Progenesis as a principle in meiofauna evolution

Abstract

The interstitial meiofaunal taxa may have evolved in very different ways, which are shortly discussed and schematically represented. Several taxa reveal the possibility of an evolutionary pathway by early maturation of larvae or juvenile stages of large size species with a pelago-benthic life-cycle. This presumed progenetic (= neotenic) evolution can be sustantiated in, for example, various Polychaeta and Crustacea by larval or juvenile morphological features. The progenetic origin of several of these taxa is also strongly supported by morphological series, of which species conforming well with typical larvae form the endpoint. The monophyletic origin of similar progenetic taxa, however, is only proved with difficulties. The theoretical available phylogenetic reconstructions for a series of progenetic taxa are discussed. It is emphasized that progenetic evolution is not regressive or degenerate but novel morphological features like derived reproductive organs are characteristic indications of the progenetic origin of a species. In general, progenesis may have a significant role in the evolution of higher taxa in the interstitial habitat.