Monoporeia affinis (Lindström 1855) (Crustacea)

Compiled by Fritz Gosselck, Germany

1. Description / Ecological demands / Autecology

M. affinis is a glacial relic species of freshwater origin. The small amphipod (11 mm in length) is restricted to deep cold water zones and tolerates reduced salinities from 18 psu down to 0 psu. M. affinis is a deposit feeder, living mainly in the top 5 cm of the sediment. The species M. affinis and P. femorata are the dominant deposit feeding invertebrates over wide areas of soft sediments in the Baltic Proper. The lifespan is 2–4 years. M. affinis is an active bioturbator able to rework the sediment. Sometimes dense populations of 10.000–20.000 ind./m² are observed. M. affinis shows strong oscillations in the abundance with a cycle of 6-7 years. M. affinis is found to prevent the recruitment of the Baltic tellin Macoma balthica. The amphipod is prey to the large isopod species skorv (Saduria entomon), the polychaete scale worm (Harmothoe sarsi) and cod.

2. Distribution

M. affinis is distributed over the Baltic Sea, but not in the Belt Sea, where salinity is too high. The species forms the major part of the benthic fauna in the Gulf of Bothnia. In the Bothnian Sea, the abundance reaches the highest values of macrofauna in the open parts of the Baltic. In the western Baltic Sea (Arkona Basin, Bornholm Basin) its distribution is restricted to areas deeper than 10 m (needs colder water), whereas in the eastern and northern sea areas it is also found in the shallow water.

3. Importance

M. affinis is an endemic species to the Baltic Sea.

4. Status of threat

M. affinis is red-listed in the German part of the Baltic Sea as “endangered”.

5. Threat factors

The loss of oxygen in the Baltic Sea - especially on the bottoms of the Gulf of Finland - has reduced the abundance of M. affinis. Reducition of nutrient loading improves living conditions of the species.