Invertebrates and Mammals of the Baltic Sea

How large can a jellyfish grow in the Baltic Sea?
Which animals live on the seabed in the Baltic Sea?
What is the difference between the grey and the ringed seal?

Zooplankton

In spring, when the water has warmed up, and there is enough phytoplankton, the amount of zooplankton will also start to increase rapidly. During this time, the fry of many fish hatch and find a wide variety of foods. Zooplankton is the main food for juvenile fish in the Baltic Sea. The Baltic herring and the European sprat also feed on zooplankton as adults.

Zooplankton can also be much larger invertebrates, such as the common jellyfish because it also floats freely in the surface layers of the sea. This jelly-like animal with an umbrella-like body moves in the sea to where the waves lead it. It can move by contracting and elongating its body rhythmically. The common jellyfish is a typical saltwater animal that cannot reproduce in the fresher parts of the Baltic Sea.

The common jellyfish is almost entirely translucent and has transparent extensions hanging below its body, with which it directs the tiny zooplankton into its mouth. The common jellyfish can grow up to a few tens of centimetres in our waters

The common jellyfish can move forward by rhythmically pulling its body together

Predatory fish
Phytoplankton
Zooplankton

Think!

Why is zooplankton important in the Baltic Sea?

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Benthic animals

The seabed is home to a variety of small invertebrates: worms, snails, bivalves, and crustaceans. Most of them feed on decaying plant and animal remains that have fallen to the seabed. The diversity is the highest in the shallow water between the algae.

Bivalves living in the Baltic Sea, such as the Baltic clam and the blue mussel, may be abundant on the seabed. The tiny white shells of the Baltic clam with a pinkish interior wash out on the shore and can be seen on the beach in large streaks. This bivalve can tolerate fresh water, and lives at a depth of about a few meters. The edible blue mussel, located a little deeper, has an elongated, black shell. Both of these species are eaten by many waterfowl.

There are several species of bivalves in the Baltic Sea. The Baltic clam is about the size of a fingertip. Edible blue mussels hardly grow to the size of a teaspoon (concave part) in the east side of the Baltic Sea, but on the other side of the Danish Strait, they can grow larger than a tablespoon. The blue mussel is sold in grocery stores for food, however, only the larger mussels grown in saline water are eaten.

Saduria entomon are small invertebrates (about 5 cm long) who often end up in the Baltic herring nets and may accidentally reach the shop with fish

The Chinese mitten crab gets its name from the fact that its legs are covered with hair

There are many small crustaceans in the sea. Among them, the benthic fauna of the Baltic Sea includes, for example, Saduria entomon - one of the largest native crustaceans in the Baltic Sea. This finger-long animal can swim as well as crawl along the bottom of the sea. It eats smaller fish and invertebrates living on the seabed. There are no native lobsters or crabs in the Baltic Sea. At present, however, the Chinese mitten crab that came here with ships has become quite common.

European eel
Seafish
Eastern jellyfish
Common jellyfish
Chinese mitten crab
Blue mussel
Round mussel
Baltic clam

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Mammals of the Baltic Sea

The grey seal and the ringed seal live in the Baltic Sea. Often, seals live offshore in the open sea and roam over a fairly large area. For example, they can swim from the Swedish coast to Estonian waters in a few days. Seals can dive to a depth of one hundred meters to catch their prey. They can stay underwater for up to half an hour. Both grey and ringed seals usually give birth on spring ice, but they have to birth on land in ice-free winters, usually on more isolated islets. Initially, the pups will remain on the ice bank, where the mother will feed them.

The grey seal is the largest mammal in the Baltic Sea

The ringed seals prefer to live near the coast that is rich in islands and bays

In mid-summer, seals gather into colonies and often spend time together, laying on their resting sites on islets or reefs

Over the last hundred years, the number of grey seals in the Baltic Sea have decreased 90%: initially due to overfishing, later due to seawater pollution. At present, there are fewer toxins harmful to seals in the Baltic Sea and seal hunting is banned, so the number of grey seals is gradually increasing again.

The ringed seal is much smaller than the grey seal. The number of ringed seals in the Baltic Sea has decreased even more sharply than grey seals.

Think!

Compare the appearance of seals and fish. What are the similarities and differences caused by?

Seals are fish.
Seals are mammals.
Seals spend most of their life offshore.
Seals live near the coast most of the time.
Seals dive underwater to give birth.
Seals come to sea ice to give birth.

Bonus: The ringed seal and the grey seal

The male grey seal can grow more than 2 meters long, though the females remain smaller. The ringed seal is smaller, about 1.5 meters long. The adult ringed seal has light ring-shaped spots on its back; the grey seal spots have an indeterminate shape. Seals cannot move well on dry land because their hind flippers are directed backwards and do not bend forward.

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I now know that…

In spring, when the fry of most fish in the Baltic Sea hatch, there is a lot of food in the sea for them – zooplankton. Bivalves in the Baltic Sea remain smaller in growth than their counterparts in the Atlantic Ocean. Ringed and grey seals are examples of mammals in the Baltic Sea.

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