Is a shark a fish or a mammal?MsC Marine Biology
What is a shark?
From studying fossils, scientists believe that sharks have been around for more than 400 million years, long before even the trees on our planet! These top predators have evolved into over 500 species that come in all different sizes, shapes and colours, making each one unique in its own way.
But what are they? Fish? Mammals? Reptiles?
Sharks are fish, even though some people believe they are mammals due to how big some of them can get and because some give birth to live young. Fish are aquatic animals that were among the first vertebrates (animals with a backbone) to evolve on earth. They are divided into two broad groups:
- Teleosts (tel-ee-ost) – they have a bony skeleton and symmetrical tail.
- Elasmobranchs (el-as-mo-branch-es) – they have a skeleton made of cartilage and include sharks, rays and skates.
So, while all sharks are fish, not all fish are sharks!
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.
Marine mammals behave in many fascinating ways and have very diverse social structures. Large-brained mammals, like dolphins, have a remarkable capacity for innovation (such as the Australian dolphins that teach their offspring how to protect their snouts with sponges while foraging) and for passing on a particular culture. From the unique songs between humpback whales to the specific dietary preferences of orca (some chase schooling fish, others hunt sharks or seals, and some prefer to dine exclusively on salmon), scientists are finding that these are all learned behavioural traits that are passed down through generations, leading to cultural differences once thought to be unique to humans. Figuring out what factors really drive them to act the way they do is probably just as complex as explaining why we behave as we do!
Many people believe that sharks are ‘lone hunters’, but in fact a large number of species are social and live and hunt in sizeable groups. Lemon sharks, for example, have been found to even have ‘friends’. But not all are like this; some species prefer a solitary lifestyle and only cross paths with other sharks during mating or at common feeding grounds.
How long do they live?
According to NOAA, the bowhead whale has the longest lifespan of all marine mammals, living for up to at least 200 years. That’s a lot of birthday candles!
But what about sharks? Scientists have studied the maximum ages of only some of the 500-plus shark species, but it seems that the Greenland shark is not only the longest-living shark species, it is also the longest-living vertebrate on earth! Researchers estimated that one female was about 400 years old and that she reached sexual maturity when she was about 150.
Humans pose the biggest threat to marine mammals and sharks. Today, these animals face the following threats, among many others, to their well-being:
- Fishing industry (overfishing, by-catch, bottom-trawling)
- Climate change
- Habitat loss
- Boat strikes
- Noise pollution
- Chemical pollution
- Oil spills
- Toxic algal blooms
- Plastic pollution
- Slow reproduction
- Hunting and capture (e.g., finning for shark-fin soup)
The threats in the above list are complicated. Some can be tackled relatively easily (by saying ‘no’ to plastic bags that can end up in a shark or whale’s stomach, for example, or not purchasing seafood that was caught by questionable or unsustainable methods). Others, such as the shark-finning situation, are more complex. It can be sad to learn about all these threats, but there is some good news: you can help to mitigate them by learning about the issues, choosing a few ways to help marine animals in your day-to-day life, and doing it!
Ellen C. Garland, et al., 2017, Song hybridization events during revolutionary song change provide insights into cultural transmission in humpback whales
NOAA, 2021, What is the longest-lived marine mammal?.&text=Scientists%20agree%20that%20the%20bowhead,Mammal%20Permit%20782%2D1719).)