Aquatic animalsPosted on May 18, 2018 - Last modified: November 12, 2019
We consider aquatic animals those that live fully, or much of their life in water. By using the term aquatic, we include animals that inhabit both salt water and fresh water.
We also consider aquatic animals to be mammals that cannot survive out of water, although they breathe oxygen, such as cetaceans (dolphins, killer whales, etc.), or quadruped mammals such as beavers. Therefore, seabirds such as pelicans, gulls, ducks, etc. are also included.
However, amphibians are not classified as aquatic animals because they do not require an aquatic life habitat. For the most part, amphibians begin their life stage in water, like tadpoles, but when they grow up they have a terrestrial life returning only to the water to mate.
Table of Contents
- 1 Classified by type
- 2 Habitats
- 3 Common features
- 4 Differences between saltwater and freshwater aquatic animals
Classified by type
Within aquatic animals we can make two large divisions according to their morphology: vertebrate animals and invertebrate animals.
Vertebrate aquatic animals have a spinal column and a bone system. In this group we have fish that use gills to breathe and mammals such as dolphins or whales, that use lungs.
Fish have the characteristic of changing the temperature of their body with respect to that of the environment, called the ectodermal body, they breathe through gills and are divided into three groups: The agnate or jawless fish in which few current species are included (lampreys and mixines). The chondricts or cartilaginous fish, composed of sharks, rays and chimeras that are characterized by having visible external gills and a skeleton composed only of cartilage. And finally, the osteíctios or bony fish that have a skeleton and gills protected by a fin of hard bone called an operculum. The vast majority of fish that inhabit the seas and oceans are classified in this group.
They are aquatic animals that have lung respiration, a double-circuit circulatory system, and scaly skin. They are oviparous or ovoviviparous animals.
Aquatic reptiles are made up of sea turtles, which inhabit water for 90% of their lives), sea snakes, marine iguanas, crocodiles, and alligators.
Characteristics for the plumage that covers their body to keep them dry and suitable for flight. They feed on small marine animals such as fish and crustaceans.
Waterfowl are made up of penguins, pelicans, albatrosses, gulls, and herons.
Mammals have adapted to all kinds of environments and we can also find them in the sea. They all have lung respiration and suckle their young. They may or may not have hair. Aquatic mammals can be classified into five groups: Cetaceans, pinnipeds, sirenians, pachyderms and rodents.
In the group of cetaceans we have whales, killer whales, belugas, dolphins, porpoises and sperm whales. They have a body similar to that of fish called fusiform bodies which makes them more hydrodynamic. Their legs have become fins.
The elephant seal, sea lions and walruses belong to the group of pinnipeds. They are carnivorous animals with an elongated body, short legs, and fin-shaped hands and feet.
The sirenians characteristic for being herbivores. They are the only mammals fully adapted to aquatic life, along with cetaceans. They are very heavy and large animals with slow movements. Its closest relative is the elephant.
Invertebrate aquatic animals do not have any kind of vertebral column and neither do they have any articulated internal skeleton. Within this group we can find the cnidarians. This group is made up of jellyfish, anemones, corals, and hydras.
10.000 species are known, they are relatively simple and are made up of nerve cells and sense organs. They live exclusively in aquatic environments.
They can adopt two morphological forms: the form called polyp (sac form) that is immobile and the free form called jellyfish.
They belong to the edge of deuterostomous animals, exclusively marine and benthic. Its name is due to its exclusive internal skeleton formed by calcareous ossicles (composed of carbon). Their skin is usually covered with thorns or other hard and rough structures. In this group we find starfish, sea urchins and cucumbers.
Sponges or poriferous are mostly marine, sessile and devoid of tissues. They grow in large colonies that have a water filter called an aquifer system, made up of pores, channels and chambers through which they feed. Waste and excess water are eliminated through a hole called an osculum.
There are 9.000 species of sponges, of which 150 species live in fresh water.
90% of the seabed is made up of these creatures that are represented by polychaetes, flatworms, equiurans and nemerteans. Their bodies are soft, elongated, and have no limbs.
They are microscopic animals that inhabit fresh water, moist soil, mosses, lichens, fungi, and even salt water. They have their mouths in their bellies and attract food by creating currents around them. Others adhere to corals, forming a jelly-like mass to live within them.
This group is made up of some 67.000 species that belong to arthropods. They have a quirinous and calcareous shell that they shed as they grow. They lead an exclusively nocturnal life.
In this group we can find sea crabs, galleys, prawns, spider crabs and oxen.
It is estimated that there may be about 100.000 species, and about 35.000 already extinct. They are characterized by having a soft body, sometimes naked and sometimes with shells.
They live practically all over the planet, they can be found in polar or tropical waters. They populate the areas of great heights more than 3.000 m above sea level and reach the high ocean depths of more than 5.000 meters.
Within the molluscs we find bivalves that have a shell and two usually symmetrical lateral valves, joined by hinges and ligaments. They are found on the seabed. Among the species we can find clams, oysters, mussels etc.
Gastropods have a muscular ventral foot and a dorsal shell that protects their entire body. Here we find snails and sea slugs.
Cephalopods are the most evolved and are made up of about 700 species commonly known as octopus, squid, cuttlefish, and nautilus. Its body has tentacles and suckers, from 8 that octopuses have to 90 that some nautiluses can have. The latter lacks suction cups on its tentacles.
Different aquatic animals can live in different habitats that can be divided into three groups:
- Marine animals: Some are prepared to withstand the different types of prison and the salinity of the waters.
- River animals: Withstand large currents and high temperatures. They are freshwater, therefore, they do not support the salinity of the water.
