Marine invertebrates

Marine invertebrates are animals that live in the ocean and do not have a backbone. They make up most of the big life we find in the sea. These creatures include many different groups, such as lancelets, sea squirts, and salps.

Because they do not have a backbone, marine invertebrates have found many clever ways to protect themselves. Some have hard shells or outer coverings, while others use pressure inside their bodies to stay upright and move around.

These animals have evolved many different shapes and ways of living, and scientists have grouped them into more than 30 major categories. Their diverse body plans and adaptations help them thrive in all parts of the ocean, making them an important part of marine life.

Evolution

The earliest animals were marine invertebrates. These are animals without backbones that lived in the ocean. They are made of many cells and are different from plants and fungi because they do not have cell walls. Marine invertebrates live in the sea and do not have a backbone, though some have hard shells or outer coverings.

One of the earliest known animals might be Dickinsonia. It lived between 571 million and 539 million years ago. It looked like a ribbed oval and stayed anchored to the ocean floor. Another early group of animals includes jellyfish and sea anemones. There were also interesting creatures called the Ediacara biota, which lived before a big change in animal life called the Cambrian explosion. These creatures had strange shapes, and scientists are still figuring out exactly what they were. Later, small shells appeared, showing that some animals had started to protect themselves. This was a sign that animals were beginning to evolve in new ways.

Classification

Invertebrates are grouped into different phyla. Think of phyla as ways to group animals by how their bodies are shaped and built.

Some marine invertebrates include Acoela, Annelida like sea leeches, Brachiopoda with hard shells, Bryozoa or moss animals, Chaetognatha or arrow worms, Cephalochordata like lancelets, Cnidaria such as jellyfish, sea anemones, and corals, and Crustacea including lobsters, crabs, shrimp, and crayfish.

Others include Ctenophora or comb jellies, Echinodermata like sea stars and sea urchins, Mollusca such as shellfish, squid, and octopus, Porifera or sponges, and Tunicata also known as sea squirts. There are many more groups, each with special features that help them live in the ocean.

Marine sponges

Sponges are special animals that belong to the group called Porifera. Their bodies have many tiny holes and channels that let water flow through. This water gives them food and oxygen and helps remove waste. Sponges do not have nervous systems, digestive systems, or circulatory systems like other animals. Instead, they rely on water to survive.

Sponges are made of many cells that can change and move. They do not have true tissues or organs, and their bodies are not symmetrical. Most sponges live in the ocean, from shallow waters to deep sea. They usually stay in one place their whole lives. Many sponges eat tiny particles in the water, while some have tiny plants inside them that make food from sunlight, and a few even catch small animals to eat.

Marine cnidarians

Cnidarians, which means "nettle" in Greek, are special animals that have stinging cells they use to catch their food. These animals include corals, sea anemones, jellyfish, and hydrozoans. There are many kinds of cnidarians, and they all live in water, mostly in the ocean. Their bodies have a jelly-like layer between two thin layers of cells. They can either swim or stay in one place, and they have bodies that are shaped like a circle with tentacles around their mouths.

Scientists have found fossils of cnidarians that are very old. These animals are important for science because they help researchers learn about how animals develop. For example, the starlet sea anemone is easy to care for in labs and helps scientists study how animals grow.

Marine worms

Further information: Marine worm and Sea worm

Worms are long, tube-like animals without limbs. Marine worms can be very small or very large, with some reaching over 1 metre long. They live in many different ways, some inside other animals and others in the water or in the sand.

Marine worms belong to many different groups. Some have special tentacles for breathing. One type, the giant tube worm, lives near underwater volcanoes and can handle very hot temperatures. There are also flatworms, roundworms, and many other kinds of marine worms.

Echinoderms

Echinoderms, which means "spiny skin," are animals that only live in the ocean. They often have a five-point star shape and include starfish, sea urchins, sand dollars, sea cucumbers, and sea lilies. You can find them from shallow water near the shore to deep parts of the sea. There are about 7,000 kinds of echinoderms.

Echinoderms are interesting because they can change shape as they grow and can regrow lost body parts. They also help make limestone rocks, which tell us about Earth’s past.

Marine molluscs

Molluscs are soft-bodied animals and one of the largest groups of sea creatures. They include snails, clams, oysters, octopuses, and squid. Many live in the ocean, from shallow waters to deep sea areas. Some also live in freshwater or on land.

Molluscs come in many shapes and sizes. Some have shells, like snails and clams, while others, like octopuses, do not. They have different ways of moving and eating. Molluscs have simple eyes and can sense touches and smells. They have been around for a very long time, and scientists are still learning more about them.

Marine arthropods

Arthropods are animals with jointed legs and a hard outer shell called an exoskeleton. They include insects, spiders, and creatures that live in water called crustaceans. These animals have bodies made of many parts. They grow by shedding their old shell and making a new one.

Marine arthropods live in the ocean. They range from very tiny creatures to very large ones like the Japanese spider crab. They have special eyes made of many parts. They can sense chemicals and touch with tiny hairs on their bodies. Some arthropods lived millions of years ago and are now extinct. Others, like the horseshoe crab, have stayed mostly the same for a very long time.

Other phyla

Marine invertebrates include many different animal groups. Some of these are called tardigrades, lobopodians, and onychophorans. Others are non-craniate chordates, which are close relatives of vertebrates. These include cephalochordates, tunicates, and Haikouella.

One example is the lancelet, a small, see-through, fish-like animal that is the closest living invertebrate relative to vertebrates. Sea squirts, another type of tunicate, may help us understand how vertebrates, including humans, evolved. Salps are also part of this group, forming chains in the ocean.

Minerals from sea water

Some sea creatures use tiny amounts of minerals from the ocean. Scientists learned about these small amounts by using special tools. For example, some tunicates collect a lot of vanadium for their blood cells. Other tunicates also gather niobium and tantalum. Even though there is only a tiny bit of copper in the ocean, lobsters use it to make their blood blue, in a special color called hemocyanin. These minerals are found everywhere in the ocean, but it is hard for people to get them.

Glass models

See also: glass marine invertebrates

Glass models of marine invertebrates were made between 1863 and 1880. These models include creatures like the Aurelia aurita, Cnidarian, Octopus, Glaucus atlanticus, and Sea anemone. These glass creations help scientists and students study sea life without harming real animals.