Luku 2.3 (Biology 7 (Finland))


Chapter goals

  • Being able to name different aquatic invertebrates.
  • Being able to explain how different aquatic invertebrates are adapted to life in the water.

How are animals classified?

The individuals of a species often look alike: they have a similar colouring and structure. For example, the small tortoiseshell species of butterfly (Aglais urticae) is easy to distinguish from the common brimstone species of butterfly (Gonepteryx rhamni). The individuals of a species can recognise each other and reproduce with one another.

group of animals consists of multiple related species. For example, there are millions of insect species in the world. When classifying animals into groups, smaller groups can be linked together to form larger groups. For example, the common housefly belongs to the class of insects along with beetles, dragonflies, and other similar organisms.

Each species belongs to these kinds of groups. For example, the common housefly is:

  • an animal
  • an invertebrate
  • an arthropod (phylum)
  • an insect (class)
  • a fly (order)
  • a common housefly (species).
The common housefly belongs to the class of insects. Use the 3D model to study the characteristics of the species.

Invertebrates in water ecosystems

Invertebrates are animals that have no internal supporting structure or skeleton.

Invertebrates do not possess well-developed senses. Their circulatory and respiratory systems are inefficient. These factors explain why invertebrates are often quite small.

Water ecosystems are home to many invertebrate species. In addition to the very small zooplankton, many larger invertebrates are also found in water ecosystems. Many water-dwelling invertebrates, such as snails, crabs, and clams have hard shells.

The liver fluke snail is an invertebrate mollusc.

Species of crustaceans, molluscs, and worms are common in water ecosystems. In addition to these, insects and spiders can also be found living in the water. Many insects live in the water during their larval stage, but for some species water ecosystems are an important habitat also in maturity.

Most small invertebrates live on the water's surface. Pond skaters and whirligig beetles are examples of such species. Other invertebrates live under the water's surface, where algae and aquatic plants give them protection from predators. Clams, molluscs, and crabs dwell on the bottom of water ecosystems. Jellyfish also belong to the group of invertebrates.

How to get oxygen underwater?

When you dive underwater, you must hold your breath. This makes it hard for humans to stay underwater for long periods of time. If oxygen is scarce underwater, why can such a large number of species thrive in water ecosystems? How do these animals get their oxygen?

Many water-dwelling animals have gills. Gills are respiratory organs that filter oxygen from water and remove carbon dioxide from the organism.

The gills of crustaceans and insects are located in the back of the animal's body. Many worm-like organisms gather oxygen directly through their skin. Clams actively pump water in and out of their shells.

Some insects, such as water beetles, have no gills. As a result, they must occasionally come up to the surface to breathe in oxygen from the atmosphere with their breathing tubes.

A water beetle coming up to the surface to breathe.
Watch the video. Can you find gills? How do they work? Are the animals in the video crustaceans or insects?


Some insects lay eggs in the water. The larvae of these kinds of species develop in water ecosystems. The insects that do so are called water insects. Mosquitoes and dragonflies are water insects, as their eggs and larvae develop in water.

The complete metamorphosis of a mosquito 

The development from an egg into a mature insect is an example of metamorphosis. There are two main types of this process.

A mosquito hatches from an egg and develops underwater as a larva. Finally, the larva encases itself in a structure called a pupa. The pupa hatches into an adult mosquito that is once again capable of flying and laying eggs. This kind of process is called a complete metamorphosis. In the image gallery below, the complete metamorphosis of a mosquito is explained in pictures.

Like a mosquito, a dragonfly larva is hatched from an egg. The dragonfly larva grows under the water before climbing a plant stem to the surface. When the larva is above the surface, it hatches into an adult dragonfly. This process is called an incomplete metamorphosis.

An incomplete metamorphosis differs from a complete metamorphosis in that it lacks a pupal stage. 

In the image gallery below, the complete metamorphosis of a mosquito and the incomplete metamorphosis of a dragonfly are explained in pictures.

Dragonfly larvae live underwater.
The complete metamorphosis of a mosquito
An adult mosquito. Mosquitoes lay their eggs in water.
Mosquito larvae grow in water. They eat algae growing on leaves that have fallen into water, among other things.
The mosquito larva comes up to the water's surface to breathe.
The larva has pupated. At this stage, the pupa floats freely in the water.
The mosquito reproduces efficiently. If the ponds where their larvae grow do not dry up in early summer, mosquitoes can be present in annoyingly large numbers later in the summer.
Mosquitoes often hatch from their pupae almost simultaneously.
The incomplete metamorphosis of a dragonfly
Like mosquitoes, dragonflies lay their eggs in water. After hatching, the dragonfly larva climbs a plant stem above the water's surface. Eventually, the larval skin splits and the adult individual emerges.
After hatching, the dragonfly is small and fragile. It begins to pump fluid into its wings, which begin to straighten.
Gradually, the wings grow larger.
When the wings have almost reached their full size, it is the abdomen's turn to grow.
Eventually, the wings open and begin to harden. The other parts of the dragonfly also harden at the same time.
The dragonfly is ready to take off on its maiden flight. Dragonflies continue to develop after their first flight. Most dragonfly species reach maturity within a week of hatching.

