Evolutionary palaeoecology of serpulid (Polychaeta, Annelida) symbiosis

Throughout their evolutionary history, serpulids, the calcareous tube-dwelling suspension-feeding polychaetes, formed symbiotic relationships with cnidarians (32 relationships), molluscs (six relationships), brachiopods (three relationships), bryozoans (two relationships), echinoderms (one relationship), foraminifers (one relationship) and worms (one relationship). The absolute majority of symbiotic relationships with serpulids had cnidarian partners (micropredators) in the Mesozoic and Cenozoic. There is no observed escalation in the evolution of symbiotic relationships in serpulids. There may be an optimal frequency of symbiotic relationships for serpulid fauna and when that value was achieved in the Jurassic, it stayed constant during the entire evolution of the group. However, the number of serpulid genera involved in symbiosis increased from the Jurassic (six genera) to the Cretaceous (thirteen genera). It seems that the ability to form symbiotic relationships was in general a useful trait as it spread to various branches of the serpulid phylogenetic tree.