the same struts aren't employed by sharks. It seems that in the Horn shark (Heterodontus francisci) -- a medium-sized tropical and subtropical shark that feeds on sea urchins, crabs, worms, anemones and bony fishes -- the shape of the jaws provide the support for their crushing plates. (Back to Menu)

Q: Do Sharks Have Any Predators Besides Man?

A: It may come as something of a surprise that sharks do have natural predators. Elasmobranchs are subject to the dangers of predation from their first moments in this world. Several fascinating papers by David Cox and Thomas Koob, both at Shriners Hospital for Children in Florida, during the mid-to-late 1990s described predation on elasmobranch eggs. In one particular paper to the journal Environmental Biology of Fishes during 1993, Cox and Koob presented data on ten species of shark and ray whose eggs had drill holes from gastropods (molluscs, such as snails, limpets, whelks and slugs). It seems that these molluscs use their drill-like radula (tongue) to bore into the eggcase and feed on the energy-rich yolk. Cox and Koob report that, based on collection of beached eggcases, predation frequency on elasmobranch eggs may range from 50 to 95%. The same paper also describes the incubation of Little skate (Raja [Leucoraja] erinacea) in Frenchman Bay off the coast of Maine (USA) for one year, of which 22% showed signs of boring. In a 1999 paper, Cox, Koob and Paddy Walker report on boreholes found in the eggcases of Thorny skates (Amblyraja radiata) trawled off the Danish coast during June of 1994. The scientists collected 217 eggcases, 39 (18%) of which had “perforations of apparent biological origin”.

Gastropods aren’t the only organisms known to prey on elasmobranch eggs – other elasmobranchs, bony fishes, seals, whales and even monkeys are known to consume shark and ray eggs. A paper in the journal Copeia back in 1931 reports on the finding of an egg-capsule of the Chain catshark (Scyliorhinus rotifer) in the stomach of a Sea bass (Serranidae), while a paper by John McEachran and two colleagues in the journal Marine Biology during 1976 reports on an intact egg-case of the Smooth skate (Raja [Malacoraja] senta) in the stomach of the Thorny skate, Raja [Amblyraja] radiata. During my time at the Blue Reef Aquarium, I observed their large lobster (Homarus gammarus) to feed on any Smallspotted catshark (Scyliorhinus canicula) eggcases that fell off the supportive netting at the top of the tank. Eggcases have also been reported from the stomachs of Sperm whales (Physeter macrocephalus), an Elephant seal (Mirounga angustirostris) and a Northern Sea lion (Eumetopias jubatus). The fascinating Wild Africa series by the BBC in 2001 contained footage of Chacma baboons (Papio ursinus) -- from the Cape Peninsula National Park on the south-west tip of Africa -- making their way down to rocky shore coastline during the turn of the Spring tide to feed on shark eggs. For the baboons, the eggs represent a rare delicacy, rich in protein and energy.

If the development completes and the newborn shark or ray makes it out of the eggcase alive, its problems are far from over. Even if the pup manages to avoid trawl nets or baited hooks and survives the pollution, there are a host of critters that wouldn’t pass-up the opportunity to digest it. It is well known that larger sharks and rays will often eat their smaller kin. For example, the mighty Great White (Carcharodon carcharias) will eater smaller carchahinid (reef) sharks. There is also some evidence that whales may eat sharks. The cetacean species most commonly cited for their ‘selachivorous’ tendencies are the Killer whale (Orcinus orca) and the Sperm whale (Physeter macrocephalus - right). In a short communication to the Journal of Mammalogy in 1966, Richard Backus at the Woods Hole Oceanographic Institution in Massachusetts, reports on “a large shark in the stomach of a sperm whale” caught in the Azores. Based on the remains, and the shape of the tail, Backus considered the fish to be either a Great White or Basking shark (Cetorhinus maximus) – of the two, he deemed the latter more likely. Similarly, a pod of three Sperm whales were observed harassing a 5m (16.5 ft) Megamouth shark (Megachasma pelagios) in July 1998 off Indonesia. The frequency of shark predation by whales is largely unknown and, although Sperm whales are well known to consume smaller sharks, how often they attack larger species is unclear. Records of Killer whales attacking sharks are more profuse in the literature than those for Physeter.

