Sandy Plains: No Place to Hide
Broadnose Sevengill Shark
The Broadnose Sevengill Shark (Notorynchus cepedianus) is unique among cow sharks in being the only member of its order to inhabit relatively shallow, coastal waters year-round. All other members of the order Hexanchiformes typically inhabit the relentless cold and dark of the deep-sea. As a consequence of its accessible habitat, the life history, ecology, and behavior of the Broadnose Sevengill are better known than those of any of its relatives.
Just the Facts:
Habitat: Intertidal, Estuaries, Sandy Plains, Rocky Reefs, Kelp Forests
Depth: Surface to 150 ft (46 m)
Distribution: Temperate Eastern Pacific, Tropical Eastern Pacific, Chilean, South Atlantic, Argentinean, Southern African, Indian, South East Asian, Western Australian, Southeast Australian/New Zealand, Japanese
The life history of the Broadnose Sevengill Shark has been most studied in northern California waters. Large litters of Broadnose Sevengill pups are born in shallow coastal areas during late spring and early summer. At this time, captures of newborn Broadnose Sevengill pups are frequent in San Francisco Bay, suggesting that it is an important nursery area for this species.
Unlike other cow sharks, the Broadnose Sevengill adapts well to captivity, providing fascinating glimpses into aspects of its life in the wild. Growth rates, age estimation, and feeding habits of Broadnose Sevengills have been extensively studied at the Monterey Bay Aquarium. Results from these studies indicate that newborn Broadnose Sevengills grow quite rapidly, adding about 15 inches (38 centimetres) and 25 pounds (16 kilograms) during their first year. By three or four years of age, their growth rate slows to about 7.5 inches (19 centimetres) and 9.5 pounds (4 kilograms) per year.
Feeding habits of captive Broadnose Sevengill Sharks are also carefully monitored at the Monterey Bay Aquarium, revealing that adults of this species feed sporadically every 5 to 7 days, consuming an average of about 0.2% of their body weight each day. In contrast, newborn Broadnose Sevengills ate far more frequently and consumed about ten times as much food as adults. The ravenous appetite of Broadnose Sevengill pups may be required to support the remarkable growth rate during their first year of life. The fast growth rate of new-born Broadnose Sevengills and large litter size suggests high mortality of this species in the wild.
The White Shark (Carcharodon carcharias) is apparently an important predator of adult Broadnose Sevengills, and it seems likely that it would take the large, inefficient-swimming juveniles at every opportunity. Despite such environmental hazards, Broadnose Sevengill Sharks seem to have a well-developed sense of where ‘home’ is. A large female Broadnose Sevengill, measuring almost 10 feet (3 metres) in length and weighing about 275 pounds (125 kilograms), was captured in Humboldt Bay by Monterey Bay Aquarium staff in July 1990. Due to snout abrasions caused by repeated collisions with tank walls and artificial reefs at the Aquarium, the shark was tagged at the base of her dorsal fin and released in Monterey Bay during June 1994. In October 1996, the shark — positively identifiable by her tag — was re-captured by a sport angler back in Humboldt Bay — some 312 miles (503 kilometres) north of where the Aquarium released her. As an ominous footnote, just prior to being landed by the angler, our homing Broadnose Sevengill was attacked by a larger, unidentified shark. Her belly bore a 10-inch- (25-centimetre-) wide bite wound that is consistent with the distinctive dental pattern of a White Shark.
Although it is not immune from attack by larger sharks, the Broadnose Sevengill is a formidable predator in its own right. In California’s Humboldt and San Francisco Bays, Broadnose Sevengill Sharks feed primarily on Brown Smoothhounds (Mustelus henlei) and Bat Rays (Myliobatis californica), but also consume a wide variety of marine creatures and even such unusual items as rats. Broadnose Sevengills longer than about 8.5 feet (2.7 metres) also commonly take Harbor Seals (Phoca vitulina). Human remains have also been recorded from Broadnose Sevengill stomachs, but it is not known whether these they were taken alive or scavenged. Although Broadnose Sevengills are known to be quite aggressive toward divers in captivity, I suspect that humans only become ‘fair game’ after death. In any case, there can be little doubt that the Broadnose Sevengill is among the top predators in shallow Californian bays.
