10.3 Measurements of byssus formation by the mussel Mytilus edulis 

 (Olle Linden and Maria Foberg) 



Methods : Oil-affected mussels were collected in the littoral zone 

 off Toro about one week after the grounding of the Tsesis . At this time 

 large quantities of oil were floating on the surface. The mussels were 

 collected by a diver at 4-5 m depth and brought to the laboratory in 

 plastic bags. Care was taken to avoid contamination by the surface oil 

 film. Reference mussels were collected close to the Asko laboratory in 

 a similar but unpolluted biotope. 



In the laboratory the mussels were allowed to acclimatize for a 

 couple of hours. After this period mussels of two length-classes (ju- 

 veniles: up to 10 mm, adults: over 10 mm) were placed on petri-dishes 

 which were immersed in unpolluted sea water. Twenty to 25 animals of 

 each length-class from each locality were exposed to a continuous flow 

 of sea water in ~10 litre plastic jars. 



The number of mussels byssally attached to the dish after 3, 4, 5 

 and 6 hours was noted. 



Results and discussion : Fig. 10.3.1 shows the percentage of 

 mussels byssally attached during the course of the experiment. Indi- 

 viduals from the impacted area showed decreased tendency to attach to 

 the substrate. This tendency is more pronounced among adult individuals 

 compared to juveniles. 



Reduction or absence of byssus thread production in mussels ( Mytilus 

 edulis ) or related species under the influence of oil, has previously 

 been observed under laboratory conditions (Smith 1968, Swedmark et al . 

 1973, Eisler 1973, Linden 1977). It is obvious that affected byssal 

 activity under natural conditions must be considered a serious stress 

 syndrome. Mussels incapable of byssal thread production will be unable 

 to remain in their natural habitat as threads are used as mooring lines. 

 They will consequently be washed away and their chances of reattachment 

 at a suitable place are probably rather small. The experiments, although 

 performed under laboratory conditions, indicate that the mussels in the 

 impacted area were subjected to such effects. 



210 



