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Baltic isopod

Idotea balthica

Photo: Joakim Hansen/AzotePhoto: Joakim Hansen/Azote

Baltic isopod, Idotea balthica, is a key species in shallow-water Baltic Sea habitats. It is an important food source for coastal fish, and also the most important grazer on Fucus (bladder wrack and narrow wrack). Sometimes this isopod becomes extremely abundant, resulting in the complete removal of Fucus in that area.

Conservation genetic management advice

  • Maintain current distribution of Fucus and Zostera spp. This is because the abuncance of Idotea balthica depends primarily on the availability of Fucus and, to some extent, of Zostera.
  • Do not to transport Idotea individuals across the Baltic. Idotea shows a genetic population structure in the Baltic Sea, with most geographic areas hosting unique genetic components, see map below. This pattern could allow the populations to adapt to local environmental conditions. Care should thus be taken not to destroy these possibilites through "genetic mixing".
  • In management plans, consider that Idotea populations on the Swedish Bothnian Sea coast seem to be small and fragmented.

Threats to genetic biodiversity 

Future salinity reduction in the Baltic Sea might affect the Idotea populations either directly or indirectly through the loss of Fucus (habitat loss).

Since Idotea lacks a long-distance dispersal life stage, fragmentation of the habitat (Fucus) will reduce connectivity among geographic areas. Thus, habitat fragmentation will reduce genetic diversity and might threaten local and isolated populations, especially on the Swedish Bothian Sea coast. 

Knowledge on genetics in the Baltic (2017)

Idotea shows a strong genetic population structure in the Baltic Sea, with most geographic areas hosting unique genetic components, see map below. This pattern could allow the populations to adapt to local environmental conditions.

The Estonian coast seems to be a hotspot of genetic diversity, while especially the Swedish Bothnian Sea coast is poor in diversity, harboring isolated and small populations.

Genetic diversity is strongly correlated with distance from the entrance of the Baltic Sea. This is likely a result of the demographic history of Idotea balthica: Idotea colonized the Baltic from the North Sea stepwise through several multiple bottleneck events, with each event further reducing genetic diversity.

Population structure in Idotea, Baltic isopod.Population structure in Baltic isopod, based on genetic data. Colours represent genetically distinct populations. Non-coloured coastlines lack data. Illustration: Elsa Wikander/Azote

Tolerance to climate change & potential to adapt

An experimental study in BONUS BAMBI simulating future salinity and temperature conditions of the Baltic Sea reduced the survival of Idotea from all geograpic regions. The negative effect was strongest for populations from the marginal region, indicating that the population sizes here will strongly decrease in the future Baltic Sea, and even that entire populations can be wiped out.

However, there were differences in the survival among populations from the same region, suggesting that there is a geographic variation in the tolerance to climate change conditions among populations. Results also show that there are differences in the tolerance between individuals from the same population. This could indicate it may be possible for the isopods to adapt to the future conditions.

References

CONTRIBUTOR  

Pierre De Wit
pierre.de_wit@marine.gu.se

 

Additional information

There are two other isopod species in the Baltic Sea; Idotea chelipes and Idotea granulosa. They are less genetically diverse than Idotea balthica, and might thus find it more difficult to adapt to environmental changes.

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