Effects of substratum instability on locomotion and pedal laceration in Metridium senile (Anthozoa:Actiniaria)

Abstract

On the west coast of Sweden, populations of the small morph of the sea anemone Metridium senile (L.) are often found associated with beds of the mussel Mytilus edulis (L.). We tested the hypothesis that instability of the secondary substratum provided by mussels affects pedal disc laceration of M. senile by stimulating locomotion. The instability of a subtidal bed of M. edulis was quantified by recording movements of individual mussels. Rates of pedal disc laceration of M. senile were investigated in field and laboratory experiments using live and eviscerated mussels as substrata. Field experiments showed that M. senile produced significantly more lacerates on live mussels than on mimic (eviscerated) mussels. In a laboratory flume experiment, the rate of laceration on mimic mussels increased significantly with the frequency of substratum perturbation (overturning) with maximum laceration rates of groups overturned once every 12 and 48 h. Furthermore, the rate of laceration was found to be a function of anemone body size per se. During periods between overturning of the mimics, anemones showed a significant net migration from the protected lower shell to the exposed upper shell in pace with the frequency of overturning. The presence of a vertical flow velocity gradient between and above the mimics supported the hypothesis that migration was due to a rheotactic response in M. senile. The significance of this behavioral pattern and stimulated clonal growth in structuring populations of M. senlle on unstable substrata is discussed.