The shape of the shark

When you hear the word shark, what is the first picture that comes to mind? I suppose we tend to think of the species we are most familiar with, like most people think of the Bottlenose dolphin (Tursiops truncatus) when dolphins are mentioned. I am almost sure that the first shark you may think of will be one of the more well-known species such as the Great White Shark (Carcharodon carcharias), the Tiger Shark (Galeocerdo cuvier) or the Mako Shark. What few people are aware of, is the huge diversity within this hugely misunderstood group of creatures. 

So exactly makes a shark a shark? Generally this group of elasmobranch fish is characterised by a cartilaginous skeleton, five to seven gill slits on the sides of their heads and pectoral fins that aren’t directly fused with their heads. The genetic makeup and evolutionary history of sharks is obviously also very different to that of bony fish but they share several common characteristics with their sister group, the rays. Within their 420 million year old evolutionary history sharks have diversified into over 500 species, a list that grows longer each year with new discoveries. Just recently, scientists discovered two new species of Saw sharks in the Indian Ocean, a group of species bearing a long saw-like snout lined with sharp teeth that are used to slash and disable their prey. 

Saw sharks are just one example of how our typical picture of a shark doesn’t quite cutie and how these predators range in size, physical appearance and habitat. Some species have even adapted to life in fresh- or brackish waters such as the Bull shark (Carcharhinus leucas) while others roam the deep, dark oceans like the infamous Greenland shark (Somniosus mircorcephalus) who scavenges on whale carcasses in the abyss. Not all sharks have sharp teeth; the beautiful Zebra sharks (Stegostoma fasciatum) use their specially molar-shaped teeth to crush the shells of molluscs while the two largest sharks, the Basking shark (Cetorhinus maximus) and the Whale shark (Rhincodon typus) filter-feed on plankton and small fish.

To emphasise the beauty of shark diversity, here are three selected shark species that occur in Madeira’s waters and some details on their biology.

Shortfin mako shark

Shortfin Mako shark (Isurus oxyrinchus)

As an oceanic island, Madeira is visited by a variety of pelagic shark species that roam the open ocean in search of prey. The list of pelagic sharks currently includes 53 species and would be utterly incomplete without the Mako Shark, known locally as Marracho. On record, Makos are the fastest swimming sharks in the world, reaching speeds of up to 68 km\h!  Makos are large sharks that may reach lengths of almost 4m and prey on swordfish, tuna, bonito as well as turtles, seabirds and even other sharks. The fact that these phenomenal hunters pursuit commercially sought after fish such as tuna, means they often fall victim to Big game fishermen or are caught as By-catch on long-lines.

The species is frequently sighted further offshore with frequent encounters reported by fisherman. Just last year, however, a tourist leisure boat was lucky to enjoy a sighting with a Mako gorging on a fish at the surface.

Portuguese dogfish

Portuguese dogfish (Centroscymnus coelolepis)

The Portuguese dogfish belongs to one of the widest-ranging deep-sea sharks and is patchily distributed around the world, commonly occurring between depths ranging from 400-2000m. These largely benthic feeders use their acute vision to detect the bioluminescence of their prey, which includes cephalopods, fish, invertebrates and cetacean carrion. The species was once commercially sought-after by the Portuguese fishing industry, a reputation which became its namesake. It is even thought that the fishing tradition for Black scabbard fish (Aphanopus carbo) in Madeira began after scabbards were accidentally caught as by-catch while fishermen were pursuing Portuguese dogfish. Since both these deep-sea predators share the same habitat, it is fairly inevitable that one becomes the by-catch of the other and this is a source of concern for researchers studying this critically endangered species today.

Smooth hound shark

Smooth hound shark (Mustelus mustelus)

Smooth hound sharks roam the deep coastal waters of Madeira and are large, slender and timid creatures inhabiting depths ranging from 5-625m. The animals show preference to the temperate waters of the East Atlantic and the Mediterranean and feed on crustaceans, cephalopods and bony fish with their low-crowned teeth. The smooth hound sharks got their name from their tendency to aggregate in large numbers like a pack of dogs. In fact, some time ago, groups of these sharks were sighted on the coastline close to Praia Formosa near Funchal. Like most sharks, smooth hounds often fall victim to by-catch and discarded carcasses of these slender creatures are often found lying on beaches. 

All these three different shark species are as different as they are beautiful. This short example should also show you that, despite their incredible diversity, they are all united by a common threat; fishing activity.