The brown algaFucus radicanssuffers heavy grazing by the isopodIdotea baltica

In 2005, a perennial brown alga in the Baltic Sea was recognized as a species on its own and named Fucus radicans (Bergström & Kautsky). This fucoid forms belts like does bladderwrack, Fucus vesiculosus L., its closest relative. These seaweeds are inhabited by many small animals, for example the isopod Idotea baltica (Pallas, 1772). Along the Swedish coasts of the Baltic Sea, F. radicans is found primarily in the northern half, i.e. the Gulf of Bothnia. I. baltica is common in the seaweeds of the southern half of the Baltic Sea, the Baltic proper, but is uncommon further north in the Bothnian Sea and nonexistent in the northernmost Bay of Bothnia. I. baltica is well able to graze down seaweeds in an area. In a field experiment, we here show that I. baltica, given a choice between the two algae species, prefers to graze on F. radicans rather than on F. vesiculosus. This may be one of the most important factors restricting F. radicans to the northern areas of the Baltic, where I. baltica is uncommon.