- Why doesn't the water flow carry away the animals living in the river?
- Who are the casemakers?
- How does the burbot look for food?
The invertebrates of the river have adapted to life in the flowing water
Only animals that can cling to rocks or the riverbed can live in running water. A flat or streamlined body shape also helps to keep them still. For example, snails are stuck to the bottom via a foot with sticky mucus, while many insect larvae hold onto plants or other objects. Some animals hide under rocks, or dig into the bottom of the river, such as river mussels and the freshwater pearl mussels, where only a part of their shells extend upwards.
Among the insect larvae, it is easy to recognise the casemakers, or caddisfly, living at the bottom of the river. They have built a protective shell made of pieces of plants, stones, or small snail shell pieces around their delicate body.
Bonus: Caddisflies
Most people who have been fishing are familiar with casemakers, as they are often used as bait. Casemakers are insect larvae that live on the riverbed. Insect scientists, known as entomologists, call them caddisfly larvae. The adult caddisflies are slightly reminiscent of butterflies. In case of danger, the larva pulls into its "house", and the hard top of its head becomes a security door. Casemakers eat algae, decomposing parts of plants, but also small animals. Most of the larvae of caddisflies are then, as a part of their food chain, eaten by many fish species.
At the bottom of the river, the larvae of mayflies climb around, which are easy to recognisable by their three long tails. The adult mayflies fly along the river and slightly resemble dragonflies, but are smaller and more fragile, and have three long thread-like tails.
Bonus: Mayflies
The name Mayfly comes from the fact that adult mayflies emerge when the mayflower is in bloom. The adult mayflies live only between a few hours and a few days. During this time, they mate and lay eggs. The larvae of mayflies usually move a few kilometres downstream from where their mother laid eggs. That is why the mayflies tend to fly a few kilometres upstream while dancing over the river. Adult mayflies do not need to eat, but they offer a feast to the fish when they dance above the water surface in spring. Trout can be seen jumping out of the water to catch insects straight out of the air during a flight of the mayflies.
The European crayfish lives in some rivers. They used to be found in almost every river, and many lakes, but now crayfish are quite rare. They are active mainly at night; they hide in the underground cavities and under rocks during the day. They feed mainly on plant and animal remains at the bottom of the river. In this way, they accelerate the decomposition of dead creatures. In rivers with a lot of crayfish, the bottom is cleaner.
- Flowing water
- Large fluctuations in water levels during the year
- Oxygen poor water
Think!
- Can you give examples of adaptations of river invertebrates? How do they stay still in the river? Why doesn't the running water carry them away?
Fish
For the most part, the same fish who live in rivers also live in lakes. However, there are some fish species that live primarily in the river. They need cooler and more oxygen-rich water, and have adapted to life with a fast flow. Such fish are, for example, river trout and minnow. The burbot also prefers rivers as its habitat, although it can also be found elsewhere. However, many marine fish species, such as the brown trout and salmon, spawn in rivers. The first years of life for the juveniles of these species also pass in the river. As the young brown trout and salmon grow, it gets harder to find enough food in the river, so they move to the sea. There, brown trout can reach the weight of up to ten kilos, and salmon can be even several times larger. In order to increase the number of trout and salmon, they are raised in fish farms and released into rivers.
River trout have green sides with black and red spots. River trout live in clean, cold water, and fast-flowing rivers. A trout's main diet is made up of the amphipods living on the riverbed. It is the substances in their shells that give the trout meat a beautiful pink colour. Older trout hunt fish; they especially prefer minnows. Trout can also catch insects flying above the water. When the mayflies fly above the water's surface, trout can often be seen jumping out of the water to catch them. River trout is a close relative of the brown trout and, according to many fish scientists, they are even the same species.
- They come there to spend their old age.
- They come to spawn.
- They come there to hunt amphipods.
Burbot prefers cold water
Burbots live in rivers, as well as in lakes and freshwater bays. They are more numerous in rivers than in other bodies of water, because they prefer cold water. They are long-bodied, dark and slippery fish. They have a single barbel on their lower jaw, and one tube-like projection for each nostril. These are sensory organs that the burbot uses to feel their surroundings. They are active at night and therefore need these aids to find food at the bottom of the water. During the day, the burbots hide under rocks and in caves. Young burbots feed mainly on the benthic fauna of the river, while adults catch smaller fish and eat the spawn of other fish. Unlike most fish, they spawn in the winter. The burbot does not grow very large in smaller rivers, it is rare for them to weigh more than a kilogram there. On the other hand, some burbots that have been caught from Lake Peipsi and Emajõgi have weighed as much as five kilograms.
– has three tails
– has built a case around its body
– has a slippery body
– has spots on its body
– has a shell
– has claws
I now know that…
The river is inhabited by invertebrates that can cling to rocks or the riverbed. Some of the most common invertebrates in rivers are casemakers, mayfly larvae, and amphipods. The most common fish is the minnow. Brown trout and salmon spawn in rivers.
