FISHES OF THE GULF OF MAINE 



35 



Figure 10. — Smooth dogfish (Mustelus canis), male, about 31 inches long, Woods Hole. A, tooth band of right-hand 

 side of upper jaw, about 1.8 times natural size; B, teeth of another specimen, about 6 times natural size. From 

 Bigelow and Schroeder. Drawings by E. N. Fischer. 



Description. — The smooth dog is easily identi- 

 fied among Gulf of Maine sharks by having two 

 large spineless dorsal fins, the second only a little 

 smaller than the first, combined with low, flat, 

 pavement-like teeth. So different, indeed, are its 

 teeth from the awl-like or blade-like teeth of all 

 our other sharks that a glance at the mouth is 

 enough to separate this species from the young of 

 any larger Gulf of Maine shark. In form this 

 little shark is slender, flattened below, with taper- 

 ing but blunt snout. Its first dorsal fin originates 

 nearly over the hind angle of the pectorals. The 

 second dorsal fin is about twice as large as the 

 anal, over which its stands. The tail is of typical 

 "shark" shape, i. e. with upper lobe much longer 

 than lower. The hind margin of the upper lobe 

 of the caudal is deeply notched near the tip; the 

 lower caudal lobe is very small. 



Color. — Upper surface grayish olive, slaty gray 

 or brown, lower surface yellowish or grayish 

 white. Newborn specimens have the upper part 

 of the first dorsal fin edged with dusky gray; the 

 apex of the second dorsal sooty edged or tipped, 

 but with the rear edge white; the tail fin with a 

 sooty blotch above near the tip, but white edged 

 below. But these markings have mostly faded 

 out by the time the little "dog" has grown to a 

 length of two feet or so. Smooth dogs have a 

 greater ability than most sharks to change shade 

 to suit their surroundings, paling to a translucent 



pearly tint above white sand, but darkening on 

 dark bottom. 80 



Size. — Smooth dogs range from about 11% 

 inches to about 14% inches long when born. They 

 mature sexually at about 3 feet, most of the ma- 

 ture females with young are between about 3 

 feet 3 inches and 4 feet 4 inches long; and a few 

 grow to a length of about 5 feet. 



Habits. — The smooth dog is most familiar as a 

 shore fish and a bottom swimmer, commonly 

 entering shoal harbors and bays, and even coming 

 into fresh water. But fishermen also report them 

 as far offshore as the "tile fish" grounds off 

 southern New England and down to a depth of 

 80 to 90 fathoms. They reach the northern part 

 of their range only as warm-season visitors; at 

 Woods Hole they arrive sometime in May, to 

 withdraw in late October or in November. 



Food of the smooth dogfish consists chiefly of 

 the larger Crustacea, and it is perhaps the most 

 relentless enemy of the lobster, which had been 

 eaten by no less than 16 percent of the fish 

 examined by Field. Large crabs are likewise an 

 important article in its diet, as are the smaller 

 fishes. It has been estimated that 10,000 smooth 

 dogfish, in Buzzards Bay, might devour more 

 than 60,000 lobsters yearly, and perhaps one-fifth 



*> Eipcriments have shown that It requires only 1 to 2 hours for one to 

 darken, but as much as 2 days to pale to the extreme; see Parker (Biol. Bull., 

 vol.66, 1934, p. 31). 



