Sandy Plains: No Place to Hide
Pacific Angel Shark
The Pacific Angel Shark (Squatina californica) superficially resembles a mush-mouthed, sleepy skate. But make no mistake about it: this deceptively sluggish animal has wicked grasping teeth and is a fearsome ambush predator. When provoked, it has been known to snap at divers and fishermen. But when the two predatory species come into contact, humans generally fare far better than the Pacific Angel — when the commercial fishery for this species peaked in 1985 and 1986, some 1.2 million pounds (550 thousand kilograms) were taken annually. To date, no human has been attacked by a Pacific Angel Shark without provocation nor lost so much as a proverbial ‘pound of flesh’.
Just the Facts:
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Habitat: Sandy Plains, Rocky Reefs, Kelp Forests Depth: 10-300 ft (3-185 m) Distribution: Temperate Eastern Pacific, Tropical Eastern Pacific, Chilean |
Due to the commercial fishery for the Pacific Angel Shark, its life history is fairly well known. Unlike most sharks, Pacific Angels of both sexes mature at about the same size, between 35 and 39 inches (90 and 100 centimetres) in length. In mature females of this species, the number of pups remains relatively constant, irrespective of the size of the mother. Pacific Angel Shark pups are born from March to June in deep water — generally 180 to 300 feet (55 and 90 metres) — possibly to protect the tiny pups from predators. Estimating growth rate is frustrated by the peculiarity that in Pacific Angels vertebral growth rings do not seem to be deposited annually but are instead deposited in proportion to increased body size. Because growth rate in sharks is generally strongly correlated with feeding success, growth rate in Pacific Angels probably depends largely on the quantity and quality of its menu.
The Pacific Angel Shark is a cryptic ambush predator. It’s flattened body with expanded pectoral and pelvic fins and mottled pigmentation pattern conspire to reduce its vertical profile and enable it lie nearly invisible on the sandy bottom. To further enhance its camouflage, the Pacific Angel often buries itself in whole or in part and lies quietly on the bottom for days at a time, becoming virtually impossible for potential prey to detect.
A challenge faced by all ambush predators is that prey animals often learn quickly where local predators tend to lie-in-wait. To get around this logistical problem, Pacific Angel Sharks alternate periods of being strongly site-attached with irregular bouts of nomadic effort. In an early tracking study of Pacific Angels off Santa Catalina Island, California, nine sharks were tagged at Ship Rock and followed for periods of 13 to 25 hours. The collective activity space of the nine Pacific Angels was 6/10ths of a square mile (1.5 square kilometres). At the time, this limited movement did not surprise anyone, given the notoriously sluggish reputation of this species. A later tracking study at the same site followed Pacific Angel Sharks for periods of up to three months and found that, during this longer period, these supposedly torpid animals traveled an average distance of over 18.5 miles (30 kilometres) — in some cases, tagged sharks nearly circumnavigated the Island, a distance of about 47 miles (75 kilometres). It is now believed that a Pacific Angel Shark establishes an ambush site where it remains for several days, then suddenly — under cover of darkness — moves to a new site several miles (kilometres) away. This strategy may enable the Pacific Angel to increase its likelihood of encountering naive fishes and squids, thereby maximizing successful ambushes on prey.
A recent study investigated ambush site selection, prey detection and capture by Pacific Angel Sharks at Santa Catalina Island. Locations of buried Pacific Angels and fresh imprints of recently-vacated pits indicate that these cryptic predators carefully select the locations from which they will attempt to ambush prey. The study found that Pacific Angel Sharks usually lie-in-wait in sandy areas near rocky reefs. Rocky reefs serve as a refuge for many kinds of fishes. At Santa Catalina, important prey species for Pacific Angel Sharks include Blackfish (Chromis punctipinnis) and Queenfish (Seriphus politus), which migrate to the outer margins of the reef at dawn and dusk. Thus choosing a sandy area near a rocky reef probably increases a Pacific Angel’s chances of capturing a meal. Further, the study found that Pacific Angel Sharks are most often oriented with their heads facing up-slope. This may help the sharks avoid detection by allowing sediment to flow ‘downhill’ over their heads as well as enable them to spot potential meals silhouetted against downwelling light and to target prey that swims downslope from the reef.
Pacific Angel Shark jaws,
showing labial cartilages which increase suction
during a gape, and the needle-like lower teeth, which
ensure that prey, once grasped, cannot escape
Vision is highly important to Pacific Angel Shark predation. In a series of experiments, rubber models of various fishes were suspended from a clear acrylic rod behind a transparent acrylic partition, eliminating all scent, electrical, and vibratory cues. During daylight hours, each of the fish models was presented to buried Pacific Angels. Having only vision to go on, the sharks struck at virtually everything. High-speed videography has revealed that a Pacific Angel Shark’s predatory strikes are sudden and dramatic: in about a tenth of a second, the front half of the shark’s body snaps upward about 90 degrees from the bottom, the bear trap-like jaws protrude a remarkable distance from the head, and snap shut with audible authority. Experiments conducted at night suggest that the Pacific Angel Shark may detect prey indirectly by the faint greenish sparkle of bioluminescent plankton agitated in its wake. Physiological studies of the Pacific Angel Shark retinal pigments suggest that this species’ peak visual sensitivity occurs at wavelengths almost identical to that produced by local bioluminescent plankton. Thus, predation in the Pacific Angel Shark is visually mediated by day and probably by night as well.
In many respects, the predatory behavior of the flattened and rather odd-looking Pacific Angel Shark resembles that of the celebrated White Shark (Carcharodon carcharias). Both species take advantage of natural topography to remain cryptic, rely heavily on vision to co-ordinate predatory strikes, and employ ambush as a strategy for attacking prey. Like the White Shark, the Pacific Angel is a well-armed creature with which it is foolish to trifle. And like its infamous cousin, the Pacific Angel Shark could easily be decimated by concentrated fishing pressure.