There are reports in the literature dating back to 1956, which document Killer whales (left) feeding on Basking sharks off Southern California and a paper in the journal Morskie Mlekopitayuschie reports on the presence of a Basking shark in the stomach of a Killer whale from the “south subtropics”. Indeed, there is an anecdotal record of a pod of Killer whales attacking and consuming a Basking shark off Porthcurno in Cornwall (UK) in the early 1950s. Further reports that Killer whales may be actively preying on Basking sharks in UK waters made it into the British press at the end of the 1990s. A paper presented to the 17th Annual Conference of the European Cetacean Society in Las Palmas, Gran Canaria (held from 9th to 13th March 2003) by Lissa Goodwin (at the University of Plymouth), Nick Tregenza, Colin Speedie and Ray Dennis, assessed the question of whether orcas eat Basking sharks. Dr Goodwin and her colleagues looked at the occurrence patterns of large marine animals off southwest Britain between 1991 and 2002, finding a relationship between the occurrence of orcas and the presence of Basking sharks. From the data presented by Goodwin and her colleagues it seems that Killer whale presence was correlated with Basking shark presence. Goodwin and her colleagues consider that this correlation may reflect “a previously unidentified predator/prey relationship between orcas and basking sharks”.

In a 1996 paper for the journal Marine Mammal Science, Dagmar Fertl, Alejandro Acevedo-Gutierrtz and Forbes Darby report three Killer whales catching, killing and eating a 1.5m (5ft) shark -- which they considered was either a Bull shark (Carcharhinus leucas) or Lemon shark (Negaprion brevirostris) -- near the mouth of Golfo Dulce, in the Pacific Region of southern Costa Rica, during May 1992. Dr Fertl and his colleagues also summarize the previous reports of Killer whales feeding on elasmobranchs, suggesting, “elasmobranchs may be taken on more occasions than originally considered”. There are also several other interesting accounts of orcas killing Great White sharks. In their 1974 book, Shark: Splendid Savage of the Sea, Jacques-Yves and Philippe Cousteau mention a report of a Killer whale breaking from its pod, diving sharply and racing back up to seize a shark that was lazily swimming near the surface about a half mile away. The orca struck the shark with such force that the it rose clear out of the water with the shark “crosswise” in its mouth. More recently, Peter Pyle of the Point Reyes Bird Observatory in California filmed a 6m (20ft) female orca and her 3m calf attack and kill a 3m (10ft) Great White shark. The attack, which took place at the Farallon Islands during October 1998, was the first video-documented account of Killer whales attacking White sharks. The orca calf played with the shark’s liver, consuming a little before both swam way leaving the shark’s carcass almost untouched.

In South Africa, ReefQuest CEO, the late R. Aidan Martin has documented predation on Puffadder shysharks (Haploblepharus edwardsii) by gulls and seals. In a fascinating paper to the Journal of Fish Biology during 2004, Mr Martin reports on 18 instances over 15 days (between 15 July and 10 August 2002) where these small (max. 60cm / 2ft) catsharks were caught by young Cape Fur seals (Arctocephalus pursillus) or Black-Backed Kelp gulls (Larus dominicanis vetula) at Seal Island, South Africa. Aidan observed that the Kelp gulls would steal the already dead sharks from the fur seals and consume them headfirst. Although the seals were observed to chew the heads off the sharks, tear some of the flesh and toss the sharks over their heads, whole consumption was not witnessed, suggesting an element of play. (Photo: A Kelp gull on Seal Island, South Africa eating a Shyshark).

