5. Evolutionary History of Bivalves as Parasites

Bivalves commonly associate with other organisms, however, examples of true parasitic associations are described only for members of the marine superfamily Galeommatoidea (two species of approximately 500 known species, facultative parasitism) and the larvae of members of the freshwater order Unionida (almost all of 958 known species, obligate parasitism). The evolution toward such a close relationship required establishing a close association with the host’s body, resulting in being enclosed within its tissues. Clear adaptations to the host species are observed in both groups. Most galeommatoideans live in soft sediments and are associated with other benthic organisms or their burrows—settlement in a burrow or on/within the host’s body protects these little bivalves, while life activity of the host possibly ensures oxygenated water currents with a food source for the bivalve. However, a few recent examples of bivalve settlement within the body cavity of crabs (believed as accidental, nevertheless bivalves feed in the crab's hemocoel), or in the oesophagus of holothurians (common, presumably nutrition from the host is possible) indicate possible pathways for an evolutionary transition from free- or commensal-living to a parasitic lifestyle. Unionoids, large freshwater bivalves, are characterized by their tiny larvae that parasitize fish. This close relationship primarily benefits bivalves through enhanced dispersal abilities, but fish tissues may also serve as a source of nutrients for the larvae. Parasitic association likely established when close and common contact of both associates could happen, thus one may hypothesize a fish that lived close to the bottom of lakes and rivers (including durophagous species) as a likely host at the beginning of their co-evolution. Accidental contact of the larvae with the body of fishes (during predation on bivalves or caused by anti-sinking mucous or the larval threads tangled with fish) could result in increased bivalve dispersal. Subsequently, firmer attachment on fish tissues was acquired, followed by encapsulation of the larva within the host epithelium. This might have allowed for the feeding on host tissues, but required developing resistance to the host’s immune system, which might have further strengthened their association.