Monstrilloids are copepods that live freely in plankton without feeding but have parasitic immature stages that develop within infected benthic molluscs and polychaetes. Because of their incompletely known life cycles and the difficulty of matching conspecific males and females, it has been difficult to assess their true diversity anywhere on earth. The monstrilloid fauna of the Mediterranean and Black seas (MBS) has been investigated for over 140 years, during which time four phases of study can be recognized. The initial list of MBS monstrilloids recorded during the first phase (1877–1893) grew only slowly for decades afterwards during the second phase (1895–1952) because of patchy sampling and a dearth of formal taxonomic descriptions. The third phase (1957–1986) featured little new work at all. During the most recent fourth phase since 1992, a reappraisal with heed to nomenclatural rules and upgraded descriptive standards has led to the realization that many nominal species of MBS monstrilloids are invalid or doubtful. Furthermore, some that have been frequently recorded, such as Monstrilla grandis
, Cymbasoma longispinosum
, and C. rigidum
, may actually be undescribed representatives of widespread species groups. We provide an updated annotated checklist of MBS monstrilloids that includes 21 supposedly valid nominal species or species-groups. This rather high regional diversity will likely grow if future zooplankton surveys in the highly heterogeneous and extensive coastal systems of the MBS pay due attention to this intriguing group of copepods.
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