A recent study of the diet of Broadnose Sevengill Sharks demonstrates that this species also exploits a broad spectrum of food types off southern Africa. In these waters, the two most abundant prey types are smaller sharks — the Shortnose Spiny Dogfish (Squalus megalops) and the Pyjama Catshark (Poroderma africanum) — with a wide variety of teleost fishes composing most of the remainder of the Broadnose Sevengill’s menu. As off California, southern African Broadnose Sevengills often include marine mammals in their diet, such as dolphins and — especially off southern Namibia — Cape Fur Seals (Arctocephalus pusillus). However, unlike their Californian conspecifics, southern African Broadnose Sevengills do not prey heavily on marine invertebrates, only occasionally taking marine snails or octopuses. The study concluded that — with the possible exception of the White Shark — the Broadnose Sevengill has no trophic equivalents and is an apex predator in the southern African marine environments where it occurs.
Much of the Broadnose Sevengill Shark’s success as a broad-spectrum carnivore is attributable to its adaptable feeding behavior. A recently published study describes observations of the feeding behavior of Broadnose Sevengills off southern Africa and California. According to this study, the Broadnose Sevengill employs at least five distinct feeding strategies, two of which take strategic advantage of environmental conditions.
A sudden burst of speed enables Broadnose Sevengills to catch prey unawares and even pursue prey onto shore. In Humboldt Bay, California, a group of Leopard Sharks (Triakis semifasciata) was swimming leisurely near a mudbank 12 to 16 inches (30 to 40 centimetres) deep. Suddenly, from a distance of almost 100 feet (30 metres), a 9-foot (2.7-metre) Broadnose Sevengill streaked past several of the milling Leopard Sharks to seize one of them mid-body. The attacking shark actually launched itself onto the mudflats as its prey hung lifeless from its mouth, apparently killed outright by the initial bite to its backbone. The Broadnose Sevengill then wriggled back into the water and swam off. The ability of Broadnose Sevengills to change their activity level suddenly, from sluggish cruising to explosive action, swim in extremely shallow water and even pursue prey onto shore, undoubtedly contributes to their predatory success.
Conditions of poor visibility may enable Broadnose Sevengills to approach prey more closely than they could otherwise. In Humboldt Bay, this species often forages on the shallow-water mudflats before sunrise during spring tides. After sunrise, Broadnose Sevengills usually move into the deeper channels of the Bay. Sharks captured during that time typically have full stomachs. Similar after-sunset increases in Broadnose Sevengill activity close inshore are also known from Lüderitz Lagoon, Namibia, and St. Helena Bay, South Africa. Broadnose Sevengills also seem to prefer water that is highly discolored or turbid. Along the South African coast, fishermen most often catch Broadnose Sevengills in patches of “brown water”, upwelled by strong southeasterly winds or caused by river runoff, tidal movements, or wave action. When winds or currents change and the discolored water is dispersed, the sharks often cease taking baits. This pattern suggests that Broadnose Sevengills may take advantage of poor visibility to conceal themselves, enabling them to ambush prey from close range and thus increase their chances of successful feeding.
Broadnose Sevengills also increase their feeding success by co-operating. Such behavior has been observed by fishermen in southern Africa, including at Lüderitz Lagoon and St. Helena Bay. A typical co-operative attack on a Cape Fur Seal begins with a group of Broadnose Sevengills forming a loose circle around their intended victim. Any escape attempt by the seal is actively prevented by the pack, which gradually tightens around the hapless prey. Eventually, one or more Broadnose Sevengills rush in to bite the seal, stimulating other members of the pack to join in to obtain their share of the food.
Clearly, invading shallow, coastal marine environments has worked out very well for the Broadnose Sevengill Shark, providing this species with a varied diet and opportunities for social interaction that its deep-sea brethren probably do not know. Its ability to take advantage of turbid waters and low-light conditions has enabled the Broadnose Sevengill to compete effectively against other coastal sharks on subtidal sandy plains.