The Smooth Dogfishes, Mustelus canis of the western Atlantic and Mustelus californicus of the eastern Pacific, are small sharks that attain lengths of about five and two and one-half feet, respectively. They do not inhabit the open sea, but usually remain quite near shore.
What we say here applies to the Atlantic species, but the Pacific one is undoubtedly quite similar. Crabs and lobsters are its chief food, but small fish, worms, squid, and other mollusks are also eaten. Garbage is taken if available. Dogfish have about six rows of flattened, pavement-like teeth, employed for crushing rather than shearing as are the more pointed teeth of many sharks. Scientists have shown, by marking individual teeth in the rear rows, that the dogfish’s back teeth gradually move forward, replacing those lost at the front edge of the mouth. Since new ones are constantly being formed, to move forward as those in front break off or are worn away, the shark has a never-ending supply of teeth. The sand at the bottom of the shark tank in the old New York Aquarium was filled with hundreds of discarded teeth.
Other experiments have shown that the smooth dogfish seeks its prey by smell rather than sight, even though it has well-developed eyes. This is probably true of many other kinds of sharks, but whether it holds for the fast-swimming oceanic ones is questionable.
Some sharks lay large eggs covered with horny capsules. In others the eggs hatch while still inside the mother, and the young sharks complete further development before being liberated. In the smooth dogfish, which produces living young, there is an intimate, placenta-like union between the bloodstreams of the mother and her offspring, by which the embryo is provided food and other necessities in a way similar to that in the mammals. The period of development within the mother is about ten months, and the litters of four to twenty baby dogfish, about fourteen inches long, are born in late spring and early summer. Since smooth dogfish are of little commercial importance and are believed to be destructive to other fishes, their abundance is sometimes an annoyance to fishermen.