Thus, even to the exclusion of humans, sharks and rays have a plethora of would-be predators to avoid. (Back to Menu)

Q: When did the first sharks appear on Earth?

A: The Universe is thought to be about 15 billion years old, and the Earth is generally considered to be somewhere between four and five billion years old. As a group of fish*, sharks have been around considerably longer than any of the other species covered on this site. The first sharks appeared on earth about 400 million years ago. This is compared to the first canids (members of the dog/wolf/fox family), which date back to between 55 and 38 million years – the Red fox (Vulpes vulpes) appeared about 1.5 million years go, originating in Eurasia (the continents of Europe and Asia considered as a whole). The mustelids (a group a predatory mammals that include badgers, otters, weasels and ferrets) appear in the fossil record about 25 million years ago – the earliest European badger dates back about two million years, while the American badger (Taxidea) arose in North America about six million years ago. The first animal to possess features of both human (in this instance the teeth) and chimpanzee (the skull morphology) was discovered in Africa, in a small place south of Libya and north of Cameroon, called Chad. This half-man-half-ape creature walked upright and represents the oldest fossil hominid (human), dating back some 6 million years – it was named Sahelanthropus tchadensis (or “Toumai”, for short). Arguably, the first direct ancestors to modern humans (probably Homohabilis) evolved as late as 2.2 million years ago.

The first sharks are represented by only a few fossil scales (cartilaginous skeletons rarely survive the traumas of fossilization) dating back to the Ordovician period (about 455 million years ago). The oldest fossil shark teeth date back some 400 million years and belong to a shark called Leonodus, although we know little about how this shark looked. Excavations of fossil sharks back in the 19th Century found well-preserved skeletons of Cladoselache (part of a group of sharks called Cladodonts) and revealed that these sharks were not -- as had been commonly assumed -- the ancestor of all sharks; there were others that preceded them. Many prehistoric sharks were adorned with weird and wonderful spines and armour, which many palaeontologists (people who study fossils) consider served as a defensive mechanism.

Even today, nobody is sure from what modern sharks evolved – some suggest that they are derived from a 2m (6ft) bullhead-like shark from the early Jurassic (about 180 million years ago). Aidan Martin provides an informative coverage of possible modern shark ancestors on the Origin of Modern Sharks page at his ReefQuest site. Whatever, the ancestor of modern sharks was, the sharks we see today (sometimes referred to as neoselachians) had appeared by the mid-Cretaceous (ca. 100 million years ago). Modern day sharks survived the mass extinction at the end of the Cretaceous (65 million years ago) superseding the dinosaurs and swimming into the present.

Approximately 200 million years after the first sharks swam the oceans, rays arrived. Conventional wisdom had, until recently, suggested that skates and rays (collectively termed Batoids) originated from sharks that gradually became adapted to living on the seabed – an idea referred to as the Hypnosqualean hypothesis. Recent research by a team led by Christophe Douday at the Dalhousie University in Canada, however, found molecular evidence to refute the idea that batoids are derived from sharks. In their 2003 paper to the Molecular Phylogenetics and Evolution, the geneticists present data supporting the concept of shark and -- to a lesser extent -- batoid monophyly. The data published by Dr Douady and his colleagues seem to support the rejection of the Hypnosqualean hypothesis. Instead, it appears that the seven (or more) putative synapomorpies used to define this Hypnosqualean superorder are either sympleisiomorphic, or a consequence of convergent evolution. In other words, the skates and rays don't share a recent common ancestor with the sharks and the features that suggest they're closely related were either inherited from a more distant common ancestor (older than the most recent - i.e. sympleisiomorphic) or evolved independently in the batoids because it’s best suited to the environment in which they live (i.e. convergence). (Back to Menu)

* I use the term ‘fish’ here for the sake of simplicity – I am aware that it is exceedingly difficult to define exactly what a fish is and that the term ‘fish’ is, as such, a biologically imprecise noun.

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