Aquatic animals are those that live or spend a large part of their life in the water, either in a marine (ocean), lake (lakes) or river (river) habitat.
Aquatic animals can be vertebrates or invertebrates, and in turn, are found in various categories of the animal kingdom.
Vertebrate aquatic animals
In this category are fish, reptiles and some types of birds and mammals.
Fishes
Fish are classified into three types, depending on their morphology:
- Osteíctios: they have a calcified skeleton and gills protected by an operculum, a very resistant type of bone. Tuna, grouper and cod are some of the fish in this group.
- Condricts: its skeleton is formed by cartilage and the gills are external and visible. Chimeras and sharks belong to this group.
- Ágnatos: they are fish that lack jaw, like lampreys.
A grouper (Epinephelinae), a fish from the group of osteites.
Reptiles
The aquatic animals of this group are characterized by the presence of scales, pulmonary respiration and a circulatory system that allows them to be both in the water and on the ground. Iguanas, sea turtles and crocodiles are some representative animals of this category.
Sea turtles (Chelonioidea) are aquatic animals of the reptile group.
Birds
Within the group of birds are some aquatic animals such as pelicans, herons, penguins and albatrosses. They are characterized by the presence of feathers that help them regulate their temperature and because they feed on other aquatic animals, such as crustaceans and fish.
Penguins (Spheniscidae) are characterized by being skillful swimmers.
Mammals
There are several types of aquatic animals within the group of mammals.
- Cetaceans: although they are mammals, their morphology is very similar to that of fish and they have fins. Such is the case of defines, sperm whales and whales, to name a few.
- Pinnipeds: its body structure is elongated and culminates in a pair of fins, such as seals, walruses or sea lions.
- Sirenios: they are herbivorous aquatic mammals and, together with cetaceans, they are specially adapted for aquatic life. The manatees belong to this group.
Whales are aquatic mammals of the cetacean group.
Invertebrate aquatic animals
Invertebrate animals are characterized by the absence of articulated skeleton and spine. In this group, there are several categories in which aquatic animals are found.
Cnidarians
Its morphology can be free or in the form of a sac. There are more than ten thousand species within this group and all are aquatic. Jellyfish and anemones are some of the most representative animals in this category.
A jellyfish (Medusozoa), an invertebrate aquatic animal of the cnidarians group.
Echinoderms
They are a group of animals that live exclusively in the water, specifically on the seabed. They are characterized by their star shape and have the ability to regenerate their tissues. The starfish is the most representative echinoderm in this category.
A starfish (Asteroid), an echinoderm aquatic invertebrate.
Poriferous
They are marine invertebrates that can live in the sea or in fresh water. They develop in colonies that function as structures in which they filter the water and obtain food. They lack mouth and their digestion is intracellular.
Marine worms
Their morphology is elongated and they lack limbs. Almost 90% of the seabed is made up of this type of marine invertebrates.
A platelminto (Platyhelminthes), a type of marine worm.
Rotifers
They are microscopic invertebrate organisms that live mostly in freshwater and, to a lesser extent, in salt water. They can also adhere to fungi or lichens to survive. There are more than two thousand species.
Crustaceans
To this group belong arthropods (invertebrates endowed with external skeleton), such as lobsters, prawns and crabs. The exoskeleton is made up of a carbohydrate called chitin, and they will change it several times during their life, each time they increase in size.
The crab is one of the best known crustaceans.
Mollusks
They are one of the largest edges of the animal kingdom, since it has about 100,000 species. They are characterized by having a very soft body that in some cases is covered by a shell, such as snails. Other aquatic invertebrates of this group are clams, oysters, octopus and squid.
Octopuses are a type of mollusc, characterized by its ability to alter its morphology to adapt to the environment.
Characteristics of aquatic animals
In order to live in water, aquatic animals have developed a series of physical and biological characteristics that allow them to take full advantage of the resources offered by their habitat.
Breathing
Aquatic animals can breathe in two ways: taking the scattered oxygen in the water or going to the surface. In order to do so, they have developed three types of breathing: branchial, cutaneous and pulmonary.
- The gill breath is carried out in the gills, structures formed by soft tissue through which the oxygen present in water is absorbed. It is the type of fish breathing.
- The skin breathing is carried out through the skin, which is the organ responsible for the exchange of gases with the aquatic environment. The starfish, for example, has this breathing mechanism.
- Pulmonary respiration, as the name implies, is carried out in the lungs. It is the type of breathing of aquatic animals that need to surface to take oxygen from the air, such as whales, or that although they do not live in water, they spend much of it, such as birds or aquatic mammals.
Feeding
Aquatic animals have several sources of food, but phytoplankton is essential for animals in marine habitats. It is formed by autotrophic microorganisms (they synthesize inorganic material) and is at the base of the food chain of many aquatic animals that, in turn, will become food for larger animals.
Phytoplankton absorbs part of the carbon dioxide in the atmosphere and thus fulfills two essential functions: it generates oxygen through its process of photosynthesis and becomes a source of energy in the aquatic food chain.
On the other hand, the meat of other aquatic animals, as well as seeds, fruits and remains of other plants are part of the diet of aquatic animals.
Temperature
Depending on their type of habitat (marine, lake or river), aquatic animals have developed different mechanisms to maintain their body temperature.
- Antifreeze: aquatic animals from very cold waters (such as polar fish) synthesize proteins that fulfill an antifreeze function. In some cases, they do so throughout the year and in others only during the low temperature seasons, as is the case with the winter sole (Pleuronectes americanus).
- Scales: the scales are external structures that fulfill a protective and insulating function, which allows to combat the low temperatures to the animals that possess them, as is the case of fish and reptiles.
- Feathers or insulating hair: the density and distribution of feathers help birds regulate their body temperature. On the other hand, a thick layer of hair helps marine animals such as otters cope with the cold.
