Aquatic animals

Aquatic animals live in water, whether fresh or salt water, for the majority of their lifecycle. Nearly a million species of animals depend on an aquatic ecosystem to survive. While fish are commonly thought of as animals that live in the water, other examples include species of coral, plankton, insect larvae, worms, mollusks, crustaceans, amphibians, reptiles, and mammals. Aquatic animals include an enormous variety of types and species that range in size from microscopic to some of the largest animals on earth. Known species of animals account for about 20 percent of the total animal population on earth, but scientists are still discovering new animals, especially those that live in the inaccessible deepest waters of the ocean. So, the true number of species of aquatic animals is still unknown, although the number is estimated to be about one million.

Background

Aquatic animals are some of the oldest animal lifeforms on earth. About 150 million years after earth was formed, sea levels around the earth rose and likely chemical components from rock dissolved into the oceans. The first organisms, which emerged about 3.5 billion years ago, were single-cell microbes called prokaryotes. They were followed by bacteria, multi-celled algae, and plankton. Scientists believe that between 570 million and 530 million years ago an event called the Cambrian explosion occurred. This was a burst of diversification of species that occurred over a period of about 30 million years. During the Cambrian explosion, multicell organisms began to develop specialized body structures and more genetic complexity. Researchers believe this took place during a period of climate change. Organisms may have developed new features to adapt to these altered temperatures.

While the fossil record did not preserve much from the Cambrian explosion, placozoans are thought to be the first animals to emerge in the oceans about 560 million years ago. They were about a millimeter in length and had a flat, sandwich-like body made of sheets of cells filled with liquid and fiber cells for support. During this period, fossils show organisms with the ability to move and burrow into the seabed. Tracks left behind by these early animals show symmetrical structures that enabled their movement along the seabed to consume microbes. This indicates they were early ancestors of bilaterian animals. Bilaterian animals have symmetrically structured bodies. For example, they have the same number of eyes, legs, or fins on each side of their bodies.

Archeaocyatha built sponge-like mounds as they clumped together and created the first ocean reefs, which provided hunting grounds for early predators. Anomalocarids were some of the largest predators that began to roam the seas during the Cambrian period. They were as large as three feet in length and had specialized appendages to drag prey toward their tooth-lined jaws. One species of anomalocarid, Tamisiocaris borealis, was the first filter feeder that ate plankton. Early worms, such as Ottoia prolifica, made u-shaped burrows in the seabed and ambushed prey who came too close.

Animals with shells or other defenses against predators became more common. The trilobite was an early relative of insects, spiders, and crabs that dominated the Cambrian seas and developed a variety of structural characteristics that allowed it to thrive. Fossil evidence also suggests that these creatures were social and migrated in large groups. Clams and brachiopods of this period developed their signature dual shells, and cephalopods, ancestors of modern octopuses and squid, used shells as a means of protection.

The first vertebrates emerged during the following Ordovician period, but they only became prolific after millions of years. During the later Permian period, about 290 million to 260 million years ago, sharks and large bony-plated fish began to become top predators of the ocean. However, about 252 million years ago, one of the largest extinctions in the earth’s history occurred. Volcanic eruptions from plate movement changed the atmosphere and caused drastic temperature, oxygen, and pH changes in the ocean. This extinction event is estimated to have affected about 90 percent of the aquatic animals. While most life was lost, some species survived and became the ancestors of modern aquatic animals.

During the following Triassic period, reptiles, such as the plesiosaur and the mosasaur, began to take over as the top predators of the ocean. Although living at similar times as dinosaurs and sometimes similar in appearance, aquatic reptiles had different ancestors. Some evolutionary scientists believe some species may have evolved from land-dwelling lizards or other reptiles that adapted to living in the water. Aquatic turtles appeared during the Cretaceous period, about 145 million to 66 million years ago. Large species, such as the Archelon, could be as big as 15 feet (4.5 meters) long.

Plankton and coccolithophorid organisms with exoskeletons were abundant and diverse during the Jurassic Cretaceous period as they formed the base of the food chain. But as these died and accumulated on the seabed, they were buried and eventually became calcium carbonate formations, such as the white cliffs of Dover, or oil reserves that are found in many parts of the modern world.

The Cretaceous-Paleogene extinction occurred about 66 million years ago. It is thought to have been triggered by a large asteroid impacting the earth in modern-day Mexico. The resulting effects caused catastrophic climate change and lack of sunlight. These conditions caused more massive extinctions on land and in the ocean, marking the beginning of the Cenozoic era. However, as with previous mass extinctions, some species survived and thrived in the new environment. Mammals, who were previously small and scarce, became larger and more predatory. Eventually, as life began to thrive again, seals, whales, and manatees began to populate the ocean. Tectonic plate movement 34 million years ago also caused major marine climate changes, which caused more diversification in marine mammals. From this period on, marine mammals became larger and more dominant.

