Rocky Reefs: Rich Feeding in Cool Waters
Smallish, dirt-common, and sometimes highly destructive to human interests, the Spiny Dogfish (Squalus acanthias) is often unfairly dismissed as the underwater equivalent of a rat. Yet it is a full-fledged shark, remarkably successful in a wide range of marine habitats, and a formidable predator in its own right. Despite these facts, most people have no inkling about how important or downright fascinating these little sharks are.
Just the Facts:
Habitat: Intertidal, Estuaries, Sandy Plains, Rocky Reefs, Kelp Forests, Open Ocean, Deep Sea, Polar Sea
Depth: intertidal - 4080 ft (1244 m)
Distribution: Arctic, Antarctic?, Temperate Eastern Pacific, Tropical Eastern Pacific, Chilean, Western North Atlantic, Caribbean, Amazonian, Argentinean, Eastern North Atlantic/Mediterranean, Southern African, South East Asian, Western Australian, Southeast Australian/New Zealand, Northern Australian?, Japanese
The diminutive Spiny Dogfish is disproportionately important economically. It is used fresh and salted for human consumption, ground for use as animal feed, as well as a major source of shark liver oil for cosmetics and traditional medicine, shark fins for soup, and shark cartilage for alternative osteoarthritis therapies. Taken by longline or in nets in enormous numbers, the Spiny Dogfish is the only species of shark that supports commercial fisheries on anything close to the scale that teleosts do.
Due to its economic importance, the Spiny Dogfish’s life history has received a great deal of study. The gestation period of this little shark ranges from 18 to 24 months — the latter figure is longer than that of an African Elephant, making it one of the longest pregnancies in the Animal Kingdom. Spiny Dogfishes are extremely slow-growing and long-lived. For example, a two-foot (60-centimetre) individual is about ten years old, while a three-foot (1-metre) specimen is about 30 years old. The oldest reliably aged Spiny Dogfish was an incredible 70 years old.
You wouldn’t know it to look at one, but this nondescript grey little shark is also something of a biological celebrity. Virtually every scientist and physician trained in North America has dissected at least one Spiny Dogfish as part of his or her formal training. This species is tremendously important in biomedical research, too. For example, a great deal of what we know about human kidney function comes from studying renal physiology in the Spiny Dogfish. A new, broad-spectrum antibiotic, called “squalamine” — effective against all manner of exotic bacterial, protozoan, and fungal pathogens — has recently been isolated from the stomach lining of this species.
Due to its abundance and small size, the Spiny Dogfish makes a convenient subject for scientific experimentation. The anatomy and physiology of this species is probably better understood than that of any other shark. We’ve learned a great deal about basic shark biology from studying the Spiny Dogfish. For example, we discovered that fetal sharks have a great deal of difficulty manufacturing their own urea, so their mother has to donate some from her own blood supply so that her developing pups can osmoregulate in her uterus and, eventually, in the sea.
But it is on its own terms that the Spiny Dogfish really excels. This species is basically a deep-sea shark that has secondarily re-invaded shallow, near-shore waters. The Spiny Dogfish seems to prefer cold waters between 45 and 59 degrees Fahrenheit (7 and 15 degrees Celsius). It is found off the shores of every continent (with the possible exception of Antarctica), ranging from the intertidal zone down to a depth of at least 4 080 feet (1 244 metres). Tagging studies have shown that Spiny Dogfishes are seasonally migratory, routinely traveling at least 1 000 miles (1 600 kilometres) in the western North Atlantic and up to 4 040 miles (6 500 kilometres) across the north Pacific, from Washington State to Honshu, Japan. Taken together, these attributes give the Spiny Dogfish one of the largest ranges of any known shark. It’s hard to argue with geographic success like that.
Despite its small size, the Spiny Dogfish is a powerful and versatile predator. It feeds primarily on schooling teleost fishes, including such commercially valuable species as hake and salmon, but also takes a wide variety of invertebrate prey. The Spiny Dogfish is a master at exploiting virtually any abundant food source. I have seen a large aggregation of Spiny Dogfishes actively feeding on Ragworms (Nereis sp.), mass spawning at night in very shallow water — as far as the car headlights could penetrate, the entire bay was full of teensy dorsal fins and thrashing sharks. But Spiny Dogfishes are not limited to such modestly sized prey. Traveling and often feeding in packs, prey size is apparently of little consequence to Spiny Dogfish — newborns of this species have been seen attacking herring even larger than themselves and adults have been seen literally eating their way through schools of cod and haddock.
Part of the Spiny Dogfish’s predatory secret is its teeth and jaws. Its teeth have oblique cusps with overlapping bases, forming a nearly continuous cutting edge. Spiny Dogfish jaws are short, transverse, and thickly muscled — equipment for a powerful bite. Because the arches supporting its jaws are arranged perpendicular to the long axis of the skull, it has long been thought that the jaws of the Spiny Dogfish are only minimally protrusile. Yet recent experimental work has demonstrated that Spiny Dogfish jaws are much more mobile than has traditionally been thought.
I got to experience this first-hand during a baiting session I conducted at night to study competitive feeding in the Spiny Dogfish. About a half-hour into the feeding experiment, one of the excited Dogfish bumped into my shoulder and bit into my wetsuit. As it bit, I could feel a peculiar vibration having about the same frequency as the clapper of an old-fashioned wind-up alarm clock. When I checked my suit after the dive, I saw that the shark had produced a remarkably smooth-edged circular bite; the excised flap of my wetsuit remained attached by the barest of threads. Suddenly, a realization hit me: that peculiar vibration I felt was caused by the arches supporting the dogfish’s jaws oscillating back-and-forth, turning its bandsaw teeth into a piscine version of an electric carving knife! No wonder Spiny Dogfishes can so easily remove bites from prey much larger than themselves.
Well armed as they are, Spiny Dogfishes are not free from various perils. In addition to humans, their predators include other sharks, large teleosts, seals, Killer Whales, and — in one case, a Herring Gull that repeatedly dropped a small Spiny Dogfish onto the rocks until it stopped snapping, then ate it. Due to their coastal haunts near human industries, bottom-feeding habits, and extremely long lifespan, Spiny Dogfishes often accumulate staggering quantities of lead and other heavy metal contaminants in their tissues. The long-term effects of these toxins on Spiny Dogfish livers and reproductive fitness are not known.