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What are Invertebrate Species?

By Jacob Queen
Updated Mar 05, 2024
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Invertebrates are animals without backbones. They have numerous possible body shapes and fall into many different animal families, including insects, jellyfish, and worms. Invertebrates are considered more primitive evolutionarily than vertebrate animals, and the variety between different kinds is much more extreme. Most experts agree that approximately 96% of all animals on the planet are invertebrates.

The invertebrate animals fall into eight different subtypes. Some of the more well-known types are the arthropods, nematodes, mollusks, and annelids. Arthropods are the insects, arachnids, and crustaceans, and they are the most numerous kind of invertebrate species. Mollusks are also very common, and they include snails and squids. Nematodes are round worms, and annelids are segmented worms like earthworms and leeches.

Invertebrates serve important purposes in terms of planetary ecology. For example, many crops are fertilized by invertebrates, and without them, much of humanity’s agricultural efforts could potentially be ruined. They are also an important food source for creatures all over the planet. Many mammals, including humans, consume large numbers of invertebrates, and if the invertebrate species population were to suffer, starvation would be a possibility for many species.

Some invertebrate species can also be harmful to people, and many of the world’s most dangerous parasites are members of the invertebrate family. Some of the more well-known invertebrate parasites include ticks, fleas, and leeches. There are also parasites like tapeworms that live within the body of their hosts, and these can potentially be even more dangerous than their external counterparts. Some parasites, like mosquitoes, can be especially damaging because they can potentially carry diseases.

Most kinds of invertebrate species go through a process called metamorphosis during their development. This is useful, because the animals can fill different ecological niches at different times in development, which allows for maximum exploitation of food opportunities. During each phase of its life, the animal undergoing metamorphosis will generally have a totally different lifestyle and survive in a different way. A common example of metamorphosis would be a caterpillar making a protective cocoon and turning into a butterfly.

Invertebrate species can vary greatly in size. Most of them are fairly small, like insects and mites. Others, like some squids and octopi, can be fairly large. The largest invertebrate in existence is the architeuthis, also known as the giant squid, which can potentially be 65 feet (20 meters) in length. Other invertebrates like the ciliated protozoan are too small to be seen with the human eye.

Frequently Asked Questions

What defines an invertebrate species?

An invertebrate species is characterized by the absence of a vertebral column or backbone. This diverse group encompasses over 95% of all animal species, including insects, arachnids, mollusks, crustaceans, and more. Invertebrates can be found in a myriad of habitats, from the deepest oceans to the highest mountains.

How many invertebrate species are there?

There are estimated to be over 1.25 million known invertebrate species, but the actual number is likely much higher due to undiscovered species, particularly in understudied habitats. According to the World Register of Marine Species, marine invertebrates alone account for over 130,000 species, illustrating the vast biodiversity of invertebrates.

What is the largest group of invertebrates?

The largest group of invertebrates is the arthropods, which includes insects, arachnids, myriapods, and crustaceans. Insects alone make up the majority, with over 1 million described species, representing about 80% of all known animal species on Earth, showcasing their incredible diversity and adaptability.

Why are invertebrates important to ecosystems?

Invertebrates play crucial roles in ecosystems as pollinators, decomposers, and as a part of the food web. They contribute to soil fertility, break down organic matter, and serve as food for numerous vertebrates. Their activities support ecosystem functions that are vital for the survival of many other species, including humans.

How do invertebrates adapt to their environments?

Invertebrates exhibit a wide range of adaptations, such as exoskeletons for protection, specialized limbs for feeding, and camouflage for evading predators. Some have evolved complex life cycles and reproductive strategies to ensure survival in challenging environments, demonstrating their remarkable ability to thrive in diverse conditions.

What are the threats to invertebrate species?

Invertebrate species face numerous threats, including habitat destruction, pollution, climate change, and overexploitation. These pressures can lead to declines in population and biodiversity loss. Conservation efforts are essential to protect these species, as they are integral to the health of ecosystems and the services they provide to humanity.

AllThingsNature is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.

Discussion Comments

By matthewc23 — On Aug 19, 2011

@TreeMan - While the basic definition of an invertebrate usually revolves around an animal having a backbone, there are other requirements as well. Technically, the backbone itself doesn't make something a vertebrate, it's whether it has a spinal cord. The other things have to do with how the animals are formed after an egg is fertilized.

In the case of sharks and non-bony fish, they do have cartilage instead of bone. At the same time, they also have a spinal cord and meet all the other requirements to be a vertebrate. Off the top of my head, they are the only vertebrates I can think of that don't have true bones. I'm sure I could be missing something, though.

In addition to titans62's question, I'd be interested to know what the species distribution is of insects, too. What countries or continents have the most insects and the widest variety?

By TreeMan — On Aug 19, 2011

For some reason I always thought that fish were invertebrates, too, but they aren't mentioned here.

I know I have heard a lot of information about some types of fish, sharks especially, that don't have bones. Their structure is made up of cartilage. I know from experience that some fish do have bones, but I know the groups are split. I'm sure if sharks were invertebrates the article would have mentioned it. I'm just curious why they aren't when they don't have real bones.

By titans62 — On Aug 18, 2011

@cardsfan27 - Most people don't know it, but almost all of the more "primitive" invertebrates go through various changes in form. For jellyfish, hydras, and similar animals, there are actually two stages call the medusa and polyp form. Jellyfish are born from eggs and start off as polyps that cling to rocks and float around in the water.

As they get older, they grow tenticles and take the form we normally think of. The opposite is true for hydras. If you've never seen a picture of a hyrda, they are very interesting.

Does anyone know what the insect species abundance is? I know as a group they make up a huge percentage of all the animals in the word, both in terms of species and numbers. I'm curious what the percentages are. I can't seem to find them anywhere.

By cardsfan27 — On Aug 17, 2011

Wow, I have seen pictures and read stories about giant squids, but I never realized how huge they could be. It's definitely impressive that something without bones can grow to be that large and still function.

The article says that a lot of invertebrates go through metamorphosis. Does this happen with anything besides insects? I have never heard of anything like a worm or snail doing through different life stages.

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