- Lake animals: They are calmer waters, where there is not much movement or high pressure. They are also freshwater.
The various aquatic animals carry out their life in the water and have a series of common characteristics that allow them to survive in their habitats. You can find a wide variety of body types, ways to reproduce, different types of teeth, and different ways to survive.
The feeding of aquatic animals can vary depending on the habitat and the species.
All marine animals depend on phytoplankton. Phytoplankton is a microscopic plant that lives at the bottom of the ocean, and converts sunlight into energy through chlorophyll, and like land plants, they consume carbon dioxide and release oxygen necessary for the life of marine animals.
Phytoplankton is the base of the aquatic food chain as it is the main diet of zooplankton (microscopic animals) which in turn is eaten by a crustacean (such as Antarctic shrimp) that is eaten by a fish. The fish in turn encounters predators such as the seal, shark, orca etc.
To feed, they have developed various techniques such as carnivorous animals that tend to stalk their prey such as the crocodile, the alligator or the matamata turtle. However, others prefer to wait for the current to carry the food back to where they are.
Other animals, such as the duck, jump into the water and are capable of swimming against the current thus hunting any animal that is in their path, then they let themselves be dragged to return to the starting point.
Birds that are herbivorous and omnivorous prefer to consume seeds and fruits of the various plants that grow on the edge of the shores of these ecosystems.
Animals must adapt to the water temperature, which varies according to the type of sea, lake or river and its location.
Fish in Antarctic waters keep their blood light so as not to consume a lot of energy and have a natural antifreeze, which they get thanks to their pale blood (they have neither hemoglobin nor red blood cells).
Other fish have scales or swim bladders to maintain temperature. Their skeletons are made up of cartilage, which allows them to be lighter and use less energy and store fat to modify their temperature at will (like whales) Although most mammals that live in water / sea have this characteristic.
In the case of birds, they develop a plumage that covers their entire body. Others, like the otter, have a double coat of hair that insulates them from water and cold.
Aquatic animals reproduce in two ways: sexually and asexually.
La viviparous reproduction it is the one possessed by large aquatic mammals such as whales, dolphins or killer whales. Where the adult female gives birth to a fully developed calf that is able to swim on its own, but will have to be fed by its parents until it is able to hunt on its own.
La oviparous reproduction It is the most common, since it is the one that most fish have, but it is also used by birds. It is done by fertilizing the female's eggs with sperm. And millions of eggs spawn in the water.
La ovoviparous reproduction It is the strangest and consists of a mixture between the oviparous and the viviparous. They grow inside an egg but it remains inside the mother's body until it hatches and is expelled into the sea. The shark is one of the animals that reproduce like this.
La asexual reproduction it is carried out by fractionation or division, as do starfish or without the need for the male part, as happens with sawfish where the offspring are exact clones of the female.
Other animal species leave sperms and eggs in the sea, where fertilization occurs.
In water there are two ways of breathing: through the lungs or through gills. The gills absorb oxygen dissolved in water and the lungs absorb oxygen found in the air.
The gills allow them to use the oxygen that is dissolved in the water to be used in metabolic functions. They are used mainly by fish.
Animals that live in water all or part of their life are called aerobic aquatic animals, therefore, this term is applicable to whales, seals, dolphins, seagulls, ducks, etc.
Cetaceans such as dolphins, killer whales or whales must rise to the surface of the sea to breathe, submerge again and repeat the process throughout their lives. Depending on the animal, they are able to store more or less oxygen and reduce rises to the surface.
There are also birds, such as penguins that spend a certain part of their lives in the sea. While at sea they must also store air and rise to the surface when migrating. Other birds such as ducks or pelicans do not need to store oxygen even if they spend most of their time floating in lakes or seas, since it is enough for them to submerge their beak or dive to feed.
Catfish are able to breathe out of water, as they have lungs.
Amphibians also possess lungs, as they spend most of their lives on land and go to the water to feed or mate. With some exceptions they have gills.
Differences between saltwater and freshwater aquatic animals
The main difference between freshwater and freshwater aquatic animals is salinity, water pressure, and the current they can withstand.
Saltwater animals endure high salinity and have a strong loss of water and a large amount of salts. Through their intestine it absorbs the water they swallow and thanks to the gills they eliminate excess salts. They can also be disposed of in the waste.
Waves and currents
The sea is in constant motion, suffering tides and currents caused by the wind or the lunar attraction. So animals have had to adapt to these changes in order to live.
For example, anemones have a basal disk that supports them and prevents them from being carried away by currents. Although there are some species, such as jellyfish, that are carried away by currents.
Fish, whales and pinnipeds have fins or flukes that help them to propel themselves and thus fight currents.
Animals that live near the coast or reefs are the ones that suffer the most from tidal changes and manage to adapt by growing horizontally (such as corals) or have a shell that they close at low tide and open it at high to feed (such as mussels or barnacles). Closing their shells allows them to retain water and thus avoid drying out.
As we increase the depths in the vast ocean, so does the pressure, the light disappears when the sun's rays do not arrive and the conditions to survive are higher.
To survive in these conditions, animals begin to need bodies to withstand the pressure. The ideal body is gelatinous and its organs must store a large amount of oxygen, as it is very scarce.
Freshwater animals have adaptations in the kidney so that ingested water passes through the gills that eliminate the abundance of water thanks to having renal glomeruli and absorb mineral salts such as sodium and chloride. This process is called diffusion, and it produces dilute urine.
They suffer from the high currents and are constantly swept away, since the water in rivers runs at a high speed and the currents become very turbulent. That is why the insects that inhabit it use the waters as a nest giving larvae that have suction cups that avoid being dragged. The movement of the waters provides them with food.