
Amphipod crustaceans Monoporeia and Pontoporeia – bottom substrate oxidizers
The communities characterized by amphipod crustaceans Monoporeia and Pontoporeia are mostly found on deep soft bottoms. Under favorable conditions, these amphipods can number thousands of individuals per square meter. The habitat type is classified as critically endangered.
Monoporeia affinis is one of the most common deep-bottom species in the Baltic Sea, although it also lives on shallower sandy or muddy bottoms. Monoporeia affinis thrives in both fresh and seawater, as long as the water is sufficiently oxygenated and the animals have enough food. The habitat requirements of Pontoporeia femorata are otherwise similar, but the species is only found in salty and cold waters, usually at depths of more than ten meters.
Biodiversity-rich communities
Benthos dominated by Monoporeia and Pontoporeia are generally species-rich, except in some deep areas of the open sea. The digging amphipods agitate and turn over the seabed, thus introducing oxygen into the bottom material, which also benefits other species. The oxidation increases the binding of phosphorus to the bottom sediment, which mitigates the effects of eutrophication.
Monoporeia affinis and Pontoporeia femorata also provide food for other invertebrates, such as isopod crustacean Saduria entomon, scale worm Bylgides sarsi and priapulid worm Halicryptus spinulosus. Many fish species, such as cod, Baltic herring and smelt, also prey on these amphipods.

Endangered habitat
Monoporeia and Pontoporeia habitats are typically almost or completely devoid of vegetation. The habitat is characterized by at least half of the total biomass consisting of these amphipod species – either one or both combined.
Monoporeia and Pontoporeia habitats occur along the entire coast of Finland. However, their numbers have decreased so much that the habitat is classified as critically endangered. The worst situation among marine areas is in the Gulf of Finland and the Bothnian Sea, where the habitat has decreased by more than 80 percent. The same situation may also be the case in the Åland Sea and the Archipelago Sea. In contrast, in the northern parts of the Bothnian Bay, the area of Monoporeia and Pontoporeia habitats appears to have remained unchanged.
Baltic clam and Marenzelleria worms are taking over
Where communities dominated by Monoporeia and Pontoporeia have disappeared or greatly diminished, the dominant species are often the Baltic clam (Macoma balthica) or red-gilled mud worms (Marenzelleria spp.). They are rapidly taking over the habitat vacated by amphipods.
The increase in Baltic clam habitats is probably eutrophication, which provides the clams with more food. Red-gilled mud worms, on the other hand, are a group of alien species that began to spread in the Baltic Sea in the 1980s and are now abundant in all sea areas. Eutrophication and the lack of oxygen on the bottom give these species a competitive advantage over the Amphipods, as both Monoporeia and Pontoporeia are very sensitive to oxygen depletion.
Species of Monoporeia and Pontoporeia habitats
Monoporeia affinis
Pontoporeia femorata
Scale worm (Bylgides sarsi)
Priapulid worm (Halicryptus spinulosus)
Saduria entomon
Baltic clam (Macoma balthica)
Red-gilled mud worms (Marenzelleria ssp.)
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Benthic animals
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The status of sea bottoms