Worms and cnidarians


Worms are simple animals. They have no limbs, heads or brains. They also lack many kinds of important organs, such as eyes. 

Because worms have soft bodies and no legs, they are an easy source of food for predators such as birds. Because of this, they have adapted to living in hidden places under the ground and in water.

Many phyla of the animal kingdom include worm-like species. The common earthworm belongs to the group of annelids. It is a decomposer that gains its energy by eating dead plant parts. 

Many flatworm species live as parasites inside the bodies of animals such as fish.

Earthworms act as decomposers in the ecosystem.
The common jellyfish is a cnidarian.


Jellyfish and coral belong to the group of cnidarians. Cnidarians are water-dwelling organisms that can mostly be found in warm saltwater ecosystems. 

Most cnidarian species are radially symmetric and have simple structures. Cnidarians have a primitive nervous system, and they have no separate circulatory or digestive systems. Instead, cnidarians have a digestive channel, where food intake and waste removal are achieved through a single hole.


Snails, clams and octopuses are molluscs. Many mollusc species, such as snails and clams, are protected by calcite shells. 

Most molluscs, such as snails, are mainly herbivores. Similarly, mussels eat by filtering organic matter from the water around them.

Octopuses, on the other hand, are highly evolved predators that eat fish and crustaceans, among other things.

Molluscs are common aquatic invertebrates. For example, blue mussels are a common species found in many seas and oceans.

Blue mussels are molluscs.



Crayfish, lobsters and crabs belong to the phylum of arthropods and the group of crustaceans. Like most arthropods, they are characterised by their jointed legs.

A crayfish's body consists of three parts: the head, the thorax and the abdomen. The head and the thorax form the front part of the crayfish’s body, which is covered by a continuous back shield. The back part of the crayfish's body consists of a jointed, flexible abdomen.

The crayfish is a common crustacean

The bodies of crabs, lobsters and crayfish are covered by an exoskeleton that consists of calcium, chitin and protein granules. The colour of the exoskeleton varies between different species. The exoskeleton provides support and protection for the animal's internal organs. Muscles are also attached to the inner surface of the shell. 

Crabs, lobsters and crayfish have paired legs, with the forelegs forming a pair of claws that the animal uses to hunt, to protect itself and to dig holes underground. Crabs, lobsters and crayfish breathe with gills.

Other aquatic crustaceans include smaller species such as shrimp and krill, and immobile species such as barnacles.


Spiders are arthropods that have eight legs. They belong to the group of arachnids.

Spiders are predators. Some spider species build webs to catch insects, whereas other species run after their prey. 

All spiders are capable of biting, and some of them use venom to paralyse their prey. Many spider species are capable of diving and living underwater for small periods of time.

The diving bell spider is the only spider species known to live almost entirely underwater. Because arachnids do not have gills, the diving bell spider must create a bubble of air in its underwater nest. It does so by collecting small bubbles of air with the small hairs of its body and bringing them to its nest.

The diving bell spider lives underwater. Because spiders do not have gills, it must gather oxygen from above the surface and bring it to its underwater retreat.


Insects are the largest group of animals in the world. They are found almost everywhere. They are particularly abundant in environments where there is enough warmth and moisture, such as rainforests.

All insects share the following traits:

  • All insects have six legs.
  • The body of an insect consists of three parts: the head, the thorax and the abdomen.
  • The heads of insects contain several eyes.
  • All insects have wings.
  • All insects undergo either a complete or an incomplete metamorphosis over the course of their life cycles.
  • Most insects (excluding bees and ants) live alone.

The most common groups of insects include:

  • butterflies
  • dragonflies
  • beetles
  • flies
  • Hymenoptera (such as bees and ants).
Butterflies are insects that have large wings. They act as pollinators for many flower species. A pair of Mazarine blue butterflies.

Test your knowledge

        • mosquito
        • mussel
        • dragonfly
        • snail
        • crayfish
        • beetle

        a) The common housefly is a species of butterfly. 

        • Correct
        • Incorrect

        b) Butterflies are arthropods. 

        • Correct
        • Incorrect

        c) Snails are arthropods. 

        • Correct
        • Incorrect

        d) Dragonflies are butterflies. 

        • Correct
        • Incorrect

        e) Worms have no skeletons. 

        • Correct
        • Incorrect

        f) A grasshopper is an insect. 

        • Correct
        • Incorrect


        • The following organisms are all invertebrates: worms, medusas, molluscs, and arthropods (crustaceans, spiders, and insects)
        • A clam is a mollusc, a crab is a crustacean, and a dragonfly is an insect. Crustaceans and insects belong to the group of arthropods.
        • Water-dwelling invertebrates get their oxygen by either:
          • using specialised organs called gills to filter oxygen from water.
          • occasionally coming up to the surface to breathe oxygen from the atmosphere.
        • Many species of insects undergo a process of metamorphosis. A complete metamorphosis consists of the egg, larval, pupal, and adult stages. An incomplete metamorphosis lacks a pupal stage.