The distribution of the sea anemone Metridium senile (L.) related to dredging for blue mussels (Mytilus edulis L.) and flow habitat

The abundance and size-distribution of the sea anemone Metridium senile was investigated at 6 stations in an area of Limfjorden, Denmark in May and September 2001. Two stations were located at a mussel fishing ground and four stations in an nearby area that have been closed for mussel dredging since 1988. In the closed area two stations were placed in a sheltered habitat (low flow velocity) and two stations in an exposed habitat (moderate flow velocity). A significantly reduced amount of shell debris was observed in the fished area compared to the closed area. Furthermore, M. senile had a lower density in the fished area than in the closed area. In the closed area, the moderate flow stations accommodated significantly higher densities of M. senile than the low flow stations. The mean size (pedal disc diameter) of the sea anemones was smaller in the closed area than in the fished area. It is hypothesised that the quantity and quality of the substratum determines the density and size of M. senile. In the fished area the sub-stratum was limited to a few clumps of shells. The movements of sea anamones were thereby restricted and so was their ability to reproduce asexually and energy is instead allocated into individual growth. In the closed area the sea anemones reproduced asexually at a higher rate due to the presence of larger volumes of substrata as shell debris and resulted in a population structure with a high density of many small individuals.