Sharks are odd and rays, their close relatives, are downright peculiar. Like all modern fish, they developed from an ancient lamprey group (the jawless wonders), but they are unique in a number of ways.
Sharks have been munching their fellow ocean dwellers for around 400 million years. In that time they’ve changed little. While the dinosaurs came, conquered and mysteriously vanished, sharks have been quietly going about their business.
The reasons for this are obvious: you don’t tamper with a winning formula – and neither does evolution. So next time you pit your angling wits against a blue or porbeagle – remember you’re up against the most successful eating machine in history.
With incredibly keen senses of smell and vibration, and powerful bodies, these fish fill many roles in the oceanic food webs. From the humble doggie to the mighty whale shark, from the slothful monkfish to the leaping manta ray, descendants of the early sharks have proved to be very adaptable.
Sharks and rays are close relatives. Both have a skeleton made of cartilage (not bone) and so are called cartilaginous fish. That’s where they differ from bony fish such as wrasse and tuna. Rays are a bit like sharks that have been dropped from a great height on to their bellies, completely flattening them.
Rays and flatfish are not related – despite appearances. Flatties are bony fish for a start. All they have in common is their bottom-dwelling lifestyle. Rays swim on their bellies – their broad wings are their pectoral fins. Flatfish actually swim on their sides. Arranged belly-down, they’d look like odd, deep-bodied bream.
That sinking feeling
Neither sharks nor rays have a swim bladder so, being denser than water, they tend to sink. All right for a bottom-dwelling ray – but for a shark spending most of its time in mid-water, it’s a bit of a problem. They get round this in two neat ways.
Fat floats on water, and sharks have a very large, fat-filled liver which provides buoyancy. For some deep water species, this is enough to keep the shark afloat on its own. Being marine animals helps here: since salt water is denser than fresh water, fish float more easily in the briny.
The fin arrangement provides more lift to help keep sharks off the bottom. The pectoral fins stick out from the body and act like small wings, creating upthrust. In hammerhead sharks those odd projections on the head also act like aerofoils, giving them extra upward force at the head end.
The upper tail lobe is much longer than the lower one and this produces downward thrust. This lifts the tail, keeping it on an even keel with the head end.
Most cartilaginous fish lack the bony gill cover that protects the gills in other fish. A quick look at a shark reveals a number (usually five) of naked gill slits in front of the pectoral fins.
Bony fish make good use of the gill cover for breathing. However, most sharks breathe efficiently by using their swimming speed to push water over their gills.
Shark skin is also different from that of bony fish. It is very thick and tough, with small teeth-like scales (called placoid scales), not at all like ordinary ones. This gives most sharks their rough skin. Remember that the next time a squirming doggie tries to give you a Chinese burn!
The biting teeth (also placoid scales) grow in rows from the back of the mouth to the front. They are always growing, so the new teeth at the back push the older teeth forward until they drop out of the mouth – very handy for an eating machine. As teeth get worn or broken, new ones replace them.
Sharks have a reputation for fearsome teeth and a huge bite, but the teeth depend very much on diet. Fish eaters, such as makos, have large sharp teeth, but bottom feeders, such as smooth hounds and most rays, have flat grinding teeth for dealing with shellfish and crabs. Whale sharks and other filter feeders have only tiny teeth.
Typical shark teeth are sharp-edged and adapted to gripping, cutting and shearing. As predators they don’t need to grind their food. They simply bite chunks out of their prey, gulp them down and wait for their corrosive digestive juices to do the rest.
Their jaws, which aren’t directly joined to the skull, are frighteningly effective. When a shark aims to take a chunk out of something, the whole dental arrangement can come right out of the mouth, revealing those snarling gums so beloved of film-makers. It then snakes its head, maximizing the slicing action of the teeth.
That loving feeling
Oddly enough for fish, sharks and rays produce babies more as a mammal does -rather than, say, a tuna might. Instead of having hundreds of thousands of tiny eggs, fertilized in the sea and then abandoned to their fate, cartilaginous fish produce only a few eggs which are fertilized, and in some cases develop, inside the female.
After some sort of courtship ritual, the male introduces sperm into the female’s cloaca using one or both of his claspers -part of the pelvic fins modified into tubes for mating. In dogfish the male wraps around the female to do this, though it’s hard to imagine a portly male porbeagle trying this without rupturing himself!
In some species, such as thornbacks and huss, the eggs are carefully laid in weed -often tethered by tendrils. Other species, starry smooth hounds for example, retain the eggs, so they develop and hatch inside the uterus, while still others have got rid of the egg shell altogether.
Spurdog embryos have no shell but they still rely solely on a yolk sac for food. Female stingrays and other large rays produce a milky liquid to feed the pups in the womb once the yolk sac has run out.
A few of the larger ocean-going sharks actually form a sort of placental connection between the pup’s yolk sac and the mother’s bloodstream. In this way the mother sustains the young until birth.
This is similar to the arrangement in mammals and, in the same way, it ensures that a lot of the young survive. The pups are born fully formed, so they don’t have to face the perils of the planktonic layer.
Cartilaginous fish have few natural enemies, so their numbers are not culled by predators. This means a breeding strategy that ensures the survival of a few young is ideal. However, it also means that even in a good year a few adults can only produce a few young.
Man is now a serious rival for much of the sea’s natural foodstocks. And that’s on top of catching and killing large numbers of sharks and rays themselves.
With a bony fish like cod which produces up to half a million eggs, even a decimated population can replenish its numbers if left alone for a few years. But it will take the British spurdog and thornback populations a very long time to grow back to anything like their former level.