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Laboratory experiments showed that the mussel Mytilus edulis aggregated more intensely around living organisms (the bivalve Hiatella arctica and the solitary ascidian Styela rustica, which commonly co-occur with mussels in fouling communities) than around inanimate objects. When exposed to an inanimate object, mussels attached their byssal threads primarily to the substrate, close to the object, but when exposed to a living organism, they attached their byssal threads directly to the organism. The ascidian was more intensely covered with byssal threads than was the bivalve. Mussel attachment to the ascidians was apparently determined by the physical characteristics of the tunic and to a lesser extent by the excretion-secretion products released by S. rustica. This study indicates that mussels can use byssus threads as a means of entrapment of potential competitors for space. It remains unclear why mussels preferentially attached to ascidians compared to the bivalve. This can be explained either by competitive interactions, or by attractiveness of the ascidian tunic as an attachment substratum.

The replacement of an ascidian ( Styela rustica L.) fouling community by a blue mussel ( Mytilus edulis L.) community was described for the White Sea. The alternation of populations of these two species takes place in fouling communities developing in the upper 3-to 5-m layer of water. The life span of each type of fouling probably depends on interannual climate fluctuations.

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