In the Gullmarsfjord, on the west coast of Sweden, a variety of morphs of Metridium senile (L.) are found. A small form (pedal disc diameter 0.3 cm) is found in dense clones (90% coverage) in high-flow habitats at 2 to 5 m depth, and larger forms (pedal disc diameter = 2.5 to 3.5 cm) are found abundantly in subtidal low-flow habitats where body size increases with depth. In order to investigate the adaptive significance of body size and laceration in M senile from different current regimes, individuals of a large and small morph were reciprocally transplanted between a low-flow (<5 cm S-') and a high-flow (>65 cm S-') subtidal habitat, and incidents of pedal laceration were monitored from August 1992 to January 1993. For comparison, 2 experiments (analogous to the field transplants) were carried out in laboratory flumes. The relative importance of sexual reproduction in the 2 habitats was investigated by comparing gonad sizes of mature females. The results of field and laboratory experiments showed that individuals from the high-flow habitat decreased their rate of laceration in low flow, but maintained a higher rate of pedal laceration than the low-flow population irrespective of site of transplantation, suggesting that properties of asexual proliferation possess both habitat-related and intrinsic components. Sexual reproductive allocation was positively correlated with body size and was significantly greater among females from the low-flow population When adjusted for a common body weight, however, differences in gonad sizes were insignificant. Frequent asexual replication of the genet and fertility at a small size (1 g) are argued to be important features in the high-flow habitat which compensate for the reduced reproductive output of individual anemones and eventually maximize clonal fitness.

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.