Although sharks in general are unjustly maligned throughout the world, it’s hard to find a more gentle fish than the smooth dogfish. This species (Mustelus canis) is probably the most common type of shark in the Atlantic and is found along our coast from Massachusetts to Florida.
Smooth dogs migrate seasonally, swimming in packs (this pack behavior is where the common name originates from) northward in the spring and south again each fall. They prefer shallow water and stick fairly close to shore while migrating. Packs are usually made up of like-sized individuals, with packs getting smaller in number as the age and size of the individuals increases. A group of full grown adults, consisting of fifteen year old females and ten year old males, may have as few as a dozen members, while younger packs may have hundreds of juveniles living together.
Smooth dogs have the unusual ability among sharks to change their skin color to camouflage with their surroundings. Dermal cells known as melanophores can be willfully manipulated to make the skin of the fish darker or lighter depending on the type of bottom the shark is swimming over. Especially when young this helps to protect these bottom-dwelling fish from their predators, which are mainly larger species of open water sharks.
Dogfish are bottom feeding scavengers and predators on small crabs and squid. Generally they can feed only on prey that is small enough to be swallowed whole. The reason for this is their dentition is quite different from nearly all other shark species. As you can see from the picture above (from Bigelow and Schoeder, 1948) these are not the serrated, triangular teeth usually associated with sharks, but consists of a multi-rowed band of small scale-like structures lining the jaw. These teeth are used more to grind up food rather than tear it.
The reproductive strategies employed by sharks is fascinating in its variety. Some species lay eggs, others hold the eggs inside their bodies and give birth to live "pups", while others actually nourish their young with true placentas. Although the placenta of a shark functions exactly like that of mammals, it originates from an ancestral yolk sac, an example of convergent evolution. In this species’ case it is ovoviviparous, meaning the female produces egg cases with yolks (the young are not nourished by the uterus) but these are held inside the body. A dozen or so shark pups are born to each female.
Sharks as a whole are extremely vulnerable to fishing pressures because of their slow growth rate and their strategy of giving birth to relatively few young that are cared for during their early stages via a long gestation period. Compare this to most bony fish which produce thousands, or even millions, of eggs with no gestation at all. Although dogfish mature faster than most shark species (two to three years), because only a dozen or so young are born to each female per year the ability of the population to rebound from heavy losses is greatly hampered. This strategy of producing fewer but more fully formed offspring is know as K Selection. Although K selection will result in lower mortality rates for these fish, it is the main reason they are so vulnerable to overfishing.
Fortunately these fish are not prized for food in this country, although there is currently an increasing market for them in Europe as more desirable species of fish, such as cod and haddock, are being wiped out. Order fish and chips in an English pub and you’ll probably get a plateful of this shark.
There are few regulations on the books protecting this species despite the fact that it is listed as "near threatened" by the World Conservation Union. As mentioned, they are not a highly targeted species commercially, but the larger threat is from those killed in quite large numbers as bycatch. Most kinds of sharks travel alone or in small groups, somewhat protecting them from mass accidental slaughter from fishing gear and boat landings. This isn’t the case for smoothies and an unlucky pack can be killed en masse by a single trawler or gill net.
Another threat to this fish, one that concerns me quite a bit, is from recreational fisheries, especially those deep-sea fishing tour boats that operate up and down the coast. Hundreds of thousands of dogfish are caught each year and those that aren’t deliberately killed on board will rarely survive when thrown back. Both the damage done to the internal organs from the pressure difference at the surface, as well as the hook injuries to the mouth, ultimately doom the animal once it’s been caught.
A little about those mouth injuries. Having kept these fish for the past twelve years, I can tell you that they are surprisingly delicate animals. Although sharks tend to be amazingly resistant to disease and infection, skin abrasions and cuts rarely heal. In my experience the soreness (and yes, fish feel pain) and extended healing time of a hook wound would prevent the shark from eating for quite a while. One common injury to sharks in captivity is "nose rot", caused by these open water fish bumping into things in the confines of a saltwater tank (you’ll notice in the movie below that my shark tank is circular and contains no rocks or obstructions to avoid these injuries). Once the skin is broken the wound simply won’t heal, which is baffling since there is no infection present. And yes, I’ve actually brought a shark to the vet for testing. Sitting in the waiting room with a four-foot shark on my lap, surrounded by dogs, cats and hamsters is a surreal experience.
There is a behavior that some sharks do called "spy-hopping", although this is more commonly seen in some species of whales as well. As you can see above, this is when the animal lifts its head out of the water in order to see or smell above the surface. A shark’s sense of smell is simply astounding, able to detect prey scents in the parts per billion in water. It is now known that some great white sharks will spy-hop to help them locate sea lion colonies. Although smooth dogfish are bottom dwellers, in captivity they will adopt this spy-hopping behavior. As far as I can tell this seems to be an effort to detect food when someone is near their tank. If I’m not holding food they’ll go back underwater and swim around normally. If I have food for them they can smell it and dive to the bottom waiting to be fed (even with good eyesight, these sharks are strictly bottom feeders, even in captivity). Click the image below to see my sharks and halfway through the short video they’ll spy-hop for you.
Fun Fact: The scientific name of this fish, Mustelus canis, means "weasel-like dog".
Other diaries in this series can be found here.