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, which are special types of animals that are close to having a backbone but do not actually have one.

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 the pressure of fluids 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 called phyla. 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, meaning animals without backbones that lived in the ocean. These animals are made of many cells and differ from plants and fungi because they do not have cell walls. Marine invertebrates live in sea environments and do not have a backbone, though some have hard shells or outer coverings.

One of the earliest known animals might be Dickinsonia, which 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 from being eaten. This was a sign that animals were beginning to evolve quickly in new ways.

Classification

Invertebrates are grouped into different phyla. Think of phyla as ways to group animals based on their body plans — how their bodies are shaped and built. For example, some have symmetry, some have segments, and others have special parts called appendages.

There are many types of marine invertebrates. Some 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 unique features that help them live in the ocean.

Marine sponges

Sponges are special animals that belong to the group called Porifera. Their bodies are full of tiny holes and channels that let water move through them. This water brings them food and oxygen and helps get rid of waste. Sponges don't have nervous systems, digestive systems, or circulatory systems like other animals. Instead, they rely on the water flow to stay alive.

Sponges are made of many cells that can change and move around. They don't have true tissues or organs and their bodies aren't symmetrical. Most sponges live in the ocean, from shallow waters to very deep sea. They usually stay in one place their whole lives. While many sponges eat tiny particles in the water, some have tiny plants living 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 over 10,000 different 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 about 580 million years old. These animals are important for science because they help researchers learn about how more complex animals develop. For example, the starlet sea anemone is easy to care for in labs and helps scientists study how animals grow and develop.

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 and others even longer. 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 and making babies. 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, each with their own unique ways of living.

Echinoderms

Echinoderms, which means "spiny skin," are a special group of animals that only live in the ocean. You can recognize them by their usual five-point star shape, and they include starfish, sea urchins, sand dollars, sea cucumbers, and sea lilies. These creatures can be found from the shallow areas where the ocean meets the land all the way to the very deep parts of the sea. There are about 7,000 different kinds of echinoderms.

Echinoderms are interesting because they start life with one shape but change to have a five-part symmetry as they grow up. They are very important for scientists because they can regrow lost parts of their bodies and they help form limestone rocks, which tell us about Earth’s history.

Marine molluscs

Molluscs are soft-bodied animals and one of the largest groups of sea creatures. They include snails, clams, oysters, octopuses, and squid. There are about 85,000 known types of molluscs, and 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. For example, many use a special tongue-like tool to feed. Molluscs have simple eyes and can sense touches and smells. They have been around since ancient times, and scientists are still learning more about how they evolved.

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, and they grow by shedding their old shell and making a new one.

Marine arthropods, those that live in the ocean, range from very tiny creatures to very large ones like the Japanese spider crab. They have special eyes made of many parts and can sense chemicals and touch with tiny hairs on their bodies. Some, like trilobites, lived millions of years ago and are now extinct, while others, like the horseshoe crab, have remained mostly unchanged 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 very small amounts of minerals found in the ocean. Scientists did not know about these tiny amounts until they started using special tools to look closely. For example, some tunicates collect a lot of vanadium, which they use in 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 in a special blue color in their blood called hemocyanin. These minerals are everywhere in the ocean, but it is not easy 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.