What are aquatic animals?

Aquatic animals are those animals that live all the time or much of their life in water, whether sweet or salty. They can breathe oxygen dissolved in water (through gills or skin) or they can breathe it from the air (through lungs).

The number of species that inhabit water is so vast that it has not yet been fully discovered due to the inaccessible depths of the ocean. However, aquatic animals can be categorized in the same way as terrestrial animals (in vertebrates and invertebrates), taking into account the characteristics of the organisms and their adaptation to the aquatic environment.

Types of aquatic animals

Aquatic animals are divided into two large groups according to the internal structure of their organism: vertebrates or invertebrates.

Vertebrate aquatic animals

They are those that have a bone skeleton or a skeleton made up of cartilage, and are classified as:

  • Fishes. They are those that breathe through the gills, and are divided into three types:
    • Agnes.  They do not have a jaw, like a river lamprey.
    • Chondrichthyans.  They have a cartilaginous skeleton, like the shark.
    • Osteitians.  They have an internal bony skeleton, like tuna.
  • Reptiles. They are those with a pulmonary respiratory system, double-circuit circulatory system, and scaly skin. Their reproduction can be oviparous or ovoviviparous. For example, the sea turtle, the sea snake, the marine iguana, and the crocodile.
  • Mammals. They are those that have a pulmonary respiratory system and are characterized by breastfeeding their young. They are divided into five groups:
    • Cetaceans Like the whale, the beluga, and the dolphin.
    • Pinnipeds. Like the seal, the elephant seal, and the walrus.
    • Sirenians. Like the manatee and the dugong.
    • Pachyderms.  Like the rhinoceros and the hippopotamus.
    • Rodents Like the otter and the capybara.
  • Birds. They are those that have plumage all over their body, which allows them to stay dry to be able to fly or to maintain their body temperature. They feed on fish and crustaceans. For example, the penguin, the pelican, the seagull, and the heron.

Invertebrate aquatic animals

They are those that do not have any type of internal bone or cartilaginous skeleton, and make up a group of six very different species:

  • The arthropods. They are the aquatic crustaceans that live in the depths. They usually have a shell, a gill respiratory system, a complex nervous system, and sexual reproduction. For example, the crayfish.
  • Mollusks. They are aquatic animals that may have an exoskeleton called a shell or shell (for example, the mussel) or may not have an outer shell (for example, the octopus). In both cases, they have siphons (an anatomical structure that is part of the soft organs) through which filtered water enters and leaves, which makes locomotion, feeding, respiration, and reproduction possible.
  • Echinoderms. They are those that live both in the depths of the sea and in shallow waters. They have an internal skeleton made up of calcareous plates and ossicles ( very small bones ), and their bodies have radial symmetry. For example, the starfish, the sea urchin, and the starfish.
  • The worms. They are the earthworms or marine worms that according to their anatomical shape are classified into: ” annelids ” (with a segmented body in the form of rings), ” nematodes ” (with a cylindrical body), or ” flatworms ” (with a flattened body). They have a long and soft texture because they do not have a skeleton. Some live like parasites and feed on their victims. An example of an annelid worm is the leech.
  • The poriferous. Also known as “sponges” are those that do not have differentiated tissues, nerves, muscles, or organs, and many species have radial symmetric anatomy. The irregular shape of the body allows the flow of water through their cavities and it is through the absorption of water that they manage to nourish themselves.
  • The coelenterates. They are multicellular organisms that have only one opening (called “stomodeus”) for access to a single interior cavity that serves as both the mouth and the anus. Some species live clinging to the corals on the seabed and others float freely. They have a sexual and asexual reproductive phase. For example, jellyfish, coral, hydra, and polyp.

Aquatic animal characteristics

Aquatic animals have different characteristics, depending on:

  • The type of breathing. Because dissolved oxygen in water has a lower concentration than that present in the air, aquatic animals have specialized organs to breathe and make gas exchange possible. They can breathe through:
    • Gills Like the shark and the manta ray.
    • Skin. Like the sea urchin and the starfish.
    • Lungs. Like the dolphin and the whale.
  • The type of habitat. Aquatic animals cannot live in any type of water. Each species has characteristics that adapt to the environment, which can be:
    • Saltwater. The seas and the oceans.
    • Sweet water.  The rivers, the lakes, and the lagoons.
  • The type of feeding. The diet will depend on the type of habitat.
    • In the saltwater seas.  For example, many marine animals feed on phytoplankton (a microscopic plant capable of transforming sunlight into energy).
    • In freshwater rivers or lakes.  River salmon, for example, feed on smaller fish, crustaceans, and insects. Other species base their diet on seeds and fruits found on the banks.
  • The type of reproduction. Aquatic animals reproduce in two ways:
    • Sexual.  It can be oviparous , ovoviparous, or viviparous . For example, the turtle, the white shark, and the dolphin.
    • Asexual.  By external fertilization of eggs and sperm that are deposited in the water. For example, the starfish.
  • The type of body temperature. Some aquatic animals can acclimate to the cold temperatures of their habitat thanks to their behavioral thermoregulation system that allows them to adapt to the temperature of the water or also thanks to the scales that cover the dermis.

Examples of aquatic animals

Some examples of aquatic animals are:

  • Clam. It is a bivalve-type mollusk that is protected by a shell, lives buried in sand or mud, and can withstand temperatures of up to 5 ° C. It is one of the longest-lived animals: it can live up to 200 years.
  • The vampire squid. It is a mollusk of deep, temperate, and tropical waters. It shares similarities with squid and octopus, although instead of expelling ink to confuse its attackers, it exudes a sticky substance that takes about ten minutes to dissolve in the water.
  • The dugong. It is a sirenian-type mammal that lives in coastal areas of the Indian and Pacific Oceans, East Africa, Southeast Asia, and Oceania. It feeds only on algae and can be underwater for more than fifteen minutes without coming out to breathe.
  • The beluga. It is a mammal of the cetacean type that usually lives in the Arctic. It is distinguished by its white skin, for being very intelligent, sociable, and curious. It feeds preferably on fish, although it also hunts shrimp, crabs, and squid.
  • The Penguin. It is a seabird that cannot fly, yet it is a skilled swimmer. It feeds on squid, fish, and shrimp. It has excellent eyesight that allows it to see its prey at great distances.
  • The sea serpent. It is a cold-blooded reptile, it has scales, a forked-tipped tongue and it changes its skin completely regularly (as it happens with the earth snake). It lives in tropical areas of the Pacific, Indonesia, and Australia.
  • The marine iguana. It is a scaly reptile that lives in the Galapagos Islands (an archipelago in the Pacific Ocean) and, upon reaching adulthood, its dermis turns dark green. Despite its sharp claws and teeth, it is herbivorous. It feeds on algae and aquatic plants, which it can even pluck from rocks.

Land animals

Land animals are those that live and move by land, they cannot fly or stay in the water for long. They feed on plants and other animals, depending on the habitat in which they live (such as the pole zone, forests, jungles, or deserts ).

They are classified into vertebrates and invertebrates. Some examples of vertebrate land animals are the giraffe, lion, dog, camel, and horse. Among the invertebrate land animals, the spider, the grasshopper, the snail, and the cockroach stand out.

Aquatic animals


Categories: Biology