It is estimated that the first shark species appeared on the planet 400 million years ago and they have been diversifying ever since. Almost half a thousand different shark species have been known. Despite the great antiquity of the shark species, one of the most questioned issues regarding the sea is still debated: Is shark a mammal or is it a fish? Currently there are many defenders of the two positions. There is also a neutral position that ensures that it is neither one thing nor the other.
In WeFish, we are going to solve this question that has intrigued humanity since time immemorial. To achieve this we are going to present the different characteristics of this peculiar species.
Is shark a mammal?
One of the main characteristics of mammals is that they give birth to their young. In addition, they have mammary glands with which they will feed the young during the first stage of their lives. This last point is essential, since the term “mammal” comes from “mammary glands”.
This confusion has also occurred with cetaceans, such as dolphins or whales. The great lack of awareness of the ocean in the past caused that, generally, all aquatic creatures were classified as fish. With the evolution in science and the expansion of knowledge, we began to make distinctions between marine species. Although the confusion was caused by the fact that cetaceans have fins and live underwater, they were considered mammals and expelled from the category of fish.
But what happens in the case of sharks? Aren’t there sharks that can give birth? Are there differences between a shark and a mammal like cetaceans?
Is the reproduction of sharks similar to that of mammals?
Sharks are fascinating creatures, with a lot of peculiarties. One of these characteristics that makes them so special is the ways they can reproduce. Shark reproduction methods vary depending on the species in question. As we said, about 500 different species of shark have been discovered.
Despite the existence of so many species, they all have one thing in common with respect to reproduction: fertilization is carried out internally. The males have an organ called the pterygopod, which serves to fertilize the female.
It is at the time of gestation when the differences between the different species arise. Sharks can be:
About 30% of shark species are oviparous. After mating, the female lays her eggs in safe areas as rocks or algae. The embryos continue the development process inside the eggs, feeding on the yolk. Oviparous shark species are, for example, Zebra shark, catsharks, swellsharks or the Hornshark.
This is the method of reproduction for sharks that most closely resembles that used by mammals. Females give birth to fully developed and alive young. The substances that feed the embryo come directly to the mother, through an umbilical cord. The biggest difference is that, once the moment of birth has arrived, the young is independent of its mother. Sharks don’t have mammary glands, which means that the young must search for their own food after leaving their mother’s womb. This is the main reason why the shark cannot be a mammal. An example of these is the lemon shark.
The most frequent of the reproduction methods among sharks. On this way, the eggs hatch inside the female’s womb, in the oviduct. Unlike the viviparous method, the female doesn’t provide food to her offspring directly, but they feed on the eggs of the yolk sac and the fluids from the walls of the womb. The fully developed young are expelled from the womb. In some species, such as the white shark, the competition for survival begins before birth. Intrauterine cannibalism or oophagia occurs then. This occurs when the offspring feed on their littermates, still in the mother’s womb, and they select the individuals best prepared to be born.
A peculiar case
A case that astonished the scientific community occurred at the Reef HQ Aquarium in Townsville in Australia. In 2017, a zebra shark gave birth asexually, through parthenogenesis, three years after being separated from the male. In this way, it became the first case of a shark that managed to reproduce in a sexual and asexual way. This can occur in some species when they live for a lake period of time in captivity, in pursuit of the survival of the species.
Is shark a mammal or fish?
Since aquatic mammals were differentiated from fish, many people mistakenly believe that the shark is a mammal. This is due to the apparent physical similarity between sharks and dolphins. But as we have already said, this is a mistake.
The main characteristic that differentiates mammals from all other kinds of species on the planet is that mammals suckle their young. This doesn’ t happen in the case of the shark. Shark pups, when they are born, are totally independent of the mother. Whether through oviparous, viviparous or ovoviviparous gestation, baby sharks have a hunting instinct and do not need to be fed. Which makes it one of the most dangerous animals in the world.
Another important factor for their differentiation is that mammals are warm-blooded animals and have a respiratory system with lungs. In the case of fish, in which we include sharks, the respiratory system is made up of gills and the blood of these animals is cold. Cold-blooded animals are sensitive to changes in temperatures with changing surrounding environmental conditions. Meanwhile, the warm blood of mammals allows them to maintain a constant body temperature.
Finally, mammals also regulate their body temperature thanks to the hair with which they protect themselves on the surface of their skin. Sharks have scales that help them swim.
Conclusion: A shark is not a mammal, it is a fish
As we have explained, sharks are not mammals. A shark is a cartilaginous fish. The skeleton of sharks isn’t made up of bones, but cartilage. Some species of shark can give birth to their young, but the fact that they cannot suckle them disqualifies them as mammals. Sharks share more similarities with fish than with mammals.