While sharks have been around for more than 400 million years, mammals only appeared about 178 million years ago. Like sharks, mammals diversified into many forms and species, but while the two have similar structures that help them thrive in the oceans, they are entirely different animals.
Marine mammals give birth to live young, just like we do! Sharks, on the other hand, have very diverse reproductive modes: some species lay eggs (oviparous), while others produce live young (viviparous). Oviparous species lay eggs that develop and hatch outside the mother’s body. Viviparous species are either ‘placental’ (have a placenta) or ‘aplacental’ (lack a placenta; this is sometimes referred to as being ‘ovoviviparous’). In aplacental sharks, some species’ babies (called pups) rely on their yolk sac for nutrition during the pregnancy, whereas others consume unfertilised, yolk-filled eggs (this is called ‘oophagy’).
When water gets cold, mammals need to keep warm and do so thanks to their warm blood and a thick layer of blubber that insulates them. Marine mammals are endothermic, meaning their body temperature is constant and created internally. Most sharks, like most fishes, are ectothermic – their body temperature matches the temperature of the water around them.
There are some sharks that have endothermic capabilities, though. Members of the shark family Lamnidae have the unique ability to make their internal body temperature warmer than the surrounding chilly water by using a highly developed network of blood vessels that retain the heat produced by their muscles. This family includes the white Carcharodon carcharias, shortfin mako Isurus oxyrinchus, longfin mako I. paucus, porbeagle Lamna nasus and salmon L. ditropis sharks.
Just like humans, marine mammals breathe by taking air into their lungs. They do so through one or more blowholes that are located on top of their heads. Fish, on the other hand, extract oxygen from the surrounding water using their gills. Sharks, for example, have five to seven gill slits on each side of their heads.
Fish do not have hair on their bodies, whereas marine mammals do – or did, at one point.
Not only is the blue whale Balaenoptera musculus the largest animal living on earth today, it is also the largest animal to have ever existed on our planet. A blue whale can grow up to 100 feet long (about 30 metres)! Sharks can seem rather puny compared to that, with the largest accurately measured whale shark Rhincodon typus coming in at 61.7 feet (18.8 metres) long. As the largest fish in the sea, its average length is between 18 and 32.8 feet (5.5 and 10 metres).
Marine mammals have tails or flukes that move up and down; this is because they evolved from four-legged animals whose backbones naturally flexed up and down. Fish (like sharks), on the other hand, have tails and bodies that move from side to side.
From the heavily serrated teeth of great white sharks to the conical teeth of orcas or the single long, spiralled ‘tusk’ of the narwhal, teeth come in all shapes, sizes and functions. Whereas marine mammals have one set of teeth for their entire lifespan (they don’t grow new teeth if some fall out), sharks shed and regrow teeth throughout their lives.
Most baby marine mammals are taken care of, protected and fed by their mothers for several months or even years. Sharks do not care for their pups once they have given birth – the youngsters are immediately on their own.