Abstract
The symbiosis (defined as a strict interspecific association) between bivalves and other organisms is examined. Microsymbiosis (i.e., symbiosis with microorganisms) is frequent among Recent bivalves, and has been proposed to explain the unusual characters of several fossil bivalves. However, a critical review of the morphological criteria used to infer microsymbiosis in fossil bivalves shows that their application is likely to result in a large number of false positive and false negative results. Symbiosis with macroscopic organisms (i.e., macrosymbiosis), on the other hand, has a better chance of being recognised correctly in fossils, although direct preservation of the associated organisms remains the only completely safe criterion. Recent and fossil instances of macrosymbiosis are reviewed, and new evidence is presented to clarify the adaptive significance of some of these associations.