With the appearance of humans on land, further changes in aquatic life occurred. Hunting and human-caused environmental changes have caused the extinction or drastic reduction of aquatic species. For example, only twenty-seven years after it was first recorded and named, the Stellar’s sea cow was driven to extinction through hunting and destruction of its environment. Development of coastal areas, pollution, hunting, habitat destruction, and climate change continue to threaten the biodiversity of all aquatic animal life.

Overview

Both fresh and salt waters are home to a variety of species of animals. Some, such as most fish, live their entire lives in the water. Others such as some insects are aquatic when they are young and move to the land in adult stages. Some organisms, such as amphibians, can live in both the water and on land. The biodiversity found in aquatic habitats forms the foundation for all life on earth. Because of this, scientists are especially concerned with studying aquatic animals because the fragile environments in which they live are susceptible to human-driven threats.

Aquatic animals may be vertebrates or invertebrates and either extract oxygen directly from the water or breathe air. While the term aquatic is applied to water-dwelling in general, the term marine generally is used for animals specifically living in saltwater, such as oceans and seas. Animals belong to the biological kingdom Animalia; to qualify they must be living beings that require oxygen and organic material to survive and be able to move, reproduce, and grow. Most animals are also bilaterians.

Coral are aquatic life forms that can indicate a healthy marine environment. They live on the seabed in colonies formed of calcium carbonate, which coral polyps secrete to protect themselves. They may feed on passing microscopic animals and plants, and in some cases hunt using tentacles with stinging cells. Shallow-water corals grouped together form large reefs, which are essential to maintaining a healthy aquatic ecosystem.

Microscopic life found in water is known as plankton. Some plankton are plants, while animal plankton is called zooplankton. While some species drift on the water currents for their entire lifecycle, others grow large enough to propel themselves. For example, krill has one of the largest combined biomasses on earth of any animal. It is a major component of the diet of humpback whales. Zooplankton spend the daytime in deeper waters to avoid predators, but at nightfall they move closer to the water’s surface to eat phytoplankton, microscopic organisms that use photosynthesis.

Some microscopic aquatic animals are species of insects, which lay their eggs in aquatic environments, and their larvae, which live in the water until they reach an adult stage. Other forms of zooplankton are small aquatic worms that live their entire lives in the water and keep the environment balanced by eating debris from dead plants and animals. Some worms, such as fluke worms, are parasitic and survive by attaching themselves to fish. Worms are also important sources of food for other aquatic animals, such as fish.

Fish live only in aquatic environments, since they depend on gills to absorb oxygen from the water. Fish can be found in almost all bodies of water. Most fish are ectothermic, or cold-blooded, so they are dependent on the ambient temperature of the water. Most species also rely on fins to move. They may vary in size from tiny minnows to large sharks. Fish lay eggs to reproduce. Fish eggs and microscopic larvae are important elements of the base of the aquatic food chain.

Mollusks, soft-bodied invertebrate animals, make up about a quarter of all aquatic animal species. Some, such as snails and clams, grow shells to protect their bodies. They may remain stationary and attached to rocks or other fixtures, or use a foot or other means of propulsion to move. Cephalopods, such as octopuses and squid, use propulsion, tentacles, and other tactics to move and evade predators. Cephalopods are considered some of the most neurologically advanced invertebrates and are also some of the largest invertebrates.

The variety of crustaceans found in an aquatic habitat ranges from microscopic krill to giant crabs. They are a diverse group of animals that have an exoskeleton, segmented body, antennae, and appendages. Many species also have pincers or claws that they use for movement, defense, and hunting. Some species, such as fish lice, are parasitic.

Amphibians are aquatic animals that live in or near water and on land in their adult stage. They usually begin life in a freshwater aquatic environment and undergo metamorphosis to mature from larvae such as tadpoles with gills to adults with lungs. Some species, such as salamanders, use their skin to absorb oxygen. Generally, amphibians have four limbs that they use for movement, although some are more worm-like. They all lay eggs to reproduce and require moisture on their skin.

Although all reptiles have lungs and must breathe air, some species of reptiles are adapted to live in the water. For example, sea turtles may spend the majority of their life in the ocean, only going on land to lay their eggs. Others, such as alligators, may spend time on land and in the water.

Aquatic mammals include cetaceans, which are whales, dolphins, and porpoises; pinnipeds, which are seals, sea lions, and walruses; sirenians, which are manatees and dugong; and marine fissipeds, which are polar bears and sea otters. Like land-dwelling mammals, these animals give birth to live young, produce milk to feed their young, are warm-blooded, breathe air, and have fur or hair at some time during their lifecycle. Since all mammals require air, they usually live in the water and on land near the water, although some live exclusively in the water and frequently surface to get air.

Bibliography

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