Many genera are exclusively stygobiont, whereas others are epigean.
A few contain both stygobionts and epigean species. At present, there is considerable taxonomic instability in this group, and phylogenetic relationships among many of the genera remain unresolved. However, approximately 25 genera are monotypic and nine others contain only two or three species. Virtually all of these genera are either exclusively stygobiont or contain at least one or more stygobionts. Pending changes to the classification system of the family, the large, predominately epigean, marine genus Melita may also be assigned to the Hadziidae.