Sharks, skates and rays (collectively the elasmobranchs) come in a wide variety of shapes and sizes, with around a thousand species known to science. Many of these are on the menu of other animals, often other sharks or rays. It may come as something of a surprise that even the most iconic of shark species, the formidable great white (Carcharodon carcharias), is not immune to predation.
Go to work on an egg
Elasmobranchs are subject to the dangers of predation from their first moments in this world. Several fascinating papers by David Cox and Thomas Koob 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 in 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 infer that, based on collection of beached eggcases, predation frequency on elasmobranch eggs ranges from 50 to 95%. The same paper also described the incubation of little skate (Raja 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 reported 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 reported 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 described an intact egg-case of the smooth skate (Raja senta) in the stomach of the thorny skate, Raja radiata. During my time at the Blue Reef Aquarium, I observed their large lobster (Homarus gammarus) feeding on any small-spotted 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 BBC’s fascinating Wild Africa series that aired in 2001 contained footage of Chacma baboons (Papio ursinus) in 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.
They might be giants
If the 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 potential predators to avoid. It is well known that larger sharks and rays will often eat their smaller kin. Tiger sharks (Galeocerdo cuvier) and great whites (Carcharodon carcharias) will, for example, take smaller carchahinid (reef) and squalid (dogfish) sharks if the opportunity arises. Similarly, there are reports of hammerhead sharks (Sphyrna sp.) having been found with stingray barbs lodged in their throats, suggesting they predate myliobatoids. 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). In a short communication to the Journal of Mammalogy in 1966, Richard Backus at the Woods Hole Oceanographic Institution in Massachusetts, reported 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) off Indonesia in July 1998. 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.
Reports in the literature dating back to 1956 document killer whales feeding on basking sharks off southern California, and a paper in the journal Morskie Mlekopitayuschie described 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. Subsequently, a paper presented to the 17th Annual Conference of the European Cetacean Society in Las Palmas, Gran Canaria during March 2003 by Lissa Goodwin, Nick Tregenza, Colin Speedie and Ray Dennis, assessed the question of whether orcas eat basking sharks around Britain. Goodwin and her colleagues looked at the occurrence patterns of large marine animals off southwest Britain between 1991 and 2002 and observed a relationship between the occurrence of orcas and the presence of basking sharks. From these data it seems that killer whale presence was positively correlated with basking shark presence. Goodwin and her colleagues considered 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 on 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. Fertl and his colleagues also summarized the previous reports of killer whales feeding on elasmobranchs, suggesting, “elasmobranchs may be taken on more occasions than originally considered”. There are also several 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.
Finally, in terms of mammalian predators, it appears that seals may sometimes predate some species of shark. In late 2012, Chris Fallows photographed a cape fur seal (Arctocephalus pusillus) catching and eating a blue shark (Prionace glauca) off South Africa. Several similar photos/videos have since emerged of fur seals eating smaller species, such as leopard sharks, including footage captured by SD Expeditions in 2015 of a cape fur seal being very protective over the a carcass. More recently, in 2018, a grey seal (Halichoerus grypus) was photographed catching and eating a tope shark (Galeorhinus galeus) near Belfast Harbour off the Irish coast.
For the birds
In South Africa, the late ReefQuest CEO Aidan Martin documented predation of puffadder shysharks (Haploblepharus edwardsii) by gulls and seals. In a fascinating paper to the Journal of Fish Biology during 2004, Aidan reported 18 instances over 15 days during July and 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. 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.
The curse of man
By far the most significant predator of (arguably threat too, as we don't always eat them) sharks is man, with humans considered responsible to significant declines in shark and ray numbers in recent decades. In the main shark article, I’ve already explored some elements of elasmobranch biology that make them vulnerable to over-exploitation, including that they’re slow growing, with many larger species not reaching sexual maturity until they’re in their mid-teens; many have long pregnancies; most produce only a couple of pups in each breeding cycle; and many species will not breed every year. This combination of factors mean that sharks are not well adapted to repopulate when numbers are depleted.
Commercial fishing/trawling, which is both targeted to provide shark steaks and “rock salmon” to restaurants and accidental (i.e. bycatch, where sharks are caught incidentally while fishing for something else), places a huge demand on shark populations and catch rates have declined worldwide in recent decades. Pollution is also a problem, with significant levels of compounds such as methylmercury present in the meat of most large pelagic species, as is demand for cartilage (sold as a health supplement), shagreen (skin used in luxury items such as shoes), teeth and jaws as curios, gill plates of manta rays (highly sought-after Chinese “medicine”) and liver oil (squalene or squalane), which is used in cosmetics and health food supplements. Plastic pollution is an increasing threat, with a largely unknown biological impact - microplastics are thought to be a particularly significant threat to filter feeding species such as the Basking and Whale sharks. A study led by Maria Fossi and published in Marine Environmental Research during 2014 estimated that Basking sharks may consume as many as 540 pieces of microplastic per hour of feeding, or 13,110 microdebris items per day. More recently, in a review published in 2018, Elitza Germanov and colleagues point out that microplastic ingestion can cause physical damage to the filter feeding apparatus, as well blocking nutrient absorption and themselves being toxic.
Perhaps the biggest demand put on shark populations is for fins. The Shark Trust suggest that the demand for fins, most of which are shipped to south east Asia for use in shark fin soup, is slowly starting to decline, although demand for meat and the gill plates of manta rays is on the rise, the latter worth an estimated US$11 million annually. Nobody knows how many sharks are killed each year. A widely referenced figure suggests at least 100 million, although a paper published in the journal Marine Policy by a team of American scientists a few years ago suggests the true number could lie anywhere between 63 million and 273 million. The threat is serious enough to convince the Convention on International Trade in Endangered Species (CITES) that some shark species are on the brink of extinction and require additional protection. Unfortunately, a series of recent studies suggest that sharks are declining even in relatively un-touched areas and one study, published in Marine Ecology Progress Series by Neil Hammerschlang and colleagues during 2018, found that reducing sharks on coral reefs results in changes to the body size and shape of the other reef inhabitants – evolving smaller tails and eyes that potentially make them less able to avoid predators.