We have studied the tube ultrastructure of 44 recent species from 36 serpulid genera. Twelve distinct ultrastructures are identified. Serpulids possess very diverse tube ultrastructures, in contrast with the traditional point of view. Most species show single-layered tubes, but 34% of these species have between two and four ultrastructurally different layers. Tubes are mostly bimineralic, and are composed of aragonite and calcite; however, one of the polymorphs is always dominant. All the studied single-layered tubes with a lamello fibrillar tube ultrastructure are exclusively calcitic; prismatic structures, both in regular or irregular orientation, are either calcitic or aragonitic in composition. There is no correlation between tube mineralogy, and ultrastructure, and marine, brackish, and freshwater environments. We find that 47% of the serpulid species studied possess a unique combination of tube structure characters.

Tube ultrastructure of Jurassic and Cretaceous Glomerula is very similar to that of Recent Calcisabella, supporting the synonymy of these genera and the early Mesozoic origin of calcification in sabellids. Tube structure of serpulids differs from that of Glomerula; calcareous tubes probably evolved convergently within Sabellida. The tube wall in Recent Glomerula piloseta is composed of subparallel lamellae of aragonitic, irregular spherulitic prisms in the inner layer, and spherulites in the outer layer. Calcified lamellae are separated by organic films of different thickness. The structure of the internal tube layer in Glomerula piloseta, and the structure of entire wall in fossil Glomerula, are similar to the tube structure of Dodecaceria (Polychaeta, Cirratulidae). The irregular spherulitic prisms of Glomerula are similar to those found in the external layer of Hydroides dianthus and the internal layer of Spiraserpula caribensis.

The Serpulidae are a large group of sedentary polychaetes inhabiting calcareous tubes. The relationships within the group are poorly understood and taxonomy of the group is very confused which is a major obstacle to accessing their phylogeny. This review provides up-to-date information on the current state of taxonomy of Serpulidae sensu lato (not including Spirorbinae). The morphology of the group is reviewed with special reference to the features that can provide characters for future phylogenetic analyses. Scanning electron micrographs illustrate the structure of the chaetae and uncini. The list of 46—in our opinion valid—genera is accompanied by detailed generic diagnoses, species composition and distribution (checklist), and remarks on major taxonomic literature. A taxonomic key to the genera and a list of invalid genera with synonymy is also provided.