Morphology of gametes of mollusks, echinoderms, and brachiopods in systematics and phylogeny

The morphology of eggs and sperm of echinoderms, mollusks, and brachiopods was studied and compared. The gametes of inarticulate brachiopods (two classes Lingulata and Craniata and two subphyla Linguliformea and Craniaformea) are shown to have significant morphological differences from those of articulate brachiopods (extant class Rhynchonellata, subphylum Rhynchonelliformea). Inarticulate brachiopods have similar sperm morphology to that of primitive brachiopods, bivalves and some polychaetes that have external fertilization. Sperm morphology of articulate brachiopods is similar to that of echinoderms, which are considered to be typical deuterostomate invertebrates. This similarity supports an early deviation of lophophore-bearing animals from Bilateria, before this lineage branched into Protostomia and Deuterostomia. Similar gamete morphology in Lingulata and Craniata supports the view that inarticulate brachiopods should be retained as a supraclass taxon for comparison with other Lophotrochozoa, in particular with phoronids, bryozoans, and mollusks. Based on the new data on the gamete morphology in inarticulate brachiopods, we propose the name Lingulophyles with the type genus Lingula, and for articulate brachiopods Coptothyrophyles with the type genus Coptothyris.