Progenetic species in polychaetes (Annelida) and problems assessing their phylogenetic affiliation

Progenesis is defined as the retention of ancestral juvenile characters by adult stages of descendants due to an acceleration of the sexual maturation and thus is often regarded as a fast evolutionary process. Several small, meiofaunal polychaetes, such as Dinophilidae, some "Dorvilleidae" (for example, Parapodrilus), and Protodrilida, exhibit morphological simplicity in that they lack features typical of larger polychaetes, for example, parapodia and/or head appendages. Due to the general resemblance of adult meiofaunal polychaetes to juveniles of larger forms, progenesis has been invoked to explain evolutionary origins of many smaller taxa with increasing frequency over the past 4 decades. In this review, I summarize the interstitial species of polychaetes for which progenetic origin has been suggested on the basis of morphology. However, critical examination of morphological data that includes larval features reveals that autapomorphic characters uniting supposed progenetic taxa to specific annelid lineages are often missing. Typically larval and juvenile characters, which are argued to support hypotheses of progenetic origin, are often widely dispersed, homoplastic features. Because of this situation, molecular data seem to be the most reliable source for phylogenetic inference. However, other biological data, for example, from life history and morphology, are necessities to substantiate the progenetic evolution